A constant horizontal force of 10 N pushes a box 4 m across a floor in the direction of the force. How much work does the force do on the box?
40 J
2.5 J
14 J
0 J
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WhyWork equals force times displacement along the force direction: W = (10 N)(4 m) = 40 J.
2Multiple choice · Easy
A waiter carries a tray horizontally at constant speed across a room. How much work does the upward force from the waiter's hand do on the tray?
Zero, because the force is perpendicular to the displacement
Positive, because the tray moves
Negative, because gravity acts on the tray
Equal to the weight times the distance walked
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WhyThe applied force is vertical while the displacement is horizontal, so the angle between them is 90 degrees and the work is zero.
3Multiple choice · Easy
Which of the following is the correct SI unit for power?
watt (W)
joule (J)
newton (N)
newton-meter per second squared
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WhyPower is the rate of doing work, measured in watts (W), where 1 W = 1 J/s.
4Multiple choice · Easy
A 2 kg object moves at a speed of 3 m/s. What is its kinetic energy?
9 J
6 J
18 J
3 J
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WhyKinetic energy is KE = (1/2)mv^2 = (1/2)(2 kg)(3 m/s)^2 = 9 J.
5Multiple choice · Easy
An object is lifted straight up at constant velocity. As it rises, its gravitational potential energy (relative to the ground)
increases
decreases
stays the same
becomes negative
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WhyGravitational potential energy U = mgh increases as height h increases.
6Fill in the blank · Easy
A 1500 J amount of work is done on a cart in 5 s. The average power delivered to the cart is W.
Answer:
300
WhyPower equals work divided by time: P = 1500 J / 5 s = 300 W.
7Multiple choice · Medium
A 5 kg block slides on a horizontal surface and a constant 20 N friction force acts opposite to its 3 m displacement. How much work does friction do on the block?
-60 J
+60 J
-300 J
0 J
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WhyFriction opposes the motion, so the angle is 180 degrees and W = -(20 N)(3 m) = -60 J.
8Multiple choice · Medium
A 1200 kg car accelerates from rest to 20 m/s on a level road. Ignoring losses, what is the net work done on the car?
240000 J
24000 J
120000 J
480000 J
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WhyBy the work-energy theorem, net work equals the change in kinetic energy: W = (1/2)(1200)(20^2) - 0 = 240000 J = 240 kJ.
9Multiple choice · Medium
A spring with spring constant 200 N/m is compressed 0.10 m from its natural length. How much elastic potential energy is stored?
1.0 J
2.0 J
10 J
0.5 J
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WhyElastic PE = (1/2)kx^2 = (1/2)(200 N/m)(0.10 m)^2 = 1.0 J.
10Multiple choice · Medium
A 0.50 kg ball is dropped from rest from a height of 1.8 m. Using energy conservation and g = 10 m/s^2, what is its speed just before hitting the ground?
Work: The transfer of energy to or from an object by a force acting through a displacement along the direction of that force.
Work-energy theorem: The principle stating that the net work done on an object equals the change in its kinetic energy.
Kinetic energy: The energy an object possesses because of its motion, equal to one-half its mass times the square of its speed.
Potential energy: Stored energy associated with the position or configuration of objects within a system.
Gravitational potential energy: Stored energy a system has due to the height of an object within a gravitational field.
Elastic potential energy: Stored energy in a deformed elastic object such as a stretched or compressed spring.
Spring potential energy: The elastic potential energy stored in a spring, equal to one-half the spring constant times the square of its displacement from equilibrium.
Mechanical energy: The sum of the kinetic and potential energies of a system.