### All High School Physics Resources

## Example Questions

### Example Question #51 : Energy And Work

A bodybuilder lifts a weight upward , and then returns it to its original position, all with a constant speed. What is the net work done on the weight?

**Possible Answers:**

**Correct answer:**

Work is a vector quantity equal to the change in energy in a given direction. Our net work will be equal to product of force and displacement.

We can write this equation in terms of Newton's second law to incorporate our given values.

During its motion the weight travels a distance of upward, and then downward. Its total distance will be , however, we are looking for its displacement. The initial height and final height are equal, making the displacement equal to zero.

The total work will be zero because there was no net displacement.

### Example Question #2 : Work

A crate slides along the floor for . What other piece of information do we need to find the work done?

**Possible Answers:**

Potential energy

Power

Velocity

Time

Acceleration

**Correct answer:**

Acceleration

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

Since , you can expand the work formula a little bit to see .

The problem gives us the mass and the distance traveled, but not the acceleration. Given the acceleration, along with the mass and distance, we would be able to calculate the work.

### 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?

**Possible Answers:**

Acceleration

Time

Energy

Distance

Velocity

**Correct answer:**

Distance

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?

**Possible Answers:**

A man sits in a chair.

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

A weightlifter lifts a heavy barbell above his head.

A car drives along the road at a constant speed.

A student types numerous essays for class.

**Correct answer:**

A weightlifter lifts a heavy barbell above his head.

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 #5 : Work

Which of these is a correct formula for work?

**Possible Answers:**

**Correct answer:**

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 #6 : Work

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

**Possible Answers:**

We need to know the force used to find the work

We need to know the time to find the work

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

**Correct answer:**

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 #7 : Work

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

**Possible Answers:**

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

**Correct answer:**

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

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

**Possible Answers:**

**Correct answer:**

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 #1 : Calculating Work

A 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?

**Possible Answers:**

**Correct answer:**

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

A 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?

**Possible Answers:**

**Correct answer:**

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 .

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