FORCE  
A
F
orce:  
Is a push or pull experienced by an object.  
OR  
Is a push or pull upon an object resulting from the object’s interaction  
with another object  
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The SI Unit of force is Newton (N)  
It is measured by spring balance.  
Types of Forces  
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Fundamental forces  
Non fundamental forces  
Fundamental Forces  
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Are the basic forces in nature that cannot be explained by the action of another force  
OR  
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Are the forces in which the two interacting objects are not in physical contact  
with each other.  
Types of Fundamental Force  
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Force of gravity (weight)  
Electromagnetic force  
Strong nuclear force  
Weak nuclear force  
Force of Gravity (weight)  
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Is the earth’s gravitational pull on a body , lying on near the surface of the earth  
For example, all objects fall down if thrown up due to force of gravity pulling the  
objects towards the earth surface.  
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Force of gravity, F = mass (m) x acceleration due to gravity(g)  
→ 푭 = 풎품  
The acceleration due to gravity (g) has a constant value of 10 m/s2 or 9.8 m/s2  
NB:  
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The force of gravity pulls objects towards the centre of the earth.  
The force of gravity is proportional to the mass of an object  
The force of gravity is stronger when the mass is closer  
Gravitational force  
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Is the force of attraction acting between any two bodies of the universe  
Properties of Gravitational force  
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It is always attractive  
It is the weakest force among the four basic forces  
It is a central force.  
It obeys the universe square law  
It operates over very long distances.  
Mass and weight.  
Mass  
Difference between mass and weight  
Weight  
It is the quantity of matter in a body  
It is measured in kilograms (Kg)  
It is the same everywhere  
It is the pull of gravity on a body  
It is measured in Newton (N)  
It changes from place to place  
It is measured using a beam balance  
It has magnitude only  
It is measured by using a spring balance  
It has both magnitude and direction  
Example  
1. An astronaut weighs 900 N on earth. On the moon he weighs 150 N. Calculate  
the moons’ gravitational strength. (Take g = 10 N/kg).  
Solution  
From: w = 풎품  
ퟗퟎퟎ  
ퟏퟎ  
→ 풎 = 풘 =  
= ퟗퟎ 풌품  
품
Moons’ gravitational strength = weight of astronaut  
ퟏퟓퟎ  
풘풆풊품풉풕 풊풏 풕풉풆 풎풐풐풏  
=
= ퟏ. ퟔퟕ ms-2  
gmoon  
=
풎풂풔풔 풐풏 풕풉풆 풎풐풐풏  
ퟗퟎ  
Individual Task – 1  
1. Rocket moves from the earth to a planet x. if it weighs 10, 000N and 30N on  
the earth and on planet x respectively determine the acceleration due to gravity  
on planet x (ANS: g in a planet x = 0.03N/kg)  
Electromagnetic Force  
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Is the force that associated with production field due to movement of electrons.  
It includes both electric and magnetic forces.  
For example: -  
(a) Formation of water molecules, Atoms attract each other due to electromagnetic force  
(b) In two charges placed near each other may attract or repel due to  
electromagnetic force  
Properties of Electromagnetic Force  
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It may be attractive or repulsive in nature  
It is a central force  
It is stronger than gravitational force  
It is a long-range force (operates over a very long distance)  
Strong Nuclear Force  
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Is the force which holds the constituents of the atomic nucleus  
It acts within the nucleus of the atom.  
An example of this force is the nuclear energy obtained from the splitting  
(fission) or the fusing together of the nucleus of the atom.  
Properties of Strong Nuclear Force  
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It is basically an attractive force  
It is a short-range (operates only up to a distance of the order of 10-14 m)  
It is a non-central force (it does not act at the centre)  
It is stronger than gravitation force  
Weak Nuclear Force  
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Is the force which appears only in a certain nuclear process  
For example, in formation of water from reaction between oxygen gas and  
hydrogen gas weak nuclear force is used to bond the water molecules  
Properties of Weak Nuclear Force  
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It is Stronger than gravitation force  
It is weaker than electromagnetic force and strong nuclear force  
It Operates on small ranges of up to 10 -17 m.  
Non - Fundamental Forces  
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Are the forces in which the two interacting objects are in physical contact with  
each other  
Examples are:- (a) Kicking a ball  
(b) Air resistance  
(c) Pulling a door  
(d) Tension  
(e) Compressing a spring  
(f) Friction  
(g) Elastic forces e.t.c  
Effects of Forces  
Forces have several effects on objects. These effects include:-  
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Stretching (tensile)  
Torsion  
Attraction  
Friction  
Compression  
Viscosity  
Air resistance  
Repulsion  
Stretching and Restoring  
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Stretching occurs when an object increases its length when the force is applied  
to it.  
For some objects there is a tendency to return to their original shape and size.  
This is called restoring force.  
For example, when spring is pulled the stretching force elongates the spring  
Compression and Restoring  
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Compressional force is the force which when applied to an object result in  
decreasing in its volume.  
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Example when you compress the spring  
Restoring force is the force which causes a body to return to its original shape  
and size  
Attractive force  
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Is the force that pulls objects toward each other.  
For example, A Magnet always attracts other objects like iron.  
Repulsive force  
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Is the force that pushes objects against each other.  
For example, when the same poles of magnets are closer to each other, they  
repel  
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The figures below show an example of attractive and repulsive force  
Torsional Force  
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I
s a force produced when a solid matter is twisted  
Frictional force  
This is the force which opposes motion between two surfaces of objects in contact.  
Friction occurs when one surface of an object is resting or moving over  
another. Friction is a very common force. Whenever one object slides over another  
object, friction tries to stop the movement.  
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For example, an exercise book cannot be slid on top of a table due to friction exists  
between exercise book and table  
✓Frictional force occurs depending on the nature of the surfaces of bodies  
in contact.  
✓Friction produces heat, as is the case in matchsticks. Wear and tear of car  
tyres and shoe soles are caused by friction.  
COEFFICIENT OF FRICTION  
We noticed that more force is needed to pull the brick placed on the rough surface  
than when it is placed on the smooth surface.  
This is due to the existence of a large friction on a rough surface than on a smooth  
surface. In practice, no surface has no friction, rather, some surfaces have  
minimum friction while others have maximum friction.  
On rough surfaces, friction is more noticeable because the surface has maximum  
friction while on smooth surfaces the friction is minimal. The level of friction  
caused by a surface is expressed by the Coefficient of friction.  
The Coefficient of friction is defined as the ratio of friction force to the normal  
reaction.  
Viscous force  
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Viscosity is the resistance of a fluid to flow.  
For example, water has a lower viscous force than cooking oil, since it has less  
resistance to flow  
Air Resistance  
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Is the force that resists the movement of an object through the air.  
Example of this force is viscosity  
Factors affecting Air Resistance  
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Size and shape of the body  
The speed of fluid  
The density of the fluid  
Normal Force  
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Is the force that acts in equal and opposite direction to the weight of a body  
Applied Force  
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Is the external force that causes the system or body to change position