Force and Motion

Published by: BhumiRaj Timalsina

Published Date: 24 Jan 2022

Force and Motion in Grade-7, Reference Note

Introduction

We do various types of works in our daily life like kicking a football, pressing something, twisting rods, playing cricket, pulling and pushing, lifting water from the well, etc. If we want to shift our table or anything from one place to another place then, we either push or pull the table to maintain in that place. It means that to move any object from one position to another position, that object has either to be pulled or pushed. This pushing and pulling of a body is known as force. It can also be defined as the pull or push which changes or tends to change the position (either rest or motion) of a body. It can change the state of the body. The SI unit of force is Newton (N) and the CGS unit is dyne. It has both magnitude and directions.

Types of force

Pulling force: We use force to pull a table from one place to another. We pull rope towards us while playing tug of war. Similarly, while pulling Machhindranath chariot in Newari festival in Nepal, a force is used. This force is known as pulling force. It is the force that pulls or tries to pull an object.

 

Source: www.mstworkbooks.co.za Fig: Pulling Force

 


Activity 1
At first, bring a board cleaning duster and a brick. Then pull the duster by using spring balance and note down the reading shown by the spring balance. After this, pull the brick by using spring balance and also note down the readings shown by the spring balance. Find out the difference between the force required to pull duster and the brick.
The magnitude of the force shown by the spring balance is the force required to pull duster and brick.
 

Pushing force: We use force to push our vehicles if it stops working. To push a cart a fruitier uses a force. To open the window we use a force. Such force is called pushing force. It is the force that pushes or tries to push an object.

Source: www.chegg.com Fig: Pushing Force

Centripetal force and Centrifugal force: When a body moves in a circular path, it experiences two types of force on it, the force that acts towards the centre is called centripetal force while the force that acts away from the centre is called centrifugal force. For an example, when an aeroplane flies in a circular motion then it slightly bends towards the centre due to the centripetal force.

Gravitational force: A special type of force in the universe or planets of the solar system which causes the planets to revolve around the sun in its own orbits. The moon also revolves around the earth due to the presence of force. This force is known as gravitational force. It is the force that acts between any two objects because of their masses. Similarly, if we throw something up in the sky, it returns back to the earth after few seconds due to the force of attraction exerted by the earth that pulls all object towards its centre which is known as gravity.
 

Magnetic force: It is the force of attraction exerted by the magnets. For examples when we bring the things like iron, cobalt, nickel etc near the magnets then they are pulled towards the magnet due to the magnetic force. It is used to separate iron from mixtures of dust particles and other non- magnetic substances. It is also used to remove iron dust from our eyes.
 

Electrostatic force: It is the force exerted by the electrically charged body. It produces electrical force. For examples, when we rub the measuring scale in our dry hair and bring it near the pieces of paper then it attracts the paper towards it. It is due to the development of charges on the scale by rubbing it in dry hair.

Muscular force: The muscles of the human beings and animals exert pulling and pushing force. This force is known as muscular force. For examples, a lion exerts muscular force for running, jumping, etc. We exert muscular force for kicking ball and lifting water.

Frictional force: When two body surfaces are slide just opposite to each other, a special type of force acts upon it or in-between them which opposes the motion of the body. Such force is known as frictional force. Frictional force increases as the weight and the roughness of the body increases. The molecules of the two surfaces in contact attract each other because of the electrostatic force due to which they stick each other at a microscopic level opposing the motion of one another. Friction is also called the necessary evil, as it is both useful and harmful.
 

Effects of friction

  • It opposes the motion of a body moving on the surface of another body.
  • There is heat production when both surfaces rub together.
  • Friction causes wear and tear of sole of shoes and tyre.

Advantages of Frictional force

  • It is easy to burn match stick and climb a tree without sliding due to friction.
  • We can write and draw due to the friction between pen and paper.
  • We can walk freely without sliding and falling due to the friction.
  • Vehicles move on roads without skidding due to the friction.

Disadvantages of frictional force

  • It reduces the efficiency of machines.
  • It slows down the motion of moving part of the body.
  • Due to the friction, the sole of our shoes and tyres tears away.
  • Due to the friction, noise is also produced in the machine.

Methods of increasing friction

  • Grooves are made in the tyres to increase friction that prevents from slipping on the road.
  • Spikes are made on the soles of shoes of athletes and mountaineers to increase friction, which helps to stop and run fast without slipping.

Methods of reducing friction

  • In machines where possible, sliding friction can be replaced by rolling friction by using ball bearings.
  • Proper greasing between the sliding parts of machine reduces the friction.
  • The parts of the machine, which are moving over one another, must be properly lubricated by using oils and lubricants of suitable viscosity.
  • Friction can be reduced by changing the design of fast moving objects. Streamlined shape reduces friction.

Distance and displacement

Distance

It is the space or the path between any two points covered by the body. It’s SI unit is a metre. It has the only magnitude but no direction so it is a scalar quantity.
For example, You move from point A to B (3m), B to C (1m), C to D(2m) and D to E (1m). Then total distance covered by you is AB + BC + CD + DE
= 3m + 1m + 2m + 1m
Total distance covered by you from point A to B is 7m.

