FORCE

A force is an interaction that causes a change. In mechanics, a force is an interaction that causes

a change in velocity (an interaction that causes acceleration).

Concept of Force

The Concept of Force

Explain the concept of force

For better understanding of this concept, let s ask ourselves the following question:

1.

What causes/makes a body at rest to move?

2.

What causes the same body in motion to stop?

The answers  to these questions,  is of course that a force  is required to do  so.  Here, an applied

force to a body can  cause the body at rest to move or if  already moving a body application of

force can do the following when applied to a body.

1.

Force can cause a change in the way the object moves

2.

Change its size or shape

3.

Change the direction in which an object is moving

A force is defined as a push or pulls experienced by an object. The force is usually described in

terms of its size (magnitude) and direction.

The S.I Unit of Force

State the S.I unit of force

The SI unit of force is a Newton (N).

A Newton (1N) can be defined as the amount of force required to give a mass of one kilogram

(1kg) an acceleration of 1m/s

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Measuring Instrument (Device)

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The instrument that is used to measure the amount of force that is exerted on an object is called

spring balance.

A spring balance can be used to measure small forces. It consists of a coiled spring fixed to the

other end  with a  hook at the other  end. The body upon  which the force acts is attached  to the

hook.  The  distance through  which  the  spring  is  stretched  is  directly  proportional to  the  force

applied by the balance.

A pointer is attached to the spring and the force is indicated on a calibrated scale.

Types of Force

Fundamental Types of Forces

Identify fundamental types of forces

There are two main types of forces namely:

1.

Fundamental forces

2.

Non fundamental forces

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Fundamental Forces

Fundamental  forces  are  the  basic  forces  in  nature  that  cannot  be  explained  by  the  action  of

another force.

There are four types of fundamental forces. These include:

1.

The force of gravity

2.

The electromagnetic force

3.

The strong nuclear force

4.

The weak nucleus force

Force of gravity (weight)

The force of gravity is the pull by which the earth, moon and other ver y large bodies attract other

objects  towards  themselves.  It  is  commonly  ref erred  to  as  the  weight  of  the  object  that  is

attracted.

All objects on the earth experience a force of gr avity that is directed downwards towards centre

of the earth. The earth s gr avitational pull is so great that it is able to hold most things firmly on

the ground.

In mechanics, a freely falling body in the air moves down irrespective of its mass. This is due to

force of gravity.

Example 1

A fruit fall from a tree because the earth exerts gravitational force on it. The force of gravity on

the earth is always equal to the weight of the object (body).

Force of gravity, W = mass (m) x gravitational acceleration (g).

W= mg

Where acceleration due to gr avity on the earth, g = 9.8m/s

2

Conclusion, we can say that the force of gravity has the following properties.

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1.

It pulls (attracts) objects towards the centre of the earth.

2.

It is directly proportional to the mass of the object. This means that the greater the mass

the greater the pull of gravity.

3.

It is strong when the mass is closer to the centr e of the earth.

NB:

It should be noted that the weight of an object is directly r elated to its Mass.

Weight, W = mass x g = mg

The g above has two meanings:

It is the gravitation s field strength (10N/Kg)

1.

2.

It is the acceleration of free fall (10m/s

2

Electromagnetic force

This force includes both electric and magnetic force. It is relatively strong.

Example 2

Examples where electromagnetic forces are involved include:

1.

In the formation of molecules of a substance. Atoms attract each other to form molecules.

This is due to electromagnetic force.

2.

In two par allel wires carrying current. If places near each other, the electromagnetic force

acts on the wires.

Strong nuclear force

This is the force, which holds of the constitutions (sub atomic particles) of the atomic nucleus. It

acts within the nucleus of the atom.

Weak nuclear force

This is the force which appears only in certain nuclear processes.

The Properties of Each Type of Fundamental Forces

Describe the properties of each type of the fundamental forces

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Properties of gravitational forces

The following are the properties of gravitational force

It always attracts objects.

It is the weakest force among the four basic forces.

It is a central for ce (gravitational force between two objects acts along the line joining the

centres of the objects.

It operates over very long distance.

Properties of electromagnetic force include

It may be attractive or repulsive is nature

It is a central force

It is stronger than gravitational force

It is also a long-range force (operates over a ver y long distance)

Properties of a strong nuclear force:

It is basically an attractive force

It is a non-central force (does not act at the centre)

It is stronger than gravitational force

It is a short-range force that is it operates only up to distance of the order of 10

m

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Properties of the weak nuclear force

It is much stronger than the gravitational force but weaker than the strong nuclear force

and electromagnetic force

It acts on small ranges of up to 10

-17

m

Activity 1

CLASS ACTIVITY

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1.

Define the term force and give its SI unit.

2.

Mention four fundamental types of force that you know.

3.

Define  the  following  terms:  Force  of  gravity;  Strong  nuclear  force;  Electromagnetic

force.

4.

A body has a mass of 40kg. Find its weight.

Effects of Force

The Effects of Forces

Identify effects of forces

A  force  acting  on  an  object  may  cause  the  object  to  change  shape,  to  start  moving,  to  stop

moving, to accelerate or decelerate.

When two objects interact with each other they exert a force on each other, the forces are equal

in size but opposite in direction.

Resultant force

The  forces  acting  on  an  object  can  be  replaced  with  a  single  force  that  causes  the  object  to

behave in the same  way as all the separate forces acting together did, this  one overall force  is

called the

resultant force

.

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If the resultant force acting on an object is ZERO then:

The object will  remain  stationary  if  it  was  stationary  when the  resultant  force  became

zero.

Move at a constant (steady) speed in a straight line if it was moving when the resultant

force became zero.

If the resultant force acting on an object is NOT ZERO then:

The object will accelerate or decelerate (speed up or slow down).

The Effects of Forces on Materials

Justify the effects of forces on materials

Example 3

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A rectangular bar having a cross-sectional area of  80 mm2has a tensile force of 20 kN applied to

it. Determine the stress in the bar.

Example 4

A circular section cable has a tensile force of 1 kN applied to it and the force produces a stress

of7.8 MPa in the cable. Calculate the diameter of the cable.

Example 5

A split pin requires a force of 400 N to shear it. The maximum shear stress before shear occurs

is120 MPa. Determine the minimum diameter of the pin.

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