The blue vector connects the origin (center) of the motion to the position of the particle. ). 2: (a) A wheel is pulled across a horizontal surface by a force F. 3. Consider the Earth which rotates on its axis once every 24 hours. Angular Velocity. If ω ω increases, then α α is positive. This is an AP Physics 1 topic. 4) (10. We define angular velocity ω as the rate of change of angular displacement. Consider first the angular speed (ω) is the rate at which the angle of rotation changes. Nov 22, 2011 · Courses on Khan Academy are always 100% free. The units of angular velocity are radians per second (rad/s). It uses the principle of conservation of angular momentum to detect changes in the In simple words, angular acceleration is the rate of change of angular velocity, which further is the rate of change of the angle θ θ. The Angular Velocity formula is. I've decided that it is easy to find; all you need to do is measure their revolutions per minute (n) and the carousel radius (R). Therefore, the angular velocity of the Earth’s rotation is . v = rω or ω = v r. At a particular moment, it’s at angle theta, and if it took time t to get there, its angular velocity is omega = theta/ t . The faster the change occurs, the greater the angular acceleration. º ω = 360 º 24 h. Angular velocity diagram: A vector diagram illustrating circular motion. 6. 1) v = ω × r. Formula. (b) Velocity vectors forming a triangle. Combining Equations 7. Jul 20, 2022 · Example 6. The Lesson: The angular velocity of a particle traveling on a circular path is the ratio of the angle traversed to the amount of time it takes to traverse that angle. The rate of change of angular velocity is angular acceleration. Define arc length, rotation angle, radius of curvature and angular velocity. If a ball is travelling in a circle of diameter with velocity , find the angular velocity of the ball. It is an important physical quantity because it is a conserved quantity – the total angular momentum of a closed system remains constant. | Δ→r | = 2rsin(Δθ / 2) When the angle Δθ is small, we can approximate. Angular velocity is defined as a rate of change of angle which is defined relative to some chosen reference point. Sep 12, 2022 · Law of Conservation of Angular Momentum. Suppose the particle is moving with constant angular velocity: the directions of \(\vec r\) and \(\vec v\) are constantly changing, but \(\vec \omega\) is pointing The conversion between radians and degrees is 1 rad = 57. 0 rad / s t = 0, is ω = 9. com ️📚👉 Get Any Class & Subject's Topic Video Here:- https://www. Convert units of measure. The resultent of vector product of any two vectors is a vector quantity. ω = Δθ Δt, which means that an angular rotation (Δθ) occurs in a time, Δt. Measuring Angular Velocity. Linear velocity is the measure of “the rate of change of displacement with respect to time when the object moves along a straight path. The average angular velocity is the angular displacement divided by the time interval: omega = (theta 1 - theta 0) / (t1 - t0) This is the average angular velocity during the time interval from t0 to t1 , but the object might speed up and slow down during How to Find Angular Velocity from Linear Velocity. 5): v = ω × r (7. The final angular velocity was 1. In symbols, this is. The dimension formula of linear velocity is [M]0[L]1[T]-1. Like the linear acceleration is F/m F / m, the angular acceleration is Dec 30, 2020 · Timon Idema. Think of spinning on a merry-go-round. Vector angular velocity: For an object rotating about an axis, every point on the object has the same angular velocity. A particle is moving in a circle of radius R. Radian measure and arc length can be applied to the study of circular motion. Jul 20, 2022 · Figure 6. The amount of change of angular displacement of the particle at a given period of time is called angular velocity. When configuring this constraint, it may be Note that the angular acceleration as the girl spins the wheel is small and positive; it takes 5 s to produce an appreciable angular velocity. Suppose a body is rotating around some fixed axis. The unit of angular acceleration is radians per second 2. Substituting \(\omega \) gives the following relationship between linear and angular velocity, \(v=r\omega \). In Kinematics, we studied motion along a straight line and introduced such concepts as displacement, velocity, and acceleration. Step 1: Determine which formula to use depending on the given data. In symbols, this is \[\omega = \dfrac{\Delta \theta}{\Delta t}, \] where an angular rotation \(\Delta \theta \) takes place in a time \(\Delta t\). Angular velocity, $\omega$ is the rate of change of angular displacement with respect to time. ω = ω = Δθ Δt, Δ θ Δ t, where an angular rotation Δθ Δ θ takes place in a time Δt. sin(Δθ / 2) ≅ Δθ / 2. There are two methods to achieve that: Use the definition of the angular velocity, i. Mar 27, 2022 · Look back at the formula for angular velocity. 