9702/Oct Nov/11/2016/Q11

A car has mass m. A person needs to push the car with force F in order to give the car
acceleration a. The person needs to push the car with force 2F in order to give the car
acceleration 3a.

Which expression gives the constant resistive force opposing the motion of the car?

A ma
B 2ma
C 3ma
D 4ma

Solution:

Answer: A

Resultant force = applied force – resistive force

Ma = F- R

R = F- ma —-i

3ma = 2F – R

R = 2F – 3ma —–ii

Substitute for R in eq i

2F – 3ma = F – ma

F = 2ma

Therefore, R = 2ma – ma = ma

Resistive force = ma (A is the correct option)

Reference: PYQ - Oct/Nov 2016 Paper 11 Q11

9702/Oct Nov/13/2015/Q11

Question 11

A rocket of mass 30 000 kg sits on a launch pad on the Earth’s surface. The rocket motors
provide an upward force of 330 kN on the rocket.

What is the initial acceleration of the rocket?

A     0.12 m s^–2
B     1.1 m s^–2
C     1.2 m s^–2
D     11 m s^–2

Solution:
30 000kg x 9.81 = 294 300N
330 000 - 294 300 = 35700N
ma = 35700N
a = 35700/30000
   = 1.19
   = 1.2 ms^-2

Ans: C

Reference: PYQ - Oct/Nov 2015 Paper 13 Q11