209
Exercise 4A: Force and Motion
Equipment:
2-meter pieces of butcher paper
wood blocks with hooks
string
masses to put on top of the blocks to increase friction
spring scales (preferably calibrated in Newtons)
Suppose a person pushes a crate, sliding it across the floor at a certain speed, and then repeats the
same thing but at a higher speed. This is essentially the situation you will act out in this exercise. What
do you think is different about her force on the crate in the two situations. Discuss this with your group
and write down your hypothesis:
_________________________________________________________________________
1. First you will measure the amount of friction between the wood block and the butcher paper when
the wood and paper surfaces are slipping over each other. The idea is to attach a spring scale to the
block and then slide the butcher paper under the block while using the scale to keep the block from
moving with it. Depending on the amount of force your spring scale was designed to measure, you
may need to put an extra mass on top of the block in order to increase the amount of friction. It is a
good idea to use long piece of string to attach the block to the spring scale, since otherwise one tends
to pull at an angle instead of directly horizontally.
First measure the amount of friction force when sliding the butcher paper as slowly as pos-
sible:___________________
Now measure the amount of friction force at a significantly higher speed, say 1 meter per second. (If
you try to go too fast, the motion is jerky, and it is impossible to get an accurate reading.)
___________________
Discuss your results. Why are we justified in assuming that the string’s force on the block (i.e. the
scale reading) is the same amount as the paper’s frictional force on the block.
2. Now try the same thing but with the block moving and the paper standing still. Try two different
speeds.
Do your results agree with your original hypothesis. If not, discuss what’s going on. How does the
block “know” how fast to go.