High School Physics : Energy and Work

Example Questions

Example Question #1 : Understanding Work

Miguel pushes a crate with  of force. What other piece of information do we need to find the work he does?

Energy

Velocity

Distance

Time

Acceleration

Distance

Explanation:

The formula for work is , or work equals force times distance.

The problem gives us the mass, but is missing the distance. Given the distance, we would be able to calculate the work.

Example Question #1 : Work

Which of the following is an example of work?

A skier skies down a frictionless hill with no loss in kinetic energy.

A student types numerous essays for class.

A weightlifter lifts a heavy barbell above his head.

A car drives along the road at a constant speed.

A man sits in a chair.

A weightlifter lifts a heavy barbell above his head.

Explanation:

Work is a force times a distance or

Only one of these has a force times a distance: the weightlifter lifting a weight above his head.

The car moving at a constant velocity has no acceleration and therefore no force.

Typing essays does not require a force times a distance (though it sure feels like work!).

If the skier has no loss in energy, then no work was done, as work is also the measurement of the change in energy.

The man sitting in a chair has a constant velocity of zero, no acceleration, and therefore no force.

Example Question #3 : Work

Which of these is a correct formula for work?

Explanation:

Work is produced when a force is used to create a change in distance. The amount of work is given by the product of the distance traveled and the force applied.

Example Question #1 : Understanding Work

Michael pushes a box  across the floor. If the box moves with a constant velocity, what is the total work done?

We need to know the mass of the box to find the work

We need to know the time to find the work

We need to know the force used to find the work

Explanation:

Works is equal to force times distance:

Since the box moves with a constant velocity, it has no acceleration. Remember that , so if there is no acceleration, then there must be no net force.

If there is no net force, then there is also no work.

Example Question #2 : Work

Rodrigo pushes a sofa  down a long hallway. If the couch moves with a constant velocity, how much work does Rodrigo do?

We need to know the coefficient of friction of the sofa on the floor to solve

We need to know the mass of the sofa to solve

Explanation:

Work is the product of a net force over a given distance.

In order for there to be work, there must be a net force applied and the object must have a non-zero displacement. In this question we are given the displacement, but we have to solve for the force.

The question only tells us that the couch moves with constant velocity. This tells us that acceleration is zero. If acceleration is zero, then force must also be zero according to Newton's second law.

If the force is zero, then the work is also zero.

Example Question #1 : Calculating Work

box is dropped . How much work was done on the box?

Explanation:

The formula for work is , work equals force times distance.

In this case, there is only one force acting upon the object: the force due to gravity. Plug in our given information for the distance to solve for the work done by gravity.

Remember, since the object will be moving downward, the distance should be negative.

The work done is positive because the distance and the force act in the same direction.

Example Question #2 : Calculating Work

book falls off the top of a  bookshelf. How much work is required to lift the book back to its original position, assuming the lifting is done with a constant velocity?

Explanation:

Work is a force times a distance:

We know the distance that the book needs to travel, but we need to sovle for the lifting force required to move it.

There are two forces acting upon the book: the lifting force and gravity. Since the book is moving with a constant velocity, that means the net force will be zero. Mathemetically, that would look like this:

We can expand the right side of the equation using Newton's second law:

Use the given mass and value of gravity to solve for the lifting force.

Now that we have the force and the distance, we can solve for the work to lift the book.

This problem can also be solved using energy. Work is equal to the change in potential energy:

While on the ground, the book has zero potential energy. Once back on the shelf, the energy is equal to . The work is thus also equal to .

Example Question #1 : Calculating Work

book falls off the top of a  bookshelf. How much work is required to put the book back on the top of the bookshelf, assuming it is lifted with a constant velocity?

Explanation:

Work is a force times a distance:

We know the distance that the book needs to travel, but we need to sovle for the lifting force required to move it.

There are two forces acting upon the book: the lifting force and gravity. Since the book is moving with a constant velocity, that means the net force will be zero. Mathemetically, that would look like this:

We can expand the right side of the equation using Newton's second law:

Use the given mass and value of gravity to solve for the lifting force.

Now that we have the force and the distance, we can solve for the work to lift the book.

This problem can also be solved using energy. Work is equal to the change in potential energy:

While on the ground, the book has zero potential energy. Once back on the shelf, the energy is equal to . The work is thus also equal to .

Example Question #1 : Calculating Work

book falls off the top of a  bookshelf. How much work is required to put the book back on the top of the bookshelf, assuming it is lifted with a constant velocity?

Explanation:

Work is a force times a distance:

We know the distance that the book needs to travel, but we need to sovle for the lifting force required to move it.

There are two forces acting upon the book: the lifting force and gravity. Since the book is moving with a constant velocity, that means the net force will be zero. Mathemetically, that would look like this:

We can expand the right side of the equation using Newton's second law:

Use the given mass and value of gravity to solve for the lifting force.

Now that we have the force and the distance, we can solve for the work to lift the book.

This problem can also be solved using energy. Work is equal to the change in potential energy:

While on the ground, the book has zero potential energy. Once back on the shelf, the energy is equal to . The work is thus also equal to .

Example Question #2 : Calculating Work

A cat knocks a  toy mouse across the floor with  of force. If the cat did  of work, how far did the mouse travel?