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AP Physics
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Notes: Friction Forces
4.6 Forces of Friction

When an object is in motion on a surface or through a viscous medium, there will be a resistance
to the motion
o This is due to the interactions between the object and its environment

This is resistance is called friction

Friction is proportional to the normal force

The force of static friction is generally greater than the force of kinetic friction

The coefficient of friction (µ) depends on the surfaces in contact

The direction of the frictional force is opposite the direction of motion

The coefficients of friction are nearly independent of the area of contact

Static friction acts to keep the object from moving

If F increases, so does ƒs

If F decreases, so does ƒs

ƒs  µ n

The force of kinetic friction acts when the object is in motion

ƒk = µ n

Variations of the coefficient with speed will be ignored
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Block on a Ramp, Example

Axes are rotated as usual on an incline

The direction of impending motion would be down the plane

Friction acts up the plane

Opposes the motion

Apply Newton’s Laws and solve equations
Example 4.11: Suppose a block with a mass of 2.50 kg is resting on a ramp. If the coefficient of static
friction between the block and ramp is 0.350, what maximum angle can the ramp make with the
horizontal before the block starts to slip down?
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Example 4.12: The hockey puck in the diagram struck by a hockey stick, is given an initial speed of
20.0 m/s on a frozen pone. The puck remains on the ice and slides 1.20 x 102 m, slowing down steadily
until it comes to rest. Determine the coefficient of kinetic friction between the puck and the ice.
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Connected Objects

Apply Newton’s Laws separately to each object

The magnitude of the acceleration of both objects will be the same

The tension is the same in each diagram

Solve the simultaneous equations
Example 4.13: A block with mass m1 = 4.00 kg and a ball with mass m2 =
7.00 kg are connected by a light string that passes over a frictionless pulley,
as shown in the diagram. The coefficient of kinetic friction between the
block and the surface is 0.300.
a. Find the acceleration of the two objects and the tension in the
string.
b. Check the answer for the acceleration by using the system
approach.
Example 4.14: A block of mass 5.00 kg rides on top of a second block of mass 10.0 kg. A person
attaches a string to the bottom block and pulls the system horizontally across a frictionless surface, as in
the diagram. Friction between the two blocks keeps the 5.00 kg block from slipping off. If the
coefficient of static friction is 0.350, what maximum force can be exerted by the string on the 10.0 kg
block without causing the 5.00 kg block to slip?