A ball is projected vertically upwards and then falls back to its original position.
Once projected, the ball experiences only a single force, downwards, due to a constant gravitational field strength of 10 N/kg.
Here are three statements about the ball:
- When the ball is moving upwards it loses kinetic energy and gains potential energy.
- The magnitude of the ball’s acceleration increases as it falls.
- No vertical forces act on the ball when it is at its maximum height
Which of the statement(s) is/are correct?
A. 2 and 3 only
B. none of them
C. 1 and 2 only
D. 3 only
E. 1 only
Using the key words from this question, namely being a “vertically projected ball”, we can formulate our own statements about this system -stating what we know before verifying the mentioned answers.
For a vertically projected ball, it is true that:
A. At it’s maximum height, vertical forces act on the ball. Gravity for example, working downwards.
B. Hence, the acceleration of the ball must be constant going downwards, as it moves at a constant gravitational acceleration of 10m/s^2.
C. As the ball shoots upwards, it must gain Gravitational Potential Energy (GPE).
D. When it shoots upwards, it must eventually slow down- meaning it loses kinetic energy.
What we can now do, is refute the question’s statements using our own.
Statement 1 must be true, as it closely matches our reasoning D and C.
Statement 2 is false, and is refuted by our reasoning B.
Statement 3 is false, as it is refuted by our reasoning A.
Making our final answer Statement 1 only, E.