Question

question 20 (1 point)
when a ball is thrown straight up with no air resistance, the velocity at its point

a) reverses from downward to upward
b) is downward
c) is zero
d) is upward
e) is equal to the initial velocity

Explanation:

When a ball is thrown straight up, it is under the influence of gravity (acceleration \(a = -g=- 9.8\ m/s^{2}\), taking upward as positive). The velocity - time relation is \(v = v_{0}+at\). At the maximum - height point, the ball stops moving upward for an instant before starting to fall. Using the kinematic equation \(v^{2}-v_{0}^{2} = 2ah\) (where at maximum height \(h\), we can also think in terms of the fact that the velocity is changing from positive (upward) to negative (downward). At the exact maximum - height point, the instantaneous velocity \(v = 0\).

• Option (a): Velocity reverses from upward to downward (not downward to upward), so this is incorrect.
• Option (b): At the maximum - height, the ball is not moving downward yet (it is at rest for an instant), so this is incorrect.
• Option (c): As explained above, using the kinematic principle (or the concept of motion under gravity where the upward - moving object stops for an instant at the peak), the velocity at the maximum - height is \(0\), so this is correct.
• Option (d): If the velocity were upward, the ball would still be moving up and not at the maximum - height, so this is incorrect.
• Option (e): By the kinematic equation \(v^{2}-v_{0}^{2}=- 2gh\) (at maximum height \(h>0\)), \(v

eq v_{0}\), so this is incorrect.

Answer:

C. Is zero