BECE Force, Work, and Energy 3 — Questions and Answers
Question 1: A boy carries a heavy load horizontally across a room at constant height. How much work does gravity do on the load?
- Positive work equal to the weight times distance
- Negative work
- Zero work (Correct answer)
- Work equal to the normal force times distance
Correct answer: Zero work
Gravity acts vertically downward while displacement is horizontal, so the angle between them is 90°, giving zero work (W = Fd cosθ = Fd cos90° = 0).
Question 2: Which energy transformation occurs when a stretched rubber band is released and flies across the room?
- Kinetic → potential
- Chemical → kinetic
- Elastic potential → kinetic (Correct answer)
- Gravitational potential → kinetic
Correct answer: Elastic potential → kinetic
The elastic potential energy stored in the stretched rubber band converts to kinetic energy as it moves.
Question 3: Friction between two surfaces always acts in which direction relative to motion?
- In the same direction as motion
- Perpendicular to motion
- Opposite to the direction of motion (Correct answer)
- Vertically downward
Correct answer: Opposite to the direction of motion
Friction is a resistive force that always opposes the direction of motion or intended motion.
Question 4: A pendulum swings from its highest point to the lowest point. What happens to its energy?
- Potential energy increases; kinetic energy decreases
- Both remain constant
- Potential energy decreases; kinetic energy increases (Correct answer)
- Both decrease to zero
Correct answer: Potential energy decreases; kinetic energy increases
As the pendulum falls to its lowest point, gravitational potential energy converts to kinetic energy (ignoring air resistance).
Question 5: A force of 50 N acts on an object but the object does not move. How much work is done?
- 50 J
- 0 J (Correct answer)
- Cannot be determined
- 500 J
Correct answer: 0 J
Work requires displacement; if the object does not move (d = 0), then W = F × 0 = 0 J.
Question 6: Which of the following best describes what happens to kinetic energy when an object's speed doubles?
- Kinetic energy doubles
- Kinetic energy triples
- Kinetic energy quadruples (Correct answer)
- Kinetic energy halves
Correct answer: Kinetic energy quadruples
Since KE = ½mv², doubling v means v² becomes 4v², so kinetic energy quadruples.
Question 7: The weight of an object on the Moon is less than on Earth because the Moon has:
- Less mass than Earth
- A weaker gravitational field strength (Correct answer)
- No atmosphere
- A smaller radius
Correct answer: A weaker gravitational field strength
Weight = mg; the Moon's gravitational field strength (≈1.6 m/s²) is about one-sixth of Earth's (10 m/s²), so objects weigh less there.
BECE Force, Work, and Energy 3
A boy carries a heavy load horizontally across a room at constant height.
How much work does gravity do on the load?