newton's law of motion

Question

Harry is pushing a car down a level road at 2.0 m/s with a force of 243 N. The total force acting on the car in the opposite direction, including road friction and air resistance, is which of the following?

a. Slightly more than 243 N.

b. Exactly equal to 243 N.

c. Slightly less than 243 N.

4. A 0.50 kg model rocket accelerates from 20 m/s [up] to 45 m/s [up] in 0.70 s. Calculate the unbalanced force acting on it

Answer 1

Since Harry is pushing at a constant velocity, the acceleration is zero. Since F = ma, and a = 0, then F = 0.

So if he is pushing with a force of 243 N, there must be an equal and opposite force of 243 N to result in a net zero situation.

For problem 4)

Using v_f = v_i + at where vf = 45 m/s, vi = 20 m/s and t = 0.70 s:

45 = 20 + 0.70a

25 = 0.7a

a = 35.7 m/s²

F = ma

F = (0.50)(35.7) = 17.86 N which rounds to 18 N