Lab 5: Static and Kinetic Friction
Name: __________________________
Goals:
- Compute the coefficient of static friction between two surfaces.
- Predict and then measure the “breakaway angle” for the same two surfaces on a tilted track.
- Find the coefficient of kinetic friction for a cart sliding on a tilted track.
Background:
Friction forces are caused by the texture of two surfaces “locking together”. Friction force is always proportional to the normal
force, since the normal force tells you how hard the surfaces are pressed together. We talk about two types of friction in this
course – static (stationary) and kinetic (sliding).
Ffstatic  s FN
and
Ffkinetic  k FN
Note that s  k , because the texture on two surfaces is able to “lock together” more effectively when the object is not
moving. Also, the static friction force is given by an inequality because its value changes depending on the applied force – the
static friction force is just equal to the applied force until a maximum value Ffmax  s FN is reached – then the object breaks
away and starts sliding.
Breakaway Force:
When we apply a horizontal force to an object, the static friction force prevents the object from moving until we exceed some
minimum value of applied force. This means that the static friction force is equal in magnitude to the applied force until this
maximum value is achieved.
F f static
Fapplied
Before breakaway:
F f static
Fapplied

The static frictional force always
cancels out the applied force, so
there is no acceleration.
We can measure the coefficient of static friction for a pair of surfaces by pulling on the object with a spring scale until the
breakaway force is achieved – this tells us Ffmax , and we are able to solve for s .
F f max
Fapplied

At breakaway:
Once the applied force
exceeds the maximum static
friction force, the object
begins to move.
Kinetic Friction:
The kinetic friction force is always the same for a moving object (the approximation begins to diverge from reality when the
speed is large). The important thing to remember about kinetic friction is that it always points in the opposite direction of the
sliding of the object.
F fkinetic
v
Static Friction:
1. Place two 1 kg masses on the friction block (to increase the normal force) and use the spring scale to measure the
breakaway force when the block is resting on the track felt-side down. You will have to convert the scale reading
from mass to force, since the manufacturer thought we would only use it to measure weights.
Reading on scale: _______ g
Breakaway force for felt on aluminum:
_______ N
2. Use the breakaway forces in the last question to compute the coefficient of static friction. Show how to calculate
s in general in the space below.
s in general (in terms of breakaway force and mass): _______________
s for felt on aluminum: ___________
3. For an object of mass m with static friction coefficient s , compute the angle  at which gravity causes the mass
to break loose and slide down the ramp. At this “breakaway angle” the force of gravity down the ramp just barely
exceeds (is basically equal to) the static friction force up the ramp.
m

breakaway =_______________
4. Use the answer from part 3 to predict the breakaway angle for your wood block. Then measure the breakaway
angle and compute a percent error. Secure the two 1 kg weights with tape before you tilt the track!
breakaway (predicted) for felt/aluminum: ________
breakaway (measured) for felt/aluminum: ________
% error: _________
Kinetic Friction:
1. When an object slides up a ramp, the friction force points down the ramp (the same direction as the parallel
component of gravity). When an object slides down a ramp, the friction force points up the ramp (the opposite
direction as the parallel component of gravity). This has an interesting consequence – the acceleration of the
object is different on the way up than it is on the way down!
Compute the acceleration on the way up and on the way down in terms of m, g , , and k . Important Note: the
acceleration is downward in each case, so it is easiest to call “down the ramp” the positive direction of acceleration
for both cases!
v
Ff
v
m

Up the ramp.
aup  __________________
Ff
m

Down the ramp.
adown  ___________________
2. Set up the dynamics track at an angle of 10 . Tape two of the iron cart weights to the top of a friction block (felt
side down) and use a piece of cardboard to provide a nice reflection surface for the motion detector. Use the
motion detector to record the position and velocity of the block as you launch it up the ramp and let it come all the
way back down the ramp. The block should be sliding freely for as long as possible!
The acceleration of the block should be different on the way up than it is on the way down. Use your data to
compute the acceleration on the way up and on the way down (attach your v-t and a-t graphs to the lab and label
the time intervals you used to compute each acceleration).
aup  ______________
adown  _____________
3. Use the two measured accelerations to compute mk on the way up and on the way down. Show your work.
mk (up): ____________
mk (down): ______________
4. The coefficient of kinetic friction should not depend on the direction the block is moving: compute a percent
difference to see how close your two values are. Show how you computed the percent difference (there are a
couple reasonable approaches)!
Percent difference: __________