An object moves along a level road with constant velocity. The net force acting on the object is?

Question

a) opposite to the direction of motion
b) in the direction of motion
c) upward
d) downward
e) zero

Answer 1

The first person to answer is correct if not insightful. Here's the physics in case you want to know why (e) is correct:

f = ma = P - F = 0; where f is net force on a body accelerating at a due to a pushing force and a pulling force (e.g., friction).

Thus, when a = 0, which means velocity v = constant, the net force has to be zero because mass (m) is definitely not zero. In which case, we also have that the push (P) has an equal, but opposite force F, that gives us the net force of zero (ans e).

An interesting lesson here is that if F = 0 (e.g., no friction), no pushing force (P) is necessary to keep that object moving if it's already moving. In other words, the reason we'd need to push a car, for example, is because there is friction. Without friction, once we got the car rolling it would keep on rolling along that level road.

Answer 2

All the forces at play have the vehicle in equilibrium - it is moving at a constant velocity. Think of an airplane - There are weight force, lift force, forward force and drag. The weight equals the lift, the forward force equals drag. It is in equilibrium. Just because the car is moving doesn't mean it isn't in equilibrium. If something started to slow it down, the equilibrium would no longer be zero, would it?

Answer 3

Constant velocity means no acceleration and hence no overall forces.

The answer is e.