14. In physics, angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of linear momentum. The angle the particle makes with the positive x -axis is given by θ(t) = At − Bt3 θ ( t) = A t − B t 3 where A and B are positive constants. So recapping, the angular displacement represents the angle through which an object is rotated. Figure 11. khanacademy. Calculate the angular velocity of a car wheel spin. Understanding Linear Velocity Graphically. Measuring Unit. Note that the units for ω are rad/sec and the units of ν are m/sec. Feb 20, 2022 · The kinematics of rotational motion describes the relationships among rotation angle, angular velocity, angular acceleration, and time. where ω is the angular velocity, v is the linear velocity, and r is the radius. or. It quantifies the rate at which an object rotates or revolves, S. 4 days ago · The symbol for the linear velocity is a simple English letter, which is “v”. We want to determine the angular velocity of the disc D. This complete circle is radians. Angular velocity ω ω is analogous to linear velocity v. The Concept Builder consists of 52 total questions organized into 14 different Question Groups and spread across three Activities. ω= Δθ/Δt= 2πr/t. It is measured in radians per second. In other words, angular velocity is the angle swept out by an object in circular motion, per second. 3 years ago. The angular velocity vector is defined as the derivative of the angular position as a function of time. θ = the angular position. When she hits the brake, the angular acceleration is large and negative. Projectile motion is a Angular Velocity Introduction. Determine (a) the angular velocity vector, and (b) the velocity vector. In physics the average speed of an object is defined as: So suppose that an object moves along a circle of radius \ (r \), traveling a distance \ (s \) over a period of time \ (t \), as in Figure 4. May 13, 2021 · The angular velocity - omega of the object is the change of angle with respect to time. magnetbrains. Step 2: Substitute the 運動学において、角速度（かくそくど、英: angular velocity ）は、ある点をまわる回転運動の速度を、単位時間に進む角度によって表わした物理量である。言い換えれば角速度とは、原点と物体を結ぶ線分、すなわち動径が向く角度の時間変化量である。 Sep 12, 2022 · Angular Momentum of a Particle. Application. How fast is an object rotating? We define angular velocity \(\omega\) as the rate of change of an angle. The revolutions cancel and rad/sec is leftover. Angular velocity is denoted by ω. Note that the angular acceleration as the girl spins the wheel is small and positive; it takes 5 s to produce an appreciable angular velocity. The greater the rotation angle in a given amount of time, the greater the angular velocity. It's typically measured in radians, and it's represented with a delta theta. This could also apply Sep 16, 2022 · 4. t. ”. Representation. We define angular velocity ω as the rate of change of an angle. Mar 16, 2023 · The angular acceleration formula is derived in the same essential way as the angular velocity formula: We can find angular acceleration by finding the change in angular velocity over a certain change in time. In the context of human joint kinematics, angular velocity can be It may seem a very strange choice to have the angular velocity point along the \(z\) axis, when the particle is moving in the \(x\)-\(y\) plane, but in a certain way it makes sense. Angular acceleration is defined as the rate of change of angular velocity. You can estimate the former using the angular displacement calculator. In the first scenario, ac is inversely proportional to the radius, whereas, in the second scenario, ac is directly proportional to the radius of the circular path. I. Angular velocity is a measurement of how much an object rotates in a circle over time. Let us start by finding an equation relating ω, α ω, α, and t t. Angular velocity ω is the rate of change of an angle, ω = Δθ Δt, ω = Δ θ Δ t, where a rotation Δ θ takes place in a time Δ t. The faster the change occurs, the greater is the angular acceleration. But it may be measured in other units as well (such as degrees per second, degrees per hour, etc. In physics, the angular velocity specifies the angular speed at which an object is rotating along with the direction in which it is rotating. Angular velocity ω is the rate of change of an angle, ω = Δθ Δt, ω = Δ θ Δ t, where a rotation Δθ takes place in a time Δt. Steps to Calculate Angular Velocity. Remember in a unit circle, the radius is 1 unit, so in this case the linear velocity is the same as the angular A nice example of angular velocity is the Earth itself. 4. Ω = dv dtm s−1 Ω → = d v → d t m s − 1. The formula for angular velocity is given by, \(\omega =\frac{\theta }{t}\) Where, \(\omega\) is the angular velocity. 