### Get moving

#### Discovery question

How strong is the force of friction?

Cannot resolve external resource into attachment.
Measure the amount of force needed to make different surface materials start to move and continue to move.

#### Engage

 Eduardo was annoyed. When he described Slippery World and Sticky World to his friends, he would say "it's like being on ice", or "it's like having glue on your shoes." But they clearly didn't believe him. They thought he was exaggerating. So he decided to take real measurements in both worlds so that he could prove just what they were like, compared to this world.

He decided his experiment would look like this:

http://udl.concord.org/share/Friction/need-image

Here is a graph of force against time. Suppose you pushed very lightly on the force sensor, then harder and harder until the box started sliding. Predict what your graph would look like.

 PREDICTION GRAPH

#### Materials

• Force sensor
• Small cardboard
• Water bottle that fits into the box
• String
• Aluminum fil
• Waxed paper
• Sandpaper
• Tape
• Optional: other materials such as cloth or plastic

#### Explore

Find a small cardboard box. Find a water bottle that fits into the box and fill it with water.

Attach the force sensor to the computer. Push the box with the force sensor.

http://udl.concord.org/share/Friction/need-image

Start recording with the force sensor. Gradually push harder and harder until the box starts to slide. Keep pulling so that the box slides at a constant speed.
FORCE SENSOR GRAPH with prediction, single dataset

Stop recording. Label your graph. Mark where you first started pushing. Mark where the box first started to move. Mark the greatest force value.
GRAPH LABELING technical hints

What was the greatest force value?

#### Explain

Describe your graph. What did it look like before the box started moving? What happened just when the box started moving? What happened after that?

What would you say was the average value of the friction force needed to keep the box moving?

Do you think you would get the same results if you were pulling on the box instead of pushing?

Why do you think so?
ANDWER BOX

#### Elaborate

Now explore ways to increase or decrease the friction force. You can save several experiments on the same graph. Each button of the left is a different dataset. The first one, called "trial 1", is your data pushing the box. Use "trial 2", "trial 3", and "trial 4" for other materials.

Try putting waxed paper under the box. Attach it with tape on the sides. Select "trial 2" and repeat your experiment. Be sure to label the new line you have made on your graph.
FORCE SENSOR GRAPH - multiple datasets set up, data from trial 1 inserted.

Record the average value of the moving friction force. Does waxed paper increase or decrease the friction force?

Try another material, such as aluminum foil, under the box. Attach it with tape on the sides. Select "trial 3" and repeat your experiment. Be sure to label the new line you have made on your graph.

Record the average value of the moving friction force for Trial 3.

If you have sandpaper, put it under the box and attach it with tape to the sides. If you don't have sandpaper, find some other material that you think has a high friction value. Select "trial 4" and repeat your experiment. Be sure to label the new line you have made on your graph.

Record the average value of the moving friction force for Trial 4.

Fill out the following table.
TABLE
Trial Material Friction force
1
2
3
4