### Get moving

#### Discover question

How strong is the force of friction?

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Measure the amount of force needed to make different surface materials start to move and continue to move.

#### Engage

 Linker Coach (engage & motivate) Why? Affective

 Cannot resolve external resource into attachment. 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:
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Here is a graph of force against time. Suppose you pulled 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
• Sneaker
• 500 ml (16 oz) soda bottle that fits into the sneaker
• String
• Waxed paper
• Rubber cement
• Optional: other materials such as cloth, sandpaper, oil

#### Explore

1) Find an old sneaker that no one wants anymore. Find a water bottle that fits in the shoe and fill it with water.

2) Attach the force sensor to the computer. Hook the force sensor to the shoelaces.

3) Use the shoelaces to pull the shoe. Make sure it slides and doesn't lift up or tip over.

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4) Start recording with the force sensor. Gradually pull harder and harder until the shoe starts to slide. Keep pulling so that the shoe slides at a constant speed.

 FORCE SENSOR GRAPH. PREDICTION FROM ABOVE MAY NOT LOOK GOOD, THE SCALE MAY BE TOO DIFFERENT

5) Stop recording. Label your graph. Mark where you first started pulling. Mark where the sneaker first started to move. Mark the greatest force value.

 GRAPH LABELING SCAFFOLDS

#### Explain

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

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

#### Elaborate

1) Now explore ways to increase or decrease the friction force. You can save several experiments on the same graph. Use the new button on the left to add a new dataset.

2) Try putting waxed paper under the shoe. Create a new line and repeat your pulling experiment. Be sure to label the new line you have made on your graph.

 FORCE SENSOR GRAPH

Does waxed paper increase or decrease the friction force? Record the averarge value of the fricton force.

3) Try another material, such as newspaper, cloth or plastic, under the shoe. Create a new line and repeat your pulling experiment. Be sure to label the new line you have made on your graph.

4) If you have sandpaper, glue it to the bottom of the shoe with rubber cement. Create a new line and repeat your pulling experiment. Be sure to label the new line you have made on your graph.

#### Evaluate

A) Plain shoe
B) Wet shoe
C) Oily shoe

 image: shoe with peanut butter

2) Create a new line for each situation and repeat your pulling experiment. Be sure to label each new line you have made on your graph. Test the oil last – it's the messiest!

 NEW FORCE SENSOR GRAPH

Fill in the average sliding friction force for each in the table below.

CREATE TABLE

Material Average friction force
A: Plain shoe
B: Wet shoe
C: Oily shoe