1: (a) A particle is moving in a circle at a constant speed, with position and velocity vectors at times t and t + Δt. 57, and the angular acceleration was 0. Since P = torque * angular velocity for the final equation the article shows in that part of the article, angular velocity is just 2pi rad Sudhanwa Garud. 2 and is given by. Find out the types, units, dimensions, and applications of angular velocity with examples and problems. Linear and angular velocities are related to the speed of an object based on the perspective chosen. If your angle is measured in radians then angular frequency ω ω is given by. 0 rad / s. Angular velocity is the rate of velocity at which an object or a particle is rotating around a center or a specific point in a given time period. This is average angular acceleration: \alpha = \frac {\Delta \omega} {\Delta t} α = ΔtΔω. respectively. Use the linear velocity to angular Sep 12, 2022 · The linear velocity has units of m/s, and its counterpart, the angular velocity, has units of rad/s. Solution: Here we have t = 0. To determine this equation, we recall a familiar kinematic equation for translational, or straight-line, motion: Dec 18, 2023 · Angular velocity is a fundamental concept in physics that describes the rate of rotation of an object around a fixed axis. We related the linear and angular velocities of a rotating object in two dimensions in Section 5. e. Angular Acceleration, Velocity. It is a vector quantity, meaning it has both a magnitude and a ω = v / r. 2 7. In Rotational Variables, we saw in the case of circular motion that the linear tangential speed of a particle at a radius r from the axis of rotation is related to the angular velocity by the relation v t = r\(\omega\). But since the angular velocity is decreasing, the angular acceleration α → α → points into the page, in the opposite sense to the angular velocity. There, we also already stated the relation between the linear velocity vector and rotation vector in three dimensions (Equation 5. Hence the angular displacement is θ = s rDividing both side by t θ t = 1 r s tIn the time interval t be infinitesimally small ( t → 0), thenlim t→θ θ t = 1 r angular velocity of the point are related by the formula vr Example A phonograph record is turning at 45 revolutions per minute (rpm). The angular velocity "W" is 2pi rad/sec, meaning it will cover 2pi radians-if given the chance- in 1 second. 393 radians per second squared. In equation form, the angular speed is. [1] The SI unit of angular velocity is radians per second. The AngularVelocity constraint applies torque on an assembly to maintain a constant angular velocity. while angular velocity is. =angular velocity, =linear velocity, and =radius of the circle. The initial angular velocity was zero. . It is generally represented by a symbol \(ω\) \((Omega)\) & measured in \(rad/s\) \((radian per second)\). The two triangles in the figure are similar. , the angle change and time ratio. Angular velocity is often represented by the symbol ω (omega), while linear velocity is represented by the symbol v. Objects which have angular velocity are shows. The final angular velocity is zero, so the average angular acceleration is Define arc length, rotation angle, radius of curvature and angular velocity. If the body is rotating at a constant rate, the average angular velocity is used, Angular frequency (or angular speed) is the magnitude of the pseudovector quantity angular velocity. The tangential velocity of any point is proportional to its distance from the axis of rotation. The equation for average angular velocity is presented in relation to the equation for average linear velocity. Well, the key difference here is that one is a vector quantity while the other is a scalar. So, the linear velocity is equal to the radius times the angular velocity. Consider the complex rotating configuration shown below. While I was watching the bags on the carousel, I've thought about the rate of speed they have. 5 sec, r = 3 m, and θ = \ (\frac {π} {3}\) rad. Now that we have discussed the variations in linear and angular motions, let us look at the formulas for both ️📚👉 Watch Full Free Course: https://www. We measure angular velocity as the amount of rotation in a period of time. The unit of angular speed is radian per second. tangential acceleration. If you only need initial angular velocity, set the AssemblyAngularVelocity method directly on the assembly. t = time. Dec 30, 2016 · To see this relationship, you start with the fundamental definition of the angular momentum of a single particle, $$ \vec L= \vec r \times \vec p = \vec r\times m\vec v, \tag 3 $$ and then you use the fundamental definition of the angular velocity vector - that any particle rotating about the origin with (vectorial) angular velocity $\vec Angular motion should not be interpreted as circular motion. 