Question 7
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
Reference: Past Exam Paper – June 2015 Paper 12 Q10
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.
Wouldn't the acceleration be 0 when there is no fuel left??
ReplyDeletethe fuel provides energy, but not force.
Deletethe upwards force is still present. and as explained, since the mass is no longer changing, the acceleration is constant.
Could you plz explain this part in more detail
Deleteit is as explained above. As the mass is not changing, the acceleration is constant as the same force is present.
Deleteyou need to identify the difference between force and energy in physics.
energy is the ability to do work.
e.g. where there is a force but no energy transferred: pushing a wall. a force is applied on the wall but the latter does not move - so no energy is transferred.
the same applies here: there is no fuel left (fuel is the source of energy) but there can be a force (this force causes an acceleration).
What about gravitational force ?
ReplyDeleteeven if the weight acts, the above explanation would be similar
DeleteThank you so much..... I was so confused in this mcq......
ReplyDelete