Friction

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In this presentation you will: Explore the concepts of friction and the

coefficient of friction.

Friction

Friction affects our everyday lives.

It can explain why we make some things of particular substances and why they have particular shapes.

In this presentation you will find out why most vehicle wheels are made of rubber, why high speed vehicles have aerodynamic profiles and why space shuttles need to be covered with special thermal protective tiles.

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Friction

You will also notice friction if you rub the palms of your hands rapidly and repeatedly against each other, since friction generates thermal energy (heat).

Friction is the force that opposes the motion of two bodies in contact. You may clearly observe this effect every time you drag something heavy.

What is Friction?

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Friction

When an object sits on a slope without slipping, there is a force that maintains that object on the top of the slope.

What is Friction?

The forces acting on the object are balanced.

The diagram shows the forces acting on the weight sitting on the slope.

These forces are: the weight W, the normal force N of the weight and the frictional force F.

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Friction

Friction is caused when two materials that are not completely smooth are rubbed together.

What is Friction?

Of course, the rougher the surface, the greater the friction.

Even very smooth surfaces can be rough at a microscopic level so there will always be a degree of friction.

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Friction

1

What is friction?

Question

A) The heat of an object.

B) A force that makes an object slip.

C) An object in motion.

D) A force that opposes motion.

Friction

You may have noticed how hard it is to start moving a fully loaded shopping cart, but once it is moving it seems easier to keep in motion.

There are two main types of friction: static and dynamic.

Types of Friction

Static friction is the resistance of a stationary object to start moving.

Dynamic friction is the resistance of a moving object to continue its motion.

This occurs because static friction is normally higher than dynamic friction.

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Friction

Frictional force opposes the motion (or attempted motion).

There are three facts to remember about friction:

Factors Affecting Friction

Friction is not related to the area of contact.

Friction is directly proportional to the weight of the object.

weight

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Friction

2

If a brick is flipped over to lie on one of its smaller sides, how would the friction compare with the friction on one of the larger sides?

Question

A) Since the contact area is smaller, the friction will be smaller too.

B) Friction will be the same since friction is not related to area of contact.

C) Friction will increase due to the same mass lying on a smaller area.

D) It will depend upon the material that the brick is made of.

Friction

This coefficient depends on the materials used. For example, the coefficient of friction of rubber on tarmac is much higher that the coefficient of friction of ice on metal.

where µ is the coefficient of friction, F is the force of friction and N is the normal or perpendicular force.

This coefficient is defined by the formula:

The coefficient of friction is a constant value that determines the level of friction between two bodies.

Coefficient of Friction

Fμ

N

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Friction

3

What is the coefficient of friction between two materials?

Question

A) It is the sliding coefficient between those two materials.

B) It is the normal reaction N of an object made of those two materials.

C) It is the angle at which these two materials would begin to slide.

D) It is a constant that determines the level of friction between those two materials.

Friction

where F is the force of friction in newtons, the mass will be in kg and 9.8 is the acceleration due to gravity g in m/s2.

Applying the previous formula, the calculation of the coefficient of friction on a horizontal plane, where normal force N equals the weight of the object, is very simple.

Coefficient of Friction on a Horizontal Plane

Fμ

m × 9.8

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Friction

4

How is the coefficient of friction between two materials calculated?

Question

A) Divide the force of friction by the normal force of the object.

B) The coefficient of friction equals the mass of the object.

C) Measure the angle of the slope only.

D) Multiply mass of the object by gravity only.

Friction

Angle of Friction

On an inclined plane of angle θ at which the block is just about to slip, the three forces acting on the block are:

• Its weight W.

• The normal force N of the surface.

• The limiting frictional force F.

The forces are in equilibrium.

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Friction

Fμ =

Nmg sinθ

μ = mg cosθ

therefore

μ = tan θtherefore

Angle of Friction

If mg (W) is resolved into components:

mg sinθ along the surface.

mg cosθ perpendicular to the surface.

Therefore N is defined by mg cosθ and the value of the friction force F applied to the object can be calculated by mg sinθ.

Hence μ can be found by measuring θ, called the angle of friction.

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Friction

Since friction generates heat, vehicles that operate under huge levels of friction (therefore, very high heat), need to be built with protection against heat.

Air itself creates friction. This is why vehicles moving at high speeds are designed aerodynamically so the friction with the air is reduced and energy can be saved.

Friction of Air

Friction against air is also what burns meteoroids when they enter Earth’s atmosphere and we get to see them in a clear night sky as ‘shooting stars’.

This explains why the bottom face of the space shuttle is covered with ceramic tiles. These insulate the vehicle from heat when it re-enters the atmosphere.

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Friction

When a tire rotates on a road, the friction between the tire and the road helps the tire grip and transforms the energy from the engine to the motion of the car. If the tire was rotating on ice, the friction would be low and the tire would just spin. It would not grip.

Fiction is not always to be avoided. Materials that generate a high level of friction such as rubber, are extensively used in the tire industry where friction is essential for motion and control.

Benefits of Friction

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Friction

Show knowledge and understanding of friction between solids and different types of friction.

Show knowledge and understanding about the laws related to friction.

Show knowledge on how to calculate the coefficient of friction between different materials.

After completing this presentation you should be able to:

Summary

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