DR. MOHAMMED MOSTAFA EMAM

INAYA MEDICAL COLLEGE

(IMC)PHYS 101 - LECTURE 6

Friction Force

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Key terms

• Friction force (Ff) – a force that tends to oppose

motion

• Coefficient of friction (µ) – the ratio of the friction

force to the normal force

Force of Friction = μ · Normal Force

Ff = μ · FN

FRICTION

Weight

μ is the coefficient of friction!

Normal Force

Key terms

• Normal force (Fn) – the force pressing two contact

surfaces together

• Friction is a force between two surfaces that are sliding and they must in contact with each other.

• Examples: • The tyre of the lorry

is in contact with the

road.

• Friction always works in the opposite direction from the direction of the moving object.

• Can slows a moving

object downMoving

Friction

FORCE

• Friction Force:

Friction force results from the interactions of surfaces.

Irregularities in the structure of the matters causes friction

force.

These irregularities can be detected in micro dimensions.

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FORCE

• Friction force is always opposite to the direction of motion

and tends to decrease net force.

• All materials have their own friction constant in other words

friction force depends on the type of materials.

• Another factor affecting friction force is the normal force

(force is the reaction to the perpendicular force exerting

on it).

• When you apply a force to an object, then friction force

becomes active and resists with the force of having opposite

direction to your net force. 8

Friction depends on…

Roughness of the surfaces.

Force that objects are beingpressed one against the other.

Weights of the objects.

Friction produces heat.

Friction wears out the rubbing surfaces.

Friction produces noise.

Disadvantages of friction

What is Friction?

• Normal force equals weight on a level surface

mg

FN

Ff = mkFN = mk mg

MotionFriction

What is the Friction Force?

• Block on an inclined plane

mg

θ

θ

What is the Friction Force?

• Block on an inclined plane

mg

θ

θ

How many types of Friction?

There are four types of friction

namely:

Static friction

Sliding Kinetic friction

Rolling friction

Fluid friction

Key terms

• Static friction – the type of friction that exists between

the contact surfaces of two bodies at rest.

• Kinetic friction – the force that opposes sliding

motion.

• Rolling friction – the force that opposes rolling motion.

• Fluid friction – the force opposes propagation of any

item in fluid.

FORCE

•Example

50 N of force is applied to the 6 kg box. If the coefficient of friction is

0.3, find the acceleration of the box.

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- Free body diagram of the

system is given above.

- From this diagram we find the

normal force of the surface

and friction force.

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- Free body diagram of the

system is given above.

- From this diagram we find the

normal force of the surface

and friction force.

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- Free body diagram of the

system is given above.

- From this diagram we find the

normal force of the surface

and friction force.

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- Free body diagram of the

system is given above.

- From this diagram we find the

normal force of the surface

and friction force.

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