Your 1000 kg car stops on the road while driving home. It refuses to start so yo

Question

Your 1000 kg car stops on the road while driving home. It refuses to start so you need to push it off the road to the grassy side of the street.

- You successfully push it on the asphalt at constant velocity. If the kinetic coefficient of friction of rubber on asphalt is .5, what is the kinetic friction force you need to overcome to keep the car moving at constant velocity?

- You reach the grassy part and you keep on pushing your car at constant speed. If the kinetic coefficient of friction of rubber on grass is .7, what is the kinetic friction force you need to overcome to keep the car moving at constant velocity?

- Which force is larger and why?

Explanation / Answer

Since the car is moving at constant velocity both in a) and b) the sum of the forces in x and y must be equal to zero.

Fx=0 =>0=Fpush-Ffriction ------>Fpush=N

Fy=0 =>0=Fnormal-Fgravity ---->Fnormal=N=mg=(1000)(9.8)=9800 Newtons

For a), =0.5. Thus, the force of push on the asphalt is equal to 9800*0.5=4900 Newtons.

For b), =0.7. Thus, the force of push on the grass is equal to 9800*0.7=6860 Newtons.

The force of push on the grass is higher since the coefficient of kinetic friction is higher.

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