A model rocket is fired vertically from rest. It has a net acceleration of 19.5

Question

A model rocket is fired vertically from rest. It has a net acceleration of 19.5 m/s2. After 1.0 s, its fuel is exhausted and its only acceleration is that due to gravity.

The model rocket has a mass of 81 g and you may assume the mass of the fuel is much less than 81 g.
(a) What was the net force on the rocket during the first 1.0 s after liftoff?
magnitude ________ N
direction (pick one) upward or downward?

(b) What force was exerted on the rocket by the burning fuel?
magnitude _________ N
direction(pick one) upward or downward?

(c) What was the net force on the rocket after its fuel was spent?
magnitude__________N
direction (pick one) upward or downward?

(d) The rocket's vertical velocity was zero instantaneously when it was at the top of its trajectory. What were the net force and acceleration on the rocket at this instant?
Net force: magnitude __________ N
direction (pick one) upward or downward
Acceleration: magnitude________ m/s2
direction (pick one) upward or downward

Explanation / Answer

a) F = ma = 0.081 * 19.5 =1.5795 N; UPWARD b) by burning fuel F = m(a-g) = 0.081(19.5-9.8)= 0.7857 N; UPWARD; c) net force after fule is spent = F= mg = 0.081*9.8 = 0.7938 N DOWNWARD; d) at the top velocity is zero and accleration is g (downward) F = mg = 0.7938 N DOWNWARD;

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