Forces
What are forces?
Inertia
Gravity
Newton’s Laws
Projectile motion
What is a force?
A
push or a pull
 May be unbalanced or balanced
What does a force do?
 Gives
energy to
an object
 Can set an
object in motion,
make it stop, or
change its
motion
Unbalanced forces change motion
Unbalanced forces change motion
Unbalanced forces change motion
Forces are represented by vectors
Forces are added with vectors
Friction
A force which opposes motion, bringing
objects to rest
 Types:

Inertia
Inertia
The tendency of
an object to stay
at rest or in
motion.
 Objects with
greater mass
have greater
inertia

Inertia keeps astronauts in motion
Seat belts protect against inertia
Gravity
Gravity
The attractive
force between
all objects in the
universe
F = M1 M2
d2
 More
mass means greater gravity
 More distance means less gravity
Massive objects have
lots of gravity
My gravitational
attraction is small
no matter what the
distance!
The earth exerts a force on the student
and the student
exerts a force
on the earth
because
of GRAVITY!
Newton’s Laws of Motion
1. Inertia
2. F=ma
3. Action-reaction
Newton’s First Law of Motion
The law of
inertia:
An object at rest
stays at rest
an object in
motion stays
in motion
unless acted
upon by an
outside force.

Inertia keeps astronauts in motion
Inertia keeps satellites in orbit
Inertia keeps you at rest
Newton’s Second Law of Motion
 Force = Mass x acceleration
 A force is needed for acceleration
 Acceleration will occur in the direction of
the force.
Which cars are accelerating?

If you watch carefully, the green and blue
cars are accelerating. The red car is
traveling at constant speed.
A force is needed for acceleration
Will these accelerate?
 A skier going downhill?
– What force is acting on
the skier?
 An astronaut on a
spacewalk?
– Are any forces acting
on the astronaut?

Which of the following will
accelerate?
A bullet shot from
a gun?
– What forces are
acting on the
bullet?
A hammer and
feather dropped on
the moon?
– What force is acting?

Newton’s
nd
2
Law
Force = mass x acceleration
F_____
M
A
Force is measured in newtons. A newton is a
kg m
s2


Example: A car with mass 1000kg hits a tree
and decelerates at 50m/s/s. What force
does it exert on the tree?
Answer:
M
F_____
A
1000kg * 50m/s/s =
50,000 kgm or 50,000N
s2
F = ma
A car is traveling at constant speed. Is a
force acting on it?
 Answer: although there are forces acting
on the car (gravity, friction, air
resistance,etc…) since there is no
acceleration, the net force in the direction
of motion must be zero.

More
nd
2
Law Problems:
1. What is the acceleration on a mass of
50kg if a force of 10N is applied?
 2. An object accelerates due to gravity at
a rate of 10m/s/s. If its mass is 15kg,
what force is acting on the mass?

Answers:

_F_
M A
1. _10N_
0.2 n/kg= 0.2 kg m = 0.2 m/s2
50kg A
s2/kg
2.
F______
15kg 10m/s2
150 kgm/s2 or 150N
Gravity is a force that causes an
acceleration
On earth, ALL objects accelerate at
9.8m/s2 (ignoring air resistance) because
of gravity.
 No matter what the mass, ALL objects on
earth accelerate at 9.8 m/s2

Newton’s

3