Physics Form One Notes – Force
Full text version of Physics Form One Notes on Force, transcribed from the supplied source document. Source images are intentionally omitted; all recognized text from the topic pages is included below.
Full text: This page contains the complete recognized text for source pages 31-38. For the original PDF layout and illustrations, use the WhatsApp button below.
‘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 eausesmakes 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.1 unit of force
The SI unit of force is a Newton (N)
‘A Newton (IN) can be defined as the amount of force required to give a mass of one kilogram
(tkg) an acceleration of Ins
Measuring Instrument (Device)
31
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 witha 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.
‘SPRING BALANCE O
eck O—$(_
rn
Types of Force
[Etna area NT ERTS ERE
‘entfy fundamental types of forces
‘There are two main types of forces namely:
1 Fundamental forces
2. ‘Non fundamental forces
2
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 forve 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 very large bodies attract other
objects towards themselves. It is commonly referred to as the weight of the object that is
attracted.
All objects on the earth experience a force of gravity that is directed downwards towards centre
of the earth, The earth’s gravitational pull is so great that itis 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 gravity on the earth, g = 9.8m/s*
Conclusion, we can say that the force of gravity has the following properties.
33
1. Itpulls (attracts) objects towards the centre of the earth.
2. It is direetly 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 centre of the earth
NB: It should be noted that the weight of an object is directly related to its Mass.
Weight, W = mass x g= mg
The g above has two meanings
1. Itis the gravitation’s field strength (1ON/Kg)
2. Its the acceleration of free fall (10m/s*
Electromagnetic force
This force includes both electric and magnetic force. Its relatively strong,
Example 2
Examples where electromagnetic forces are involved include:
1, Inthe formation of molecules of a substance, Atoms attract each other to form molecules.
This is due to electromagnetic force.
2. In two parallel 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
3a
Properties of gravitational forces
‘The following are the properties of gravitational force
+ Ttatways attracts objects
«This the weakest force among the four basic forces
+ Ttisa central force (gravitational force between two objects acs along the line joining the
centres ofthe objects.
+ operates over very lng distance
Properties of electromagnetic force include
+A may be attractive or repulsive is nature
+ Itisa central free
+ Itis stronger than gravitational free
«This also long-range force (operates over avery long distance)
Properties of a strong nuclear force:
+ Ttis basically an attractive force
. It is a non-central force (does not act at the centre)
+ Itis stronger than gravitational force
© Iisa short-range force that is it operates only up to distance of the order of 10°’4m
Properties of the weak nuclear force
+ tis much stronger than the gravitational force but weaker than the strong nuclear force
and electromagnetic force
© Tracts on small ranges of up to 107m
CLASS ACTIVITY,
3
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, Arbody has a mass of 40kg. Find its weight.
Effects of Force
The Effects of Forces
Hdenify 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 forees 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 theresultant force
36
Oriagnal Resultant
Forces Force
wow oO 2ON,.
ed
5N > 10W, oF
10
1 458 “om
ps
Jow oO 35N
Sw
Inthe resultant force acting on a objects ZERO then:
+The objet wil remain stationary if twas sationary when the result force became
+ Move at cnstant steady) speed in a sraght ine it was moving when the resutant
force became zero.
Ir the reaultan force acting onan abject is NOT ZERO then
«The objet wil accelerate o decelerate (peed up o slow down,
The Effects of Forces on Materials
“fy the ef of forces on materiale
Example 3
a
‘A rectangular bar having a cross-sectional area of 80 mm2has a tensile force of 20 kN appliedto
it. Determine the stress in the bar
3
forceF — 20×10
6
Stress 6 = ——— = ———= = 250x 10° Pa = 250 MPa
areaA 80×10
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.
force F forceF 1×10? suit
sess = SF hence, cross-sectional area, A = = =128.2%10%m
” area A ” stress 78×10″
(Circular area = °= 128210″?
from which, r? = uae 10″ and radius r= 28 axe = 6.38810″ m= 6.388 mm
and iameter d= 22 6.388 = 12.78 mm
Example
A split pin requires a force of 400 N to shear it. The maximum shear stress before shear occurs
{s120 MPa, Determine the minimum diameter of the pin.
stress o= !2F hence, cross-sectional area, A= 12 __400__ 3335,<10″4m!
area A stress 120%10°
Circular area = mr? = 3.3333%10%m?
2 _ 3:3333×10% 5.3333%10″
fiom whieh, x* = 335% 10″ and radius r= = 10″ =1.030«10″ m= 1.030 mm
and diameter d= 2 «1=2 1.030 =2.06 mm
38