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# How Easily Do Objects Move Over Surfaces?

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### Talk It Over

Does it take more force to move an object over a smooth surface or a rough one? How can you find out?

### Get

• Small block of wood
• Drill
• Sandpaper
• 4 pieces of cardboard, about 8½ in. x 11 in.
• 3 new pieces of sandpaper: 1 each fine, medium, and coarse
• Glue stick
• Hole punch
• Paper cup
• String
• Table
• Paper clips

### Go

1. Ask an adult to drill a hole in the block of wood, close to a corner, like this:
2. Sand rough edges smooth.
3. Glue the sandpaper pieces, one of each type, to a separate cardboard backing. Make sure the sandpaper is glued smooth and level. Leave the other piece of cardboard smooth.
4. Punch three holes in the paper cup. Thread strings through the holes and tie them like this, leaving one long piece:
5. Put the smooth cardboard piece on the edge of the table. Place the block of wood on the cardboard, like this:
6. Tie the cup to the block with the string, letting the cup hang below the table's edge, like this:
7. Start adding paper clips to the cup, one at a time, counting as you go. Record the number when the block first begins to move. Also record the number that moves the block the full length of the cardboard to the edge of the table.
8. Replace the smooth cardboard with a sandpaper-covered board, and repeat steps 5–7. Keep testing until you have collected data for the block's movement on all the boards.

### Stay Safe

Handle sandpaper carefully. It can scratch your skin.

### Go Easy

Test smooth cardboard against a single grade of sandpaper.

### Go Far

Friction is the force that resists the relative motion of objects that are in contact with each other. Inertia is an object's tendency to remain at rest or in motion. Inertia depends on the mass and shape of the object. You had to overcome inertia to get the block moving. You had to overcome friction as the block moved the length of the board. (In the "Go" procedure, you measured both forces in grams. Why? Because each paper clip has a mass of approximately 1 gram.)

Use the same method to measure and compare the inertia of other objects such as a small cardboard box, a stapler, a glue stick, or toy cars with wheels. You might also measure the friction of other surfaces such as wood, carpet, plastic, glass, or other materials.

Put numbers of paper clips in a data table like this for "Go Easy":

Material Tested Block First Moved(Number of Paper Clips) Block Moved Length of Board(Number of Paper Clips)
Cardboard
Sandpaper

Make a bar graph that compares your results. Explain the differences you see. For "Go," add rows to the data table for each kind of sandpaper you test and make bar graphs.

For "Go Far," make data tables for the surfaces and objects you test. Explain how the forces of inertia and friction vary with the surface.

### Tips and Tricks

• Make sure the string is not under the block. It can get in the way and affect your results.
• You'll have a better project if you repeat each test several times and average your results.