1) (7. 1 7. http://mathispower4u. t = change in time. If an object rotates through a greater angle of rotation in a given time, it has a greater angular speed. v = r ω or ω = v r. ω≡ d θ /dt ( angular velocity) V = R ω If the arrow tip is accelerating a = d2Vdt2 = R d2 θ / dt2 = R α a = R α (α ≡angular acceleration) θ, ω , α are measured in radians (rd), rd/s, rd/s2. Angular velocity is a vector quantity that expresses both direction and magnitude, while angular speed describes magnitude only. The units of angular acceleration are or If increases, then is positive. Consider a particle moving with uniform circular motion in anticlock wise direction with centre O and radius r the particle cover an arc of length s in time t moving from A to B. the acceleration in a direction tangent to the circle at the point of interest in circular motion. Linear velocity v and angular velocity ω are related by. The units of angular acceleration are (rad/s)/s, or rad/ s2 s 2. The connection between an arc on a circle and the angle it subtends measured in radians allows us to define quantities related to motion on a circle. Possible Answers: Correct answer: Explanation: Using the equation, where. This is very similar to how the linear acceleration is defined. Feb 20, 2022 · Angular Velocity. Step 2: Convert the units to be consistent, if necessary. At t = 0 , it is located on the x -axis. \(\theta\) is the angular displacement. In most cases you should not modify it directly, as this can result in unrealistic behaviour. 2 Displacement vector for circular motion. The red vector is the angular velocity vector, pointing perpendicular Description. This could also apply to The angular velocity (ω) can be derived using the following formula: ω = v / r. In simple words, the rate of change of its angular position is called the angular velocity. The average angular velocity (omega_bar) of an object can be calculated by dividing the change in angle of Dec 20, 2022 · Angular velocity is a vector quantity and has both a magnitude and a direction. Acceleration – We use ‘a’ to denote linear acceleration, while we use ‘α’ to mean angular acceleration. So is the centripetal acceleration directly or inversely proportional to the radius of the circular path, since ac=v^2/r=w^2*r. Jul 28, 2023 · Where ω 1 is the initial angular velocity at time t 1, and ω 2 is the final angular velocity at time t 2. Denoting the angle by θ, the angular velocity is defined as the rate of change of the angle of the body. Alternatively: If you want to control the amount of torque applied, use a Torque constraint. Think of it as your car's speedometer, it shows you're travelliing at 40km/h even though you haven't actually travelled 40km, but you're Oct 19, 2021 · Angular Velocity = dθ / dt = 180 degrees / 3 seconds = 60 deg/s. In both cases, the relationships are analogous to what happens with linear motion. ω = Δθ Δt, ω = Δ θ Δ t, where an angular rotation Δθ takes place in a time Δt. Feb 20, 2022 · In equation form, angular acceleration is expressed as follows: α = Δω Δt, (10. The angular momentum →l of a particle is defined as the cross-product of →r and →p, and is perpendicular to the plane containing →r and →p: →l = →r × →p. Projectile motion is a Mar 14, 2021 · Table 13. Recall that radians are actually unitless, which is why in the formula ν = ωr the radian units disappear. Objects do not need to be rotating or moving in a circle in order to have angular velocity and momentum. The angular momentum of a system of particles around a point in a fixed inertial reference frame is conserved if there is no net external torque around that point: d→L dt = 0. Jul 9, 2024 · What is Angular Velocity? Angular velocity is a measure of how quickly an object rotates or revolves around a central point or axis. The formula for the Linear Velocity is. Angular velocity is a vector quantity and is measured in rad s –1. 1 gives the Euler angular velocities required to calculate the components of the angular velocity ω for the body-fixed (1, 2, 3) axis system. Angular velocity can be measured using various instruments, such as: Gyroscope: A gyroscope is a device that directly measures the angular velocity of an object. Then the body changes its angle with time. The Find its linear and angular speed over that time period. 1: In three-dimensional space, the position vector →r locates a particle in the xy-plane with linear momentum →p. com/ Example Question #1 : Solve Angular Velocity Problems. (1) and the direction is perpendicular to the direction of rotation with orientation given by the right-hand rule. Angular velocity is measured in angle per unit time or radians per second (rad/s). Collecting the individual components of ω, gives the components of the angular velocity of the body, relative to the space-fixed axes, in the body-fixed axis system (1, 2, 3) ω1 = ˙ϕ1 In symbols, this is. So, even though the catapult's arm has NOT covered the whole 2pi radians, it will do so in 1 second. The initial angular velocity of the turbine, setting t = 0, is ω = 9. 