Forces, Movement, Shape, and Momentum
5 marks · NormalA 1500 kg car is travelling at 18 m/s when the driver sees a hazard and reacts, then brakes to a stop.
Calculate the momentum of the car before the driver reacts.
If the driver’s reaction time is 0.6 s, calculate the thinking distance (distance travelled during reaction).
After braking, the car stops in 5.0 s. Calculate the average braking force acting on the car (assume constant deceleration).
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 27,000 kg m/s
(b) 10.8 m
(c) -5400 N (direction: opposite to motion) N
Common mistakes: Using wrong values for time or mixing reaction/braking phases
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Momentum calculated as mass × velocity.
M1 (b) · 1 mark · States correct formula and substitutes values.
A2 (b) · 1 mark · Correct thinking distance value (10.8 m).
M2 (c) · 1 mark · Correct use of F = ma or F = change in momentum ÷ time with substitution. (follow-through)
A3 (c) · 1 mark · Correct value and sign of braking force (-5400 N). (follow-through)
Worked solution
a) Momentum = mass × velocity = 1500 kg × 18 m/s = 27,000 kg m/s
b) Thinking distance = speed × reaction time = 18 m/s × 0.6 s = 10.8 m
c) Braking force: First, change in velocity = 0 − 18 = -18 m/s acceleration = (final − initial velocity)/time = (0 – 18)/5.0 = -3.6 m/s² F = ma = 1500 × -3.6 = -5400 N (opposite to motion) or F = change in momentum / time = (0 – 27,000) / 5.0 = -5,400 N