Posted by **Kaylee** on Tuesday, June 24, 2014 at 10:24pm.

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 .

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

- physics -
**MathMate**, Wednesday, June 25, 2014 at 7:08am
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

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