# AP Physics C: Mechanics : Using Torque Equations

## Example Questions

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### Example Question #1 : Using Torque Equations

An object starts from rest and accelerates to an angular velocity of  in three seconds under a constant torque of . How many revolutions has the object made in this time?

Explanation:

Since it is experiencing a constant torque and constant angular acceleration, the angular displacement can be calculated using:

The angular acceleration is easily calculated using the angular velocity and the time:

Using this value, we can find the angular displacement:

Convert the angular displacement to revolutions by diving by :

### Example Question #1 : Rotational Motion And Torque

A circular disk of radius 0.5m and mass 3kg has a force of 25N exerted perpendicular to its edge, causing it to spin. What is the angular acceleration of the disk?

Explanation:

We can find the angular acceleration using the rotaional motion equivalent of Newton's second law. In rotational motion, torque is the product of moment of inertia and angular acceleration:

The moment of inertia for a circular disk is:

The tourque is the product of force and distance (in this case, the radius):

We can plug these into our first equation:

Simplify and rearrange to derive an equation for angular acceleration:

Use our given values to solve:

### Example Question #11 : Circular And Rotational Motion

A meter stick is nailed to a table at one end and is free to rotate in a horizontal plane parallel to the top of the table. Four forces of equal magnitude are applied to the meter stick at different locations. The figure below shows the view of the meter stick from above.

You may assume the forces  and are applied at the center of the meter stick, and the forces  and  are applied at the end opposite the nail.

What is the relationship among the magnitudes of the torques on the meter stick caused by the four different forces?

Explanation:

Torque is given by,

Since all of the forces are equal in magnitude, the magnitude of the torque is then influenced by the radius r and the angle theta between the radius and the force.

For ,

For ,

For ,

For ,

Combining this information yields the relationship,

### Example Question #1 : Using Torque Equations

A man sits on the end of a long uniform metal beam of length . The man has a mass of  and the beam has a mass of .

What is the magnitude of the net torque on the plank about the secured end of the beam? Use gravity .

Explanation:

The net torque on the beam is given by addition of the torques caused by the weight of the man and the weight of the beam itself, each at its respective distance from the end of the beam:

Let's assign the direction of positive torque in the direction of the torques of the man's and the beam's weights, noting that they will add together since they both point in the same direction.

We can further simplify by combining like terms:

Using the given numerical values,

### Example Question #51 : Motion

Two children sit on the opposite sides of a seasaw at a playground, doing so in a way that causes the seasaw to balance perfectly horizontal. The  child on the left is  from the pivot.

What is the mass of the second child if she sits  from the pivot?

Explanation:

A torque analysis is appropriate in this situation due to the inclusion of distances from a given pivot point. Generally,

This is a static situation. There are two torques about the pivot caused by the weights of two children. We will note that these weights cause torques in opposite directions about the pivot, such that

Consequently,

Or more simply,

Solving for ,

### Example Question #1 : Using Torque Equations

A gymnast is practicing her balances on a long narrow plank supported at both ends. Her mass is . The  long plank has a mass of .

Calculate the force the right support provides upward if she stands  from the right end. Use gravity .

Explanation:

A torque analysis is appropriate in this situation due to the inclusion of distances from a given pivot point. Generally,

This is a static situation. As such, any pivot point can be chosen about which to do a torque analysis. The quickest way to the unknown force asked for in the question is to do the torque analysis about the left end of the plank. There are three torques about this pivot: two clockwise caused by the weights of the gymnast and the plank itself, and one counterclockwise caused by the force from the right support. Designating clockwise as positive,

Consequently,

This simplifies to

Solving for ,

Solving with numerical values,

### Example Question #1 : Using Torque Equations

A square of side lengths  and mass  is shown with  possible axes of rotation.

Which statement of relationships among the moments of inertia is correct?

Explanation:

The moments of inertia for both axes  and  are equal because both of these axes are equivalently passing through the center of the mass.

By the parallel axis theorem for moments of inertia (), the moment of inertia for axis  is larger than  or

because it is located a distance

away from the center of mass.

### Example Question #1 : Using Torque Equations

A wind catcher is created by attaching four plastic bowls of mass  each to the ends of four lightweight rods, which are then secured to a central rod that is free to rotate in the wind. The four lightweight rods are of lengths , , , and .

Calculate the moment of inertia of the four bowls about the central rod. You may assume to bowls to be point masses.

Explanation:

The moment of inertia for a point mass is .

To calculate the total moment of inertia, we add the moment of inertia for each part of the object, such that

The masses of the bowls are all equal in this problem, so this simplifies to

Plugging in and solving with numerical values,

### Example Question #61 : Mechanics Exam

A long uniform thin rod of length  has a mass of .

Calculate the moment of rotational inertia about an axis perpendicular to its length passing through a point  from one of its ends.

Explanation:

For a long thin rod about its center of mass,

According to the parallel axis theorem,

where  is defined to be the distance between the center of mass of the object and the location of the axis parallel to one through the center of mass. For this problem,  is the difference between the given distance and half the length of the rod.

Combining the above,

Inserting numerical values,

### Example Question #61 : Mechanics Exam

The moment of inertia of a long thin rod about its end is determined to be .

What is the new value if the mass and length of the rod are both reduced by a factor of ?