Science experiments

PHYSICAL SCIENCE LAB
CHANGES IN THE
FORCE OF FRICTION

INTRODUCTION:

The force you need to pull something at a constant speed is the force required to balance the force of friction. The force of friction between two objects sliding past each other depends on the force pushing the objects together, and on the smoothness of the objects.

MATERIALS:

spring balance, marked in newtons
block of wood (approximately 0.5 kg) with hook
3 additional blocks of same mass
string
graph paper
table top (at least 1 m long)

PURPOSE:

To observe how the force of friction changes as the mass of a sliding object is changed.

PROCEDURE:

1. Attach a spring balance to the hook in a wood block with a piece of string.

2. Holding the string and balance horizontally, pull the wood block along a table top at constant speed. Record, in the DATA TABLE, the force reading on the spring balance. This is the force required to balance the force of friction.

3. Place a second block of wood of the same mass on top of the first block and repeat Step 2. Record the force in the DATA TABLE.

4. Repeat Step 2 first using three wood blocks, and then using four wood blocks. Record the forces in the DATA TABLE.

DATA TABLE

Number of Blocks

1 2 3 4

Force

DATA TABLE
	Number of Blocks
	1	2	3	4
Force

CALCULATIONS:

5. Make a graph that shows how the force of friction is related to the number of blocks pushing against the table. Put the number of blocks on the horizontal axis, and the force of friction on the vertical axis. Draw a smooth line through your data points. QUESTIONS:

1. Describe the appearance of the graph line.

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2. What does the shape of your graph tell you about how the force of friction changes with the number of blocks?

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3. Use your graph to predict what the force of friction would be if five blocks were used.

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4. Why is it important to pull horizontally instead of upward at an angle?

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5. How would you expect the force values to change if a substance such as ice were used?

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6. If this investigation were done on the moon (where objects have the same mass, but less weight), how would you expect the force values to change? Explain your reasoning.

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Conclusion:

How does the force of friction depend on the weight of the sliding object?

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