Q: A car is pushed with a force of $$210\ \mathrm{N} at an $$angle of $$30^\circ to $$the direction of motion for a distance of $$18\ \mathrm{m}. $$What is the work done by the force?

$$W = 210 \times 18 \times \cos 30^\circ$$

Q: A farmer pulls a sled with a force of $$5000\ \mathrm{N} at$$ $$36.9^\circ$$ above the horizontal for $$20\ \mathrm{m}. $$The friction force is $$3500\ \mathrm{N}. $$What is the total work done by all forces on the sled?

$$W = 5000 \times 20 \times \cos 36.9^\circ - 3500 \times 20$$

Q: A $$2\ \mathrm{kg}$$ object is moving at $$3\ \mathrm{m/s}. $$What is its kinetic energy?

$$KE = \frac{1}{2} \times 2 \times 3^2$

Q: A $$5\ \mathrm{kg}$$ object increases its speed from $$2\ \mathrm{m/s} to$$ $$6\ \mathrm{m/s}. $$What is the net work done on the object?

$$W = \frac{1}{2} \times 5 \times (6^2$ - 2^2$)$$

Question 1

A person pushes very hard on a heavy desk, trying to move it, but the desk does not move. Which statement is correct regarding the work done by the person on the desk?

Accepted Answer

Work is done only if the desk starts moving.

Question 2

A block is pulled a distance $$d$$ along a horizontal surface by a constant force $$F at an $$angle $$	heta$$ above the horizontal. What is the work done by the force $$F$$?

Accepted Answer

$$W = Fd \cos 	heta$$

Question 3

A person carries a bag of groceries at constant height and walks forward at constant speed. What is the work done by the upward force of the person's hand on the bag as the person walks forward?

Accepted Answer

Zero work is done because the force is perpendicular to the displacement.

Question 4

A weightlifter holds a barbell stationary above his head. What is the work done by the weightlifter on the barbell?

Accepted Answer

Zero work is done because there is no displacement.

Question 5

A car is pushed with a force of $$210\ \mathrm{N} at an $$angle of $$30^\circ to $$the direction of motion for a distance of $$18\ \mathrm{m}. $$What is the work done by the force?

Accepted Answer

$$W = 210 	imes 18 	imes \cos 30^\circ$$

Question 6

An object is thrown straight up. What is the sign of the work done by gravity as the object moves up and then down?

Accepted Answer

Work is negative on the way up, positive on the way down.

Question 7

A hiker carries a $$15.0\ \mathrm{kg}$$ backpack up a hill of height $$10.0\ \mathrm{m} at $$constant velocity. What is the work done by gravity on the backpack?

Accepted Answer

$$W = -mgh$$

Question 8

A block is pulled by a rope at an angle $$	heta$$ above the horizontal. The force is $$2f$$ and the frictional force is $$f. $$For what value of $$	heta is $$the net work on the block zero?

Accepted Answer

$$	heta = 60^\circ$$

Question 9

A farmer pulls a sled with a force of $$5000\ \mathrm{N} at$$ $$36.9^\circ$$ above the horizontal for $$20\ \mathrm{m}. $$The friction force is $$3500\ \mathrm{N}. $$What is the total work done by all forces on the sled?

Accepted Answer

$$W = 5000 	imes 20 	imes \cos 36.9^\circ - 3500 	imes 20$$

Question 10

The work done by a force that varies with position $$F(x) as an $$object moves from $$x_1 to$$ $$x_2 is $$given by:

Accepted Answer

$$W = \int_{x_1$}^{x_2$} F(x)\,dx$$

Question 11

A spring with force constant $$k is $$stretched a distance $$X. $$What is the work done by the force stretching the spring?

Accepted Answer

$$W = \frac{1}{2}kX^2$

Question 12

A block moves in a circle at constant speed under the influence of a force always perpendicular to its displacement. What is the work done by this force over one complete revolution?

Accepted Answer

Zero

Question 13

A $$2\ \mathrm{kg}$$ object is moving at $$3\ \mathrm{m/s}. $$What is its kinetic energy?

Accepted Answer

$$KE = \frac{1}{2} 	imes 2 	imes 3^2$

Question 14

A net force acts on a moving object, but the total work done on the object is zero. Which of the following is a possible scenario?

Accepted Answer

The object moves in a circle at constant speed.

Question 15

The work-energy theorem states that the net work done on an object is equal to:

Accepted Answer

The change in kinetic energy.

Question 16

A $$5\ \mathrm{kg}$$ object increases its speed from $$2\ \mathrm{m/s} to$$ $$6\ \mathrm{m/s}. $$What is the net work done on the object?

Accepted Answer

$$W = \frac{1}{2} 	imes 5 	imes (6^2$ - 2^2$)$$

Question 17

A force of $$10\ \mathrm{N}$$ acts at an angle of $$60^\circ to $$the direction of displacement of $$5\ \mathrm{m}. $$What is the work done by the force?

Accepted Answer

$$W = 10 	imes 5 	imes \cos 60^\circ$$

Question 18

Which of the following statements about power is correct?

Accepted Answer

Power is the rate at which work is done.

Question 19

An elevator is lifted at constant speed by a cable. Which statement is correct?

Accepted Answer

The cable does positive work on the elevator, and the elevator does negative work on the cable.

Work and Kinetic Energy: Concepts, Calculations, and Applications

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