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NEWTON’S LAWS OF MOTION
First law of motion ( Law of inertia )
“A body at rest will remain at rest and a
body in motion will continue in motion
unless there is a force that acts on it and
change its state”.
`
“A body will remain in motion unless there is a
force that acts on it”
“A body at rest will remain at rest unless there is
a force that acts on it”
LAW OF INERTIA
Inertia is a property, which tends to resist change in a
body’s state of motion. Mass & Inertia are directly
proportional.
 In a bus, if a driver suddenly steps on the brakes
then the body is thrown forward & backward or the bus
continue to move for some time.
 When a car moves along a circular path, the body
moves along with the curve
as well.
2nd Law of Motion: LAW OF ACCELERATION
“Acceleration (a) is directly proportional to Force (F)
but inversely proportional to Mass (m)”.
F
a
m
where a is in m/s2
F is in Newtons(N)
m is in kg
Explain the following!
a. What happens to a basketball if you dribble it using with a
stronger force?
b. What happens to a bicycle if you step on the pedals
faster?
c. Which is easier to push on a surface, a big rock
or a marble?
d. Who can run faster an obese man or a slim
man?
Problem 1: The force applied in pushing a wheelchair is 50 N.
If the mass of the wheelchair is 15 kg then find the
acceleration of the wheelchair. Suppose the same wheelchair
will be pushed with a force of 90 N this time then how fast
will the wheelchair be?
In which instance do we have greater acceleration? Why?
Problem 2:The mass of a man who collapsed is 60kg. If
he is pulled along the floor with an acceleration of 2 m/s2
then how much force must be applied in pulling the
collapsed man? Suppose the man is 80kg then and you
apply the same force then what is the acceleration
exerted?
In which case did you move faster? Why?
3rd Law of Motion: Law of Interaction
“If there is an action, then there is an equal
or opposite reaction.”
When one body exerts a force on another, the
second body exerts a force on the first, equal in
magnitude but opposite in direction.
Explain the following figures!
Figure 1
Figure 3
Figure 2
Summary
Among the laws of motion, the third law has a lot of
application in science, in other fields or in real life
situation.
There are two important things to bear in mind
about the third law: ( 1 ) Forces come in pairs &
occurs at the same time ( 2 ) Two forces act on
two different bodies which are equal in size but
opposite in direction.
Exercise: Law of acceleration
1. The combined mass of a stretcher & a patient is 100 kg. If the force
applied in pushing the stretcher carrying the patient is 300 N then what is
the acceleration of the stretcher?
2. The acceleration of a stretcher towards the emergency room is 1.2 m/s2.
Find the force needed to push the stretcher if the mass of the stretcher is
25 kg.
3. Find the mass of an object which is pulled with a force of 155 N and which
accelerates at a rate of 2.5 m/s2.
4. The mass of a wheelchair is given as 20 kg. If is accelerated at 2.2 m/s2
then how much force must be applied in pushing the wheelchair.
5. A rolling ball accelerates at a rate of 3 m/s2. How much force has been
applied if the ball has a mass of 100 grams?
6. Find the force applied in pushing a box of medical supplies which has a
mass of 5000 grams and is accelerating at a rate of 1.8 m/s2.
7. An oxygen tank placed on a trolley has a mass of 3500 grams. Find the
force needed to pull the trolley with the tank if it is accelerated at 0.85m/s2.