Newton's laws of motion describe the relationship between an object and the forces acting upon it. Newton's first law states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Newton's second law states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. Newton's laws can be used to solve problems involving forces, mass, and acceleration.

1. Newton’s Laws – L1
At the end of this session, you’ll be able to:
State Newton’s 1st Law of Motion
Describe examples of Newton’s 1st Law
State the conditions for equilibrium and be able to
solve problems involving equilibrium.
State Newton’s 2nd Law of motion (F = ma and F =
∆p / ∆t)
Solve problems involving Newton’s 2nd Law

2. The definition of force
The simplest concept of force is a push
or a pull.
On a deeper level, force is the action
that has the ability to create or change
motion.

3. Force
Force is an action that can change motion.
n A force is what we call a push or a pull, or any
action that has the ability to change an object’s
motion.
n Forces can increase the speed of an object,
decrease the speed of an object, or change the
direction in which an object is moving.
n Measured in Newtons (N)

4. Mass and Inertia
Chemists like to define mass (kg) as
the amount of “stuff” or “matter” a
substance has.
Physicists define mass (kg) as inertia,
which is the ability of a body to
resist acceleration by a net force.

5. Inertia
Inertia is a term used to measure the ability of an
object to resist a change in its state of motion.
An object with a lot of inertia takes a lot of force
to start or stop; an object with a small amount of
inertia requires a small amount of force to start or
stop.
The word “inertia” comes from the Latin word
inertus, which can be translated to mean “lazy.”

6. Newton’s laws of motion

7. Newton’s 1st law of motion
Newton’s 1st law of motion states that…
“a body will remain in a state of rest or uniform motion
unless acted on by a external force”.
n A stationary body will stay still unless a force acts upon it.
n A body moving at a constant velocity will carry on in a straight
path, unless a force acts upon it.
n No force = No change of v = No acceleration.
n The object will not change direction unless acted on by a force.
v (ms-1)

8. Implications of Newton’s 1st Law
If there is zero net force on a body
(i.e.no external forces), it cannot
accelerate, and therefore must move at
constant velocity, which means
n it cannot turn (i.e. change direction),
n it cannot speed up (accelerate),
n it cannot slow down (decelerate).

9. Newton’s 2nd law of motion
a
F µ
Newton’s 2nd law of motion states that…
“a change in motion (acceleration or deceleration) is
proportional to the applied force”.
The more force
the more acceleration
NOTE a = acceleration, - a = deceleration Units = m/s2
F = ma

10. Newton’s Second Law
A body accelerates when acted upon
by a net external force.
The acceleration is proportional to
the net force and is in the direction
which the net force acts.
a
F µ

11. Newton's Second Law
If you apply
more force
to an
object, it
accelerates
at a higher
rate.

12. Newton's Second Law
If the same
force is applied
to an object with
greater mass,
the object
accelerates at a
slower rate
because mass
adds inertia.

14. Newton's Second Law -
UNITS
a = F
m
Force (Newtons, N)
Mass (kg)
Acceleration ( m/s2 or ms-2)

15. Newton's Second Law
A force of one newton is exactly the
amount of force needed to cause a mass
of one kilogram to accelerate at one ms-2.
We call the unit of force the newton (N).

16. Question 1
Calculate the force required to cause a car
of mass 1200 kg to accelerate at 6 m/s 2
F = ma

17. Question 1
Calculate the force required to cause a
car of mass 1200 kg to accelerate at 6
ms -2.
Σ F = m a
Σ F = 1200 kg x 6 ms -2
Force = 7200 N

18. Question 2
Calculate the acceleration produced by a
force of 20 kN on a mass of 40 g.
F = ma

19. Question 2
Calculate the acceleration produced by
a force of 20 kN on a mass of 40 g.
ΣF = m a
20 000 N = 0.040 kg x a
a = 20 000 / 0.040
acceleration = 5.0 x 105 ms -2

20. Using the second law of motion
There are often many forces acting on the
same object.
The force F that appears in the second law
is the net force.
Acceleration results from the combined
action of all the forces that act on an
object.
When used this way, the word net means
“total.”

21. Positive and negative
acceleration
We often use positive and negative numbers to
show the direction of force and acceleration.
A common choice is to make velocity, force, and
acceleration positive when they point to the
right. Negative to the left.

22. Using the second law of motion
To solve problems with multiple forces, you have to
add up all the forces to get a single net force
before you can calculate any resulting acceleration.
F(net) = ma
a = F/m = = (-10 +30)/10 = 20 / 10 = 2 m/s2

23. Direction of acceleration
Speed increases
when the net force
is in the same
direction as the
motion.
Speed decreases
when the net force
is in the opposite
direction as the
motion.

24. Acceleration from multiple forces

25. Net Force = 200 – (100 + 150) = - 50N
a = F/m = -50 / 25 = - 2 m/s2 so
a = 2 m/s2 LEFT
Acceleration from multiple forces
Three people are pulling on a wagon applying
forces of 100 N, 150 N, and 200 N.
Determine the acceleration and the direction
the wagon moves. The wagon has a mass of
25 kilograms.

26. Acceleration equation 2
a = v2 –v1
t
Change in velocity (m/s)
Time (s)
Acceleration (ms-2)
Acceleration is the rate of change of velocity
v = final velocity, u = initial velocity
a = v – u
t

27. Question 1
A mass of a body accelerates from velocities
u = 2 ms -1 to v = 8 ms -1 when acted on by a
force of 400N for 3 seconds.
a) Find the acceleration
b) Find the mass
F = ma m = F/a = 400/2 = 200 kg.
a = v – u
t
F = ma
a = v – u = 8-2 = 6/3 = 2m/s2
t 3

28. Question 1
Calculate the mass of a body that accelerates
from 2 ms -1 to 8 ms -1 when acted on by a force
of 400N for 3 seconds.
acceleration = change in velocity / time
a = (v – u)/t
= (8 – 2) ms -1 / 3s
a = 2 ms -2
ΣF = m a
400 N = m x 2 ms -2
m = 400 / 2
mass = 200 kg