## force exerted by the motor?

1. The problem statement, all variables and given/known data
A remote control electric toy car of mass 0.5kg starts from rest and reaches a speed of 1.8m/s in 2s while travelling south along a straight level path. During this time the average force needed to overcome friction is 0.25N.

What force is exerted by the motor of the car during this time? (Assume that this force is constant.)

2. Relevant equations
a=del(v)/del(t)

3. The attempt at a solution
The observed acceleration is .9m/s^2 hence an observed force of .45N forwards. The car in travelling must also provide a .25N in the same direction in order to eliminate friction so that comes to a total of .7N exerted by the motor. Correct?

The answer at the back of the book suggested .45N only. Is the answer wrong?
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 While on this topic there is another similar question. An object reaches a rough surface where the force of friction is 5N. The applied force on the object is 10N and the object is dragged over a distance of 9m. What is the work done by the applied force? Is it only 45N as only the force along the direction of movement of the object is counted? So the force gone into friction is not counted as work as that applied 5N did not go into moving the mass any distance.
 Recognitions: Homework Help Science Advisor You are correct.

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