A car and driver weighing 7750 N passes a sign stating “Bridge Out 28.5 m Ahead.” She slams on the brakes, and the car decelerates at a constant rate of 10.6 m/s2 .

Question

The acceleration of gravity is 9.8 m/s2 .
What is the magnitude of the work done stopping the car if the car just stops in time to avoid diving into the water?
Answer in units of J

Answer 1

Using Newton's second law F=ma
to find the mass, then again the same law to find the frictional force.
Finally use Work done = F.D
Units are in kg, m/s² and J.

Mass m = 7750/9.8 = 790.82 kg
Frictional force = ma
= 790.82 kg * 10.6 m/s²
= 8382.7 N

Work done (assuming uniform frictional foce)
= F.D
= 8382.7 N * 28.5 m
= 238906 J
= 238.9 kJ