# Q & A: Friction force on a car

Q:
If a car has no net force acting on it, would the friction force be equal to the Newton force being applied?
- Cathy (age 16)
Iowa
A:
I don't know what a "Newton force" is, but the metric ("La Systeme Internationale" or "MKS") unit of force is the Newton, which is one kilogram meter per second squared.

"Net" force usually means the "total" force, or, the way I like to say it, the vector sum of all forces on an object. If the sum of all forces on an object is zero, that means if you consider just one of the forces on an object, the sum of all of the other forces has to be equal in magnitude and opposite in direction to the one you are thinking about.

A good example is the car example you bring up. If a car is parked on a flat road (like the ones here in Illinois), friction holds it in place. If there are no external forces pushing horizontally, then the friction force will vanish. Vertical forces balance out between gravity and what we call the "normal force", a contact force of the road on the tires which holds the car up.

Iowa has rolling hills, and other states have genuinely steep roads in them. If you park a car on a hill, the friction force of the road on the tires has to balance out the component of gravity which points down the hill. Consult any mechanics textbook to see this worked out. You can imagine other forces here, like normal and friction forces from wheel chocks or a tow-truck chain, but if the sum adds to zero, each force opposes the sum of the rest.

Tom

(published on 10/22/2007)