Peoples Physics Book Version-2

6.4. Common Forces http://www.ck12.org

There are some things about friction that are not very intuitive:

• The magnitude of the friction force does not depend on the surface areas in contact.


• The magnitude of kinetic friction does not depend on the relative velocity or acceleration of the two objects.


• Friction always points in the direction opposing motion. If the net force (not counting friction) on an object is
  lower than the maximum possible value of static friction, friction will be equal to the net force in magnitude
  and opposite in direction.

Spring Force

Finally, the last force we will cover is that exerted by a stretched spring. Any spring has some equilibrium length,
and if stretched in either direction it will push or pull with a force equal to:

F~s p=−k∆~x [ 7 ]Force of spring∆~xfrom equilibrium

Spring Example

Question: A spring with a spring constant ofk=400N/m has an uncompressed length of.23m and a fully
compressed length of.15m. What is the force required to fully compress the spring?

Solution: We will use the equation

F=kx

to solve this. We simply have to plug in the known value for the spring and the distance to solve for the force.

F=kx= (400N/m)(.23m−.15m) =32N