5. 4: Circular Motion- Linear and Angular Speed. The angular velocity does not change for uniform circular motion, and the angular acceleration is zero. The vector Δ→v points toward the center of the circle in the limit Δt → 0. where ω is the angular velocity, v is the linear velocity (tangential speed), and r is the radius of rotation. We know that the Earth makes a full rotation of º 360 º every 24 hours, so the angular velocity ω of an object on the equator of the Earth with respect to the middle of the Earth is given by. Since the change in angle is describing the same motion as the change in distance, we can connect the two using a known geometric relationship between angle, θ θ, radius, R R, and arc length, s s: s = Rθ (7. a = d2x dt2 → α = d2θ dt2 a = d 2 x d t 2 → α = d 2 θ d t 2. Start practicing—and saving your progress—now: https://www. If there is a change of angle relative to this reference point as The linear velocity has units of m/s, and its counterpart, the angular velocity, has units of rad/s. To see why P = 2pi(torque)*(angular velocity) plug in the units rad/revolution next to 2pi, and like the article says plug in rev/sec next to angular velocity. Sep 12, 2022 · To analyze rolling without slipping, we first derive the linear variables of velocity and acceleration of the center of mass of the wheel in terms of the angular variables that describe the wheel’s motion. v. Where r is position vector and linear velocity is also vector quantity. In symbols, this is ω = Δθ Δt ω = Δ θ Δ t , where an angular rotation Δ θ takes place in a time Δ t. These quantities are indedepent of R and only depend on the and θ. It is important to note that angular velocity is usually measured in radians per second, as radians are the preferred unit of measurement for angles in scientific calculations. Δ t. 2 Angular Velocity. The angular velocity vector of the rigidbody measured in radians per second. 2. For measuring the Angular Velocity, the units used are degrees and radians. By formulating equations in terms of the angular velocity components, the linear equations are written as: $$\omega_1 + i_ {12} \omega_2 = 0$$$$\omega_2 + i_ {23} \omega_3 = 0$$. For measuring the linear velocity, the unit used is m/s. Commonly-used units for angular velocity include revolutions per minute (RPM, how many trips around the circle in one minute) and radians per second (rad/s, how many radians of angle are swept in one second). 2) (7. unit is $\text{rad s}^{-1}$ or radians per second Nov 21, 2023 · Angular velocity is the rate of change of the position angle of an object with respect to time, so w = theta / t, where w = angular velocity, theta = position angle, and t = time. The direction of the angular velocity vector is perpendicular to the plane of rotation as given What is Angular Velocity? The angular velocity refers to how fast an object is rotating or revolving in a circular path relative to another point. In Rotational Variables, we saw in the case of circular motion that the linear tangential speed of a particle at a radius r from the axis of rotation is related to the angular velocity by the relation v t = r ω v t = r ω. Objects traveling along circular paths exhibit two types of velocity: linear and angular velocity. This online calculator converts angular velocity to linear velocity for given radius. The angular velocity quickly goes to zero. Jun 30, 2023 · Learn what angular velocity is, how to calculate it, and how it relates to linear velocity. Angular velocity is commonly used in fields such as physics, engineering, and robotics to describe the rotation of objects, such as wheels, gears, or propellers. Angular velocity is not constant when a spinning skater pulls in her arms, when a child pushes a merry-go-round to make it rotate, or when a CD slows to a halt when switched off. In equation form, angular acceleration is expressed as follows: where is the change in angular velocity and is the change in time. It is a vector quantity. 4. Angular velocity and linear velocity. In this case the radius is 5 (half of the diameter Dec 4, 2023 · Angular Velocity (ω) = Change in Angular Position / Time For example, if an object rotates 90 degrees in 2 seconds, the angular velocity would be 45 degrees per second. For the kinematics to be fully defined, we must impose a speed constraint on the system. Positive angular acceleration implies an increase in angular velocity, while negative angular acceleration indicates a decrease. Mar 26, 2016 · The symbol for angular velocity is omega, so you can write the equation for angular velocity this way: The figure shows a line sweeping around in a circle. 