6


FRICTION 6.1 Vocabulary paraphrasing at sentence level A Study the words in the blue box.


1 Match the words in the blue box with a meaning from the left-hand column of the table.


2 Complete the text below the table with the words from the right-hand column in Exercise A1.


3 Group the words in the blue box according to their stress pattern.


B Study Figure 1 on the opposite page. Discuss these questions using words from Exercise A.


1 What does the diagram show?


2 What happens at each stage to: l


applied force? l static friction? l kinetic friction?


C Student A has described Figure 1 on the opposite page, but there are some mistakes. Change the


blue words so the sentences are true.


D Student B has also written about Figure 1 on the opposite page. Match each sentence


with a corrected sentence from Exercise C.


E Look at Figure 2 on the opposite page. Write six sentences to describe and compare


the coefficients of friction shown.


F Choose one of these materials. l


steel in air


l glass on plastic l rubber on rubber


1 Draw a graph for its probable coefficient of friction.


2 Write some sentences to describe one of the friction coefficients.


3 Give your sentences to your partner. Your partner should try to guess which materials you have described.


4 Rewrite your partner’s sentences with the same meaning.


46 Coefficient of Friction


The coefficient of friction, µ, is a scalar It is the


. between the normal force, N, of one object


on another and the force of resistance, R. This ratio differs according to the materials has a high


low one. There are two basic kinds of Static Friction: µs


. A rubber tyre on a road


, whereas a piston inside a cylinder has a .


Static friction is what makes stationary objects difficult to move. The direction of this force is along the contact surface and it is equal and opposite to the magnitude of the


force. The friction force is given by ƒs = µsN,


where µ is the coefficient of friction and N is the normal .


Kinetic Friction: µk Kinetic (or


) friction is created by movement. The


direction of this force is opposite to the direction of motion of the object. The


by ƒk = µkN. The kinetic friction force is less than the static friction force.


of the kinetic friction force is given


kinetic / dynamic maximum


applied value friction in contact magnitude ratio resistance coefficient


force


rubbing against amount touching not moving moving size


push / pull used


relationship greatest


movement against measurement


static


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