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Forces and Motion
Newton’s Laws of Motion
1st Law – Law of Inertia
An object at rest will remain at rest until an
outside force acts upon it.
 An object in motion will remain in motion at
a constant speed and direction until an
outside force acts upon it.
 An object will just keep doing what it’s
doing already.

If it sits still – it stays still
 If it’s moving – it stays moving

Inertia
Inertia is an object’s tendency to resist
change in motion.
 All objects have inertia, because all
objects have mass.
 The greater the mass, the greater the
inertia.
 The greater the inertia, the greater the
force needed to overcome the inertia.

2nd Law of Motion
The acceleration of an object by a force is
inversely proportional to the mass of the
object and directly proportional to the
force.
 Force = Mass x Acceleration
 So…

The smaller the mass of an object, the greater
the acceleration when a certain force is
applied to the object.
 The greater the force applied to an object, the
greater the object’s acceleration.

2nd Law of Motion
bowling ball vs. golf ball
25 kg
1 kg
10 m
38 m
2nd Law of Motion
There is a relationship between
acceleration, mass, and force.
 Force = Mass x Acceleration or F = m x a

Object # 1: mass = 35 kg & acceleration = 12 m/sec
F
÷
M
÷
A
x
MxA=F
35 x 12 = 420 Newtons
Object # 2: mass = 5 kg & force = 110 Newtons
F ÷ M=A
110 ÷ 5 = 22 m/sec
2nd Law of Motion
F
÷
M
÷
A
Object # 3: acceleration = 6 m/sec & force = 90 N
x
F÷A=M
90 ÷ 6 = 15 kg
Practice Problems:
1. F = 24 Newtons
M = 45 g
A=?
2. M = 80 kg
A = 195 m/sec
F=?
3. M = 16 g
F = 35 Newtons
A=?
4. A = 52 m/sec
F = 110 Newtons
M=?
3rd Law of Motion
If one object exerts a force on another
object, then the second object’s force is of
equal strength in the opposite direction on
the first.
 For every action, there is an equal and
opposite reaction.

3rd Law of Motion

Example:
Two people are ice skating.
 One skater pushes on the other skater. Both
skaters move even though only one skater
exerted the initial motion.


Other Examples:
Trampoline
 Being in a swimming pool

3rd Law of Motion
Momentum = mass x velocity
 Momentum – measure of the motion of an
object
 Which sledgehammer has more
momentum?

A 3 kg sledgehammer swung at 1.5 m/sec?
 A 4 kg sledgehammer swung at 0.9 m/sec?

a) 3 kg x 1.5 m/sec = 4.5 kg · m/sec ***
b) 4 kg x 0.9 m/sec = 3.6 kg · m/sec
3rd Law of Motion
p
÷
m
÷
v
p = momentum
m = mass
v = velocity
x
Practice Problems:
1) m = 15 kg
v = 4 m/sec
p=?
2) v = 12 m/sec
p = 140 kg · m/sec
m=?
3) p = 21 kg · m/sec
m = 50 kg
v=?