1 and 7. Angular momentum has both a direction and a magnitude, and Angular Speed Unit. ω = 2πf (rad)s−1 ω = 2 π f (rad) s − 1. Angular velocity can be considered to be a vector quantity, with direction along the axis of rotation in the right-hand rule sense. e. First, we note that the disc is rotating with angular velocity ω May 17, 2024 · Angular Velocity. Two-Dimensional Kinematics dealt with motion in two dimensions. The situation is shown in Figure 11. Note that the total angular momentum →L is conserved. An alternate method to calculate angular velocity is by dividing the angle traveled (θ) by the time elapsed (t): ω = θ / t. The Linear and Angular Velocity Concept Builder focuses attention upon the relationship between an object location in a rotational cycle and the use of degrees and radians to describe such a position. By multiplying the angular quantities Apr 30, 2010 · Calculate Angular VelocityCalculate Linear Velocity. If the distance from the center of the record to a point on the edge of the record is 3 inches, find the angular velocity and the linear velocity of the point in feet per minute. The same formula represents both angular speed and angular velocity. The direction is the same as the the angular displacement direction from which we defined the angular velocity. 3°. So the angular speed ω is. 5. It is a vector quantity, meaning it has both magnitude and direction, and is typically measured in radians per second (rad/s) or degrees per second (deg/s). Step 1: Identify the linear velocity, v, of the object and the radius, r, of the circle that the object is moving along. The angular velocity is thus a vector and for a complex configuration, the various components can ba vectorially added to obtain the total angular velocity. The angular velocity ω of a body in circular motion is defined as: The rate of change of angular displacement. Linear Velocity is nothing more than the velocity of an object in a straight line, whereas Angular Velocity is how much an objects spins, rotates, or turns, as nicely summarized by Chegg. 4) α = Δ ω Δ t, where Δω Δ ω is the change in angular velocity and Δt Δ t is the change in time. The magnitude of the displacement, | Δ→r | is represented by the length of the horizontal vector, Δ→r joining the heads of the displacement vectors in Figure 6. 2, and substituting displacement dx d x with change in arc length Jan 2, 2021 · Linear and Angular Velocity. Note that if the Rigidbody has rotational constraints set, the corresponding angular velocity components are set to zero in the mass space (ie relative to the Jan 22, 2020 · Velocity is defined as the rate of change in position over a given period of time. Delft University of Technology via TU Delft Open. Since it is a vector, it has a magnitude Angular velocity is a vector quantity and is described as the rate of change of angular displacement which specifies the angular speed or rotational speed of an object and the axis about which the object is rotating. org/science/physics/torque-angular-mome For rotational motion about a point or axis, angular velocity is the rate of change of the angular position with time, or in other words the derivative of the angular position with time: = angular velocity, where. In all these cases, angular acceleration occurs because the angular velocity ω ω changes. 1. The units for angular velocity are radians per second (rad/s). It can also be formulated as ω = dθ/dt, the instantaneous rate of change of the angular The angular velocity of the two red dots always remains the same. [1] Angular frequency can be obtained multiplying rotational frequency, ν (or ordinary frequency, f) by a full turn (2 π radians ): ω = 2π rad⋅ν . Solution m 45 rev min Aug 2, 2023 · What is Angular Velocity? Angular velocity is defined as the rate of time at which an object revolves around its own axis. →L = →l1 + →l2 + ⋯ + →lN = constant. 2) s = R θ. com/get-topic-wise Angular Velocity. It is also known as rotational velocity. Jun 30, 2022 · With the angular velocity calculator, you can quickly work out the necessary parameters related to the angular speed. Sep 12, 2022 · Figure 4. Angular velocity is the number of radians covered per second. Nov 5, 2020 · It has the same set of vector quantities associated with it, including angular velocity and angular momentum. The units for angular velocity are degrees per second (º/s Mar 12, 2024 · a unit of angular measure defined by the arc length covered by the angle divided by the radius of the circle that the arc length is part of; one full circle is \ (2 \pi\) radians. ω = 2 π 24 rad h. Radians per second and revolutions per minute are discusses as the units for angular velocity. wordpress. nw me ud jk cj ov gy ul re ht