Coefficient of Friction Lab
(Inelastic Collisions)
Introduction
Inelastic collisions are collisions in which kinetic energy is not conserved. While momentum is
conserved in these collisions as long as no external forces act during the collision, the initial and final kinetic
energies are not equal.
This lab is a variation on the familiar ballistic pendulum lab. In this case we conserve energy in the first
segment of the lab, after which we conserve linear momentum. A further twist is that we dissipate the energy of
the system after the collision while doing frictional work.
Theory – see board
Batch’s Quick Glance
Finding the coefficient of kinetic friction.
1) Hang your sand bag from support bar so that it is level with the center of the cart.
2) Obtain a piece of graph paper and make a mark were the back of the cart will start.
3) With sand bag already hanging, place the cart on the paper and move the paper so that it is just barley
touching the back of the cart.
4) Use one piece of tape to TAPE DOWN YOUR PAPER.
5) Pull the sandbag back and release it so that it strikes the cart and pushes it evenly along the paper for several
centimeters. This might take a few trial runs to get everything lined up just right. The key is that you don’t
want your cart to swerve as it moves along the paper.
6) You want to make sure you are dropping from the exact same height every time.
7) Make 5 good trials. Measure the distance your cart is moved. You may need to throw one or two out, this is
OK. Average your data.
8) Find the mass of your sandbag and the cart using the scale.
9) Use the equation v = √2 𝑔 ℎ to solve for the velocity of the sandbag on impact.
10) Use the equation vf =
𝑀𝑏𝑎𝑔
(𝑀𝑏𝑎𝑔+𝑀𝑐𝑎𝑟𝑡
11) Use the equation ½ (mbag +
× √2 𝑔 ℎ to solve for the final velocity of the cart.
mcart) vf2
= μk (mcart)(g)(s) to solve for the kinetic coefficient of friction(μk).
Finding the coefficient of static friction
1) Use the same piece of graph paper.
2) Tape it to the outside cover of a text book.
3) Place cart on top of book.
4) Slowly increase the angle of the cover until the cart begins to slide.
5) Use a protractor to measure this angle.
6) Take five good readings and average. This is your angle theta(θ).
7) Use the equation μs = tanθ to solve for static coefficient of friction (μs).
h - height of sandbag(mbag)
mbag – mass of sandbag
mcart – mass of plastic cart
Trial
1
2
3
4
5
Avg distance
Distance mcart
traveled (m)
Trial
1
2
3
4
5
Avg angle
Angle of book – θ
Lab write up
Be sure to include your base equation, filled in equation, steps along the way, and boxed in answer.
1) v, velocity of the sandbag on impact.
2) vf, final velocity of the cart
3) μk, kinetic coefficient of friction
4) μs static coefficient of friction (μs).
5) Comparison of your μk and μs. Which one is bigger?