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Force of Friction experiment

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Introduction

AL Physics TAS 4 (A1) – FULL REPORTimage00.png

Force of Friction

(2008/12/17)

1. Objective

  1. To study the effects of the normal reaction and surface area on the force of friction using a block.
  2. To estimate and compare the coefficients of static and kinetic friction of different materials.

2. Apparatus

  • Wooden block (190.0 g) × 3
  • Elastic strings
  • Wooden plank × 1
  • Spring balance × 1
  • Plastic plate × 1
  • Balance × 1
  • Brick × 1

3. Theory

Limiting static friction image06.pngimage01.png

Friction always opposes motion. It increases with the applied force. However, it cannot increase indefinitely.

When a body is in contact with a rough surface, friction arises at the contact surface if the body is subjected to an applied force. When the applied force exceeds the limiting static friction image06.png, the body will start to slip over the rough surface. And the value of image06.png is roughly proportional to the normal reaction R.

image13.png where image10.pngis the coefficient of static friction at the contact surface.


Kinetic friction image09.png

Generally, the friction acting on a resting block is less than image06.pnguntil the block starts to move. Once the body starts to move over the rough surface, the friction would decrease slightly to a value known as kinetic friction image09.png. So image09.png is slightly less than image06.png but it is still approximately proportional to R.

image07.png where image08.pngis the coefficient of kinetic friction at the contact surface.image02.png

image09.png remains constant even the applied force is increased further.

image08.pngis slightly less than image10.png.

3. Procedure

A.

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Middle

Block sliding:

1

2

3

4

5

Reading / N

0.4

0.5

0.4

0.3

0.4

Therefore, for a wooden block on a wooden plank, the limiting static friction image06.png is 1.1 N.

image12.png

B.        Coefficients of friction for various materials

Wood on wooden plank:

Number of blocks

1

2

3

image06.png/ N

1.1

2.0

3.2

image09.png/ N

0.4

1.1

1.5

R / N

1.86

3.73

5.59

image03.png

Wood on plastic plate:

Number of blocks

1

2

3

image06.png/ N

0.9

1.6

2.3

image09.png/ N

0.3

0.8

1.1

R / N

1.86

3.73

5.59

image04.png

Wood on brick:

Number of blocks

1

2

3

image06.png/ N

2.2

4.6

6.5

image09.png/ N

1.2

2.2

3.4

R / N

1.86

3.73

5.59

image05.png

The coeffiient of friction of the wooden blocks on various materials are summarized as follow:

wooden plank

plastic plate

brick

image10.png

0.57

0.43

1.2*

image08.png

0.27

0.21

0.6

* It is abnormal for the coefficient greater than 1. Refer to the Discussion on page 7.

C.

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Conclusion

Also, we can ensure the applied force is horizontal by fixing the spring balance on a horizontal platform. If we pull the spring balance directly, the force may not be always horizontal and the useful horizontal component of the force will be smaller than the balance’s reading.

When pulling the plank, the spring balance’s reading represents the frictional force. When pulling the blocks directly, the reading represents the applied force, which may not be equal (in magnitude) to the friction.

Q.17        In pushing a car (with the engine detached from the wheels), explain briefly why it is more difficult to start the motion, but once the car is moving, it is relatively easier to maintain its motion.

A.17        Limiting static friction is greater than kinetic friction. Especially for a car with mass generally over 1000 kg, the reaction force is very large. So the difference of magnitude between these two types of friction is very significant for a human.

Q.18        In the design of the braking system in bicycles, what parameter(s) should be maximized to achieve a larger frictional force?

A.18        The reaction force exerted on the braking pads and the coefficient of kinetic friction should be maximized.

8. Conclusion

This experiment was quite successful. It demonstrated that friction is directly proportional to normal reaction but independent of contacting area. Also, coefficients of static and kinetic friction can be compared.

~ End ~

P.

AL-PHY-TAS 4 (A1) - Full Report

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