WHAT ARE SOME FACTORS THAT AFFECT MOTION OF

OBJECTS OR BODIES?

Suppose you will be asked to push a baggage on two surfaces:

In which surface will it be easier for you push the baggage? surface harder for you to push? Can you explain your answer?

Surface A Glass surface

Surface B Hollow block surface

In which surface will it be easier for you to drive? Why?

Road A

Road B

FRICTION

A force on objects or bodies in contact with each other that resists or opposes motion of the objects or bodies relative to each other.

A force that exists whenever two surfaces or bodies are in contact whether at rest or moving.

Whenever two surfaces or bodies are in contact whether at rest or moving there exists FRICTION between them.

where applied force push or a pull normal force weight friction force opposing force to the applied force

There is FRICTION between the road & the tires of automobiles

There is FRICTION between the pen or pencil & your paper

There is FRICTION between two rubbing surfaces

There is FRICTION between your shoes & the ground

Can you give other examples of bodies experiencing friction?

WHAT CAUSES FRICTION?

It is caused by the irregularities or interlocking of tiny projections on rubbing surfaces or surfaces in contact.

Not all Surfaces are smooth or rough!

NATURE OF FRICTION

It is parallel to the surface in contact.

surface

It opposes motion thus its direction is opposite the direction of motion.

NATURE OF FRICTION

Direction of Applied force

Its amount or magnitude is dependent on the nature and kind of surface in contact.

NATURE OF FRICTION

Wooden surface Rough road Concrete road Pimpled face

NATURE OF FRICTION

Its magnitude is affected by the Normal force perpendicular to the surface.

Which of the two will experience more friction?

NATURE OF FRICTION

the status of the body in contact with surface – “at rest” or in motion.

Which of the two will experience more friction?

TYPES OF FRICTION

Static Friction arises between two objects or bodies that are in contact but motionless.

Kinetic Friction arises between bodies that in contact & in motion.

TYPES OF FRICTION

What are the advantages of friction?

It increases to counterbalance forces that moves objects or bodies.

It serves as a support to moving bodies or bodies at rest.

There must be friction so bodies will not slip on surfaces easily.

What are the disadvantages of friction?

It causes fast wear and tear of materials or parts of machines.

It may cause over heating in machines.

It may dampen motion or slows down motion.

HOW DO WE INCREASE FRICTION ON SURFACES?

The use of rubber materials

HOW DO WE INCREASE FRICTION ON SURFACES?

The use of lines, designs or marks on surfaces

HOW DO WE INCREASE FRICTION ON SURFACES?

The use of spikes on surfaces

HOW DO WE INCREASE FRICTION ON SURFACES?

The use of chains on roads

HOW DO WE INCREASE FRICTION ON SURFACES?

The use of sand or creating lines on roads

HOW DO WE REDUCE FRICTION ON SURFACES?

The use of wheels

HOW DO WE REDUCE FRICTION ON SURFACES?

The use of bearings or rollers

HOW DO WE REDUCE FRICTION ON SURFACES?

The use of wax, powder etc… on surfaces

HOW DO WE REDUCE FRICTION ON SURFACES?

The use of oil, grease & other lubricants on surfaces

HOW DO WE REDUCE FRICTION ON SURFACES?

The Use of sandpaper, varnish, paint

HOW DO WE REDUCE FRICTION ON SURFACES?

The Use of streamlining

HOW DO WE COMPUTE FOR FRICTION?

Static friction ( Fs ): Fs = sN

where N Normal Force s coefficient of static friction ( constant )

HOW DO WE COMPUTE FOR FRICTION?

Kinetic friction ( Fk ): Fk = kN

where N Normal Force k coefficient of kinetic friction (constant)

The coefficient of friction is the ratio of the force that maintains contact between an object and a surface and the frictional force that resists the motion of the object.

Materials in contactCoefficient of static

friction( s )

Coefficient of kinetic friction

( k )

Metal on metal 0.03 0.03

Wood on wood 0.5 0.3

Steel on steel 0.15 0.09

Leather on wood 0.5 0.4

Rubber tire on dry concrete

1.0 0.7

Rubber tire on wet concrete

0.7 0.5

Wood on stone 0.5 0.4

Estimated coefficients of static & kinetic friction of various materials

Note! If a body or object is being pushed or pulled horizontally on a surface, the normal force ( N ) is simply the weight of the body or object (in Newtons). However, if the mass is given (in kg )the formula would be:

Fk = kmg or Fs = smg

PROBLEM 1: A wooden box, which is 250 Newtons, is pushed on a wooden floor. How much friction is present on the surfaces in contact? Suppose the wooden box will remain at rest then how much friction will be exerted on the surface?

Problem 2:A patient weighing 60kg is pushed on a wheel chair along the corridor of a hospital. Suppose the weight of the wheel chair is neglected, and the coefficient between the floor and the wheel chair is 0.8 then how much force is needed to push the wheel chair?

Problem 3:If a force of 80 Newtons is needed to pull a balikbayan box which is 200 Newtons then find the coefficient of friction on the surfaces in contact.

EXERCISE 13: FRICTION

1. A man can push a 100 – kg stretcher containing a patient across a wooden floor. Find the coefficient of friction exerted by the two surfaces if frictional force is 200N.

2. A 45kg man wearing leather shoes stands on a wooden floor. Find the friction exerted on the floor.

3. Find the force needed to pull a balikbayan box which is 200 Newtons if the coefficient of friction is 0.22.

4. The tire of a moving 1500kg – truck is in contact with a wet concrete road. Find the force needed to overcome friction.

5. Find the mass of an object made of steel which is placed on a steel surface if the friction exerted between them is 250 Newtons.

6. A 30-kg wooden cabinet stands on a wooden floor. Find the amount of friction exerted on the floor.

7. The tire of a bus which is 1400kg is in contact with a dry concrete road. Find the amount of friction exerted.

8. Find the mass of a man wearing leather shoes standing on a wooden surface if the force exerted on the surfaces in contact is 200 Newtons.

# 1 1, 4 & 7 #2 2, 3 & 8 #3 1, 5 & 6

Application of friction

Clinical: “Friction Massage”

Friction Massage Therapy for Tendonitis

Friction massage basically “scrubs” the fibers of the tendon, aiding recovery. The mechanism is probably just stimulation.

How to do friction massage

The action of friction massage is extremely simple: just strum back and forth over the inflamed tendon at the point of greatest tenderness. Your strokes should be perpendicular to the fibres of the tendon — like strumming across a guitar string.Use gentle to moderate pressure with the pads of your fingers or a thumb.

Friction massage treatments should be wrapped up by cooling the area down with an application of raw ice.

http://saveyourself.ca/articles/frictions.php

Skin Friction

Clinical Condition:

Two surfaces rubbing together and wearing away the outer layers of the skin. This type of injury is often seen on the elbows or heels due to rubbing against bed sheeting and/or from re-positioning in bed. It also can occur on the buttocks or sacrum from sliding down in bed if the head of the bed is elevated.http://solutions.3m.com/wps/portal/3M/en_US/SH/SkinHealth/solutions/skin-wound-care/friction/

Use of lubricants

Personal lubricants have long been recommended to women to improve the comfort of sexual intercourse and to reduce the risk of vaginal tearing or genital pain, yet strikingly little available data is available on women’s use of lubricants or associated vaginal symptoms.

http://www.procto-med.com/why-men-and-women-use-lubricants-during-sex-two-studies/