A firework rocket is fired vertically upwards. The fuel burns

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.