Displacement

It is the shortest distance between the initial position and final position. It has both magnitude and direction so it is a vector quantity and is measured in metre in SI system.
For example: let us consider the example of distance, If you move directly from point A to B to E instead of moving from A to B to C to D to E, then total distance covered by you will be the shortest distance then 1st one.
Distance covered = AB + BE
= 3m + 2m
= 5m

This shortest distance is the displacement which is in fixed direction.

Scalar quantity and vector quantity

Scalar quantity

These are those physical quantities that are fully described only a magnitude (or numerical value) alone. Mass, length, time, distance, area, volume, etc are some examples of a scalar quantity. Mass have magnitude but it does not have direction, so it is a scalar quantity.

Vector quantity

These physical quantities are described by both magnitude and direction. Displacement, velocity, acceleration, force, etc are some examples of the vector quantity. Force has magnitude as well as the direction of where it is applied, so it is a vector quantity.

Speed and velocity

Speed

It is the distance covered by the body in per unit time. It has both magnitude and direction. It’s SI unit is metre per second i.e. m/s. It is calculated by:
Speed =Distancetravelled(s)Timetaken(t)Distancetravelled(s)Timetaken(t)

Fast moving objects covered a large area in less time then slow moving objects. The speed of fast moving object is calculated in terms of km/sec
For example : If the motorbike travels distance of 200km in 2hour then the speed of motorbike is
Solution;
We have,
Distance covered: 200km ( 200 ×× 1000)m = 200000m)
Time taken: 2 hour (2×60×60×60×60 seconds = 7200 seconds
Speed: ?
By using speed formula,
Speed = Distancetravelled(s)Timetaken(t)Distancetravelled(s)Timetaken(t)
= 200km2hour200km2hour
= 100km/hour
Hence the speed of motorbike is 100km/hour
In meter per second,
Speed = stst
= 200000m7200s200000m7200s
= 27.78m/sec

Velocity

It is the displacement per unit time. It has both magnitude and direction. It has both magnitude as well as direction. It’s SI unit is metre per seconds. It is calculated by the given formula:
Velocity = Displacement(s)Timetaken(t)Displacement(s)Timetaken(t)

For examples: If the motorbike travels from point A to B in a fixed direction and covers a distance of 50 km in 55 minutes. Then the velocity of car is given by,
Solution:
We have,
Distance covered: 50km = 50×× 1000 m = 50,000m
Time taken: 55 minutes = 55×× 60 seconds = 3300 seconds
Velocity = ?
By using formula,
Velocity = Displacement(s)Timetaken(t)Displacement(s)Timetaken(t)
= stst
=50,000m3300sec50,000m3300sec
= 15.15m/sec
∴∴ the velocity of motorbike is 15.15m/sec.

Uniform velocity and Variable velocity

Uniform velocity

A body is said to be in uniform velocity if it covers the equal distance in equal interval of time in a fixed direction. It is a vector quantity and is measured in metre in SI system.
Suppose, a cycle is moving from point A to B. It covers 5m in every second. So, it is moving with uniform velocity.

Variable Velocity

A body is said to be moving in variable velocity if it covers the unequal distance in equal interval of time in a fixed direction. It is also a vector quantity and measured in metre in SI system.
Suppose, a cycle is moving from point A to B. It covers the unequal distance in equal interval of time or vice versa. So, it is moving with variable velocity.

Average velocity

The mean or average value of initial velocity and final velocity of a moving body is called average velocity. It is calculated by:
Average velocity= Initialvelocity(u)+finalvelocity(v)2Initialvelocity(u)+finalvelocity(v)2
= u+v2u+v2
Suppose if the initial velocity of the moving car is 30km/hour and final velocity is 40km/hour. Then its average velocity is given by:
Solution:
We have,
Average velocity= Initialvelocity(u)+finalvelocity(v)2Initialvelocity(u)+finalvelocity(v)2
= u+v2u+v2
= 30+40230+402
=702702
= 35km/hour
∴ The average velocity of the car is 30km/hour.

Acceleration

Acceleration is the rate of change of velocity with time. It’s SI unit is metre per second square. It has both magnitude and direction. The change in velocity is described in terms of acceleration. It is the rate at which object changes its velocity. Acceleration is zero when a body moves with uniform velocity as there is no change in velocity. It is calculated by given formula,
Acceleration = ChangeinvelocityTimetakenChangeinvelocityTimetaken
= Finalvelocity(v)–Initialvelocity(u)Timetaken(t)Finalvelocity(v)–Initialvelocity(u)Timetaken(t)
= v–utv–ut
Where,
u = initial velocity
v = final velocity
t = time taken

For an example, when a car starts from rest and gains a velocity of 30 m\s in a fixed direction after 10 seconds then its acceleration is given by
Solution:
We have,
Initial velocity (u) = 0
Final velocity (v) = 30m/s
Time taken (t) = 10s
Acceleration = ?
By using formula,
a = v–utv–ut
= 30–01030–010
= 3m/s2

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