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# Force of friction

Extracts from this document...

Introduction

Kwun Tong Government Secondary School

Experiment Report

Experiment No.:              1

## Title:                      Force of friction

Date of Experiment:           2006/10/31

Mark:

 Sch. Code 2114 TAS Group 15 Name: Chan Man Lok Class: S.6C Class No.: 16

Objective:

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

Apparatus:

## Instrument

### Description

Wooden block

x 4

200g x 4

Spring balance

x 1

DCS/PL/2-2

0 - 10N

The smallest division = 0.2 N

Tripe beam balance

x 1

0 – 500g

The smallest division = 0.1g

Frictional paper

x 1

/

Rubber band

Several

/

Theory:

Limiting static friction :

Friction always opposes motion. Larger forces tending to produce the motion, larger friction is. However, it cannot increase indefinitely.

For example, when a body is in contact with a rough surface frictional forces arise at the contact surface if the body is subjected to an applied force. When the applied force exceeds the limiting static friction , the body will start to slip over the rough surface. And the value of  is roughly proportional to the normal force R.

where is the coefficient of static friction at maximum at the contact surface.

Kinetic friction :

However, the friction acting on a resting block is less than until the block starts to move. For example, once the body starts to move over the rough surface, the friction would decrease slightly to a value known as kinetic friction . So  is slightly less than  but it is still approximately proportional to R.

where

Middle

Assumption:

1. The spring balance was kept in horizontal position.
2. The rubber bands had no mass.
3. The gravitational acceleration is .
4. The blocks were pulled to move at a constant speed.

Result:

1. Frictional force and applied force

The 5 readings of spring balance which represent .

 /N 2 1.9 1.8 2 2.1

The mean of : N

The 5 readings of spring balance which represent .

 /N 1.3 1.4 1.5 1.4 1.3

The mean of : N

Graph of frictional force against the applied force

1. ##### Coefficients of friction for wooden blocks
 Number of blocks 1 2 3 4 Applied force F /N Limiting static friction 1.96 3 5.4 7.3 Kinetic friction 1.38 2.4 3.4 5.2 Normal force R / N 1.96 3.92 5.89 7.89
##### Graph of the limiting static friction against the normal force R

= slope,  = y-intercept

Graph of the limiting static friction against the normal force R

Take the maximum difference of the slopes and y-intercepts as the uncertainly.

Maximum uncertainty in slope

Maximum uncertainty in slope

Thus, slope

y-intercept

Conclusion

If the spring balance were not kept in horizontal position, the pulling force would be larger than the horizontal force. And make the results inaccurate. As a result, it was important to keep the pulling force in horizontal position.

Let pulling force be F, horizontal force be x

1. Mass of the rubber bands

The rubber bands used to keep the wooden blocks side by side had their own mass. So the normal force R didn’t only involved the weight of the blocks but also the weight of the rubber bands. It would affect the results if the mass of the rubber band were neglected. To improve the accuracy, the weight of the rubber bands should be measured and involved in R too.

1. Taking readings of the spring balance

In step A3, B6, C12, readings of the spring balance were taken during the blocks were moving at a constant speed. The difficulty was to take the readings in that instantaneous motion. It was easy to take inaccurate reading and hence affect the data. The only way to improve was doing the procedure repeatedly by several people and then taking the average of the readings.

Conclusion:

Through the experiment, the coefficient of static friction was found to be . The coefficient of kinetic friction was found to be .  is smaller than , so less force is needed to keep the block moving than to start moving it. Both limiting friction and kinetic friction were directly proportional to the normal reaction but are independent of the surface areas of the contact surface.

S.6 Physics TAS Experiment 1            Chan Man Lok 6C (16)              P.

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