AP Physics 1 : Work

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #41 : Ap Physics 1

A model rocket weighing 5kg has a net propulsion force of 50N. Over a small period of time, the rocket speeds up (with constant acceleration) from an initial velocity of  to  a final velocity of . Let us assume that the loss of mass due to fuel consumption is negligible and that the net force is along the direction of motion. How much net work was done on the rocket?

Possible Answers:

Correct answer:

Explanation:

The net work done is equal to the change in kinetic energy. So we must find the kinetic energy at both the initial and final velocities and subtract.

Example Question #11 : Work

A large block is sitting on the floor. The block is . You pull the block with a rope, applying  of force at a 25 degree angle with respect to the horizon. How much work did you do on the block if you moved it ? The block moves at a constant speed while you are pulling it.

Possible Answers:

Correct answer:

Explanation:

First let's draw a free body diagram of the block with all the forces acting on it.

Fbd

The block has a weight force which is . The normal force is equal and opposite (otherwise the block would be accelerating into the ground or into the air. The applied force is at an angle, and in order to find the work done we need to find the applied force that is pulling the block across the floor. This is:

The work done is just the product of the applied force and the distance the block slides,

Work is an energy, and the units of  are equal to Joules.

Example Question #12 : Work

Consider a constant force, given below, that acts on an object as it moves along the path , also given below. Calculate the work  done on the object. The units of the force and path are  and , respectively.

Possible Answers:

Correct answer:

Explanation:

In order to find the work done on the object, we need to take the dot product of the force and the path taken. This is a direct calculation.

Example Question #13 : Work

A rocket is in space at location  when it fires it's thrusters. The thrusters provide a force of . The thusters are turned off at location .

 

What is the work done on the rocket by the thrusters?

Possible Answers:

None of these

Correct answer:

Explanation:

Because all of the force was in the Y direction, all of the work will come from the change in the Y coordinate.

We will use the definition of work

 

 

First we need to find the distance traveled.

 

 

Then we can plug that into our work equation.

 

Example Question #11 : Work

You are pulling a dolly with special frictionless wheels on a level surface. There is a rope from your hand to the dolly which you are using to pull it with. The rope makes a angle with the ground. You are applying  of force.

 

How much work is done on the dolly over a distance of ?

Possible Answers:

None of these

Correct answer:

Explanation:

The definition of work is

Where

 is Force

 is displacement

and  is the angle between the displacement and the force.

The angle will be because the dolly is traveling horizontally and the rope which is applying the force is above the horizontal.

 

We plug in our values

 

 

Example Question #15 : Work

A car of mass  is accelerated from  to  in 2s.

Determine the work done on the car in this time frame.

Possible Answers:

None of these

Correct answer:

Explanation:

Use the definition of work:

Plug in known values and solve.

Example Question #16 : Work

A pitcher throws a baseball of mass  with a velocity of . Their hand traveled a liner distance of  during the throw. Calculate the average force of the throw on the ball.

Possible Answers:

Correct answer:

Explanation:

All of the is kinetic energy

Convert  to and plug in values:

Solve for

Example Question #11 : Work, Energy, And Power

A train of mass  goes from  to  in . Calculate the magnitude of force from the brakes.

Possible Answers:

Correct answer:

Explanation:

Use work:

All energy will be kinetic.

Convert  to :

Plug in values. Force will be negative as it is directed against the direction of travel:

Solve for :

Example Question #18 : Work

Which of the following variables are involved in the calculation of work?

I. Force

II. Distance

III. Acceleration

Possible Answers:

II only 

I and II

I only

I, II and III

Correct answer:

I, II and III

Explanation:

Work is defined as:

Where  is work,  is force experienced by the object/person, and  is the distance the object/person moves as a result of the force. Recall that force is calculated as follows.

Where  is mass and  is acceleration; therefore, work depends on the mass and acceleration (force), and distance.

Example Question #19 : Work

A person pushes a heavy stone down a hill. The stone weighs  and rolls down a distance of . How much work is done by the person?

Possible Answers:

Cannot be determined from the given information

Correct answer:

Cannot be determined from the given information

Explanation:

To solve this question, we need to recall the definition of work.

 is work,  is force, and  is distance. Force is defined as:

To calculate work, we need to know about the distance travelled, mass, and acceleration. The question gives us the distance and mass but it doesn’t give us the acceleration of the object; therefore, we cannot calculate the work done by the person. Note that if the question gave us the force applied by the person, then we will be able to calculate the work without mass and acceleration.

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