Q#7 (Past Exam Paper – June 2015 Paper 12 Q10)
A firework rocket is fired vertically upwards. The fuel burns and produces a constant upwards force on the rocket. After 5 seconds there is no fuel left. Air resistance is negligible.
What is the acceleration before and after 5 seconds?
before 5 seconds after 5 seconds
A constant constant
B constant zero
C increasing constant
D increasing zero
Solution:
Answer: C.
As the fuel burns, it produces a constant upwards force on the rocket.
F = ma
Acceleration a = F / m
Total mass = mass of rocket + mass of fuel
As the fuel burns its mass decreases. (The mass of the rocket with its fuel does not remain constant.) This causes the acceleration to increase.
After 5 seconds there is no fuel left. So,
Total mass = mass of rocket
The mass of the rocket itself does not change with time.
Since a = F / m, the acceleration remains constant after 5 seconds as the mass is no longer changing.
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