Q & A: Forces vs Friction

Q:
If a car is driving down the street at a constant velocity and applying a 5000 Newton force, is this enough information to find the friction force and if it is, is that friction force less than, greater than or equal to 5000 N?
- Dennis
Richwoods High School, Illinois
A:
Dennis:

I get the feeling that that's a homework problem, so I don't think I'll answer it as such.

But I'll try to hint at the answer.

If velocity is constant, that means that there is no acceleration (by definition). So if there is no acceleration, there is no net force acting on the car.

If there is a force pushing the car forward, you'd see its velocity increase because it would accelerate, from Newton's Second Law. If friction were greater than the applied force, the car would also slow down due to the force of friction. What happens if the force of friction equals the pushing force on the car, that makes it go forward?

Think of that and remember that F = ma, and I think you might find the answer.

Jason

(published on 10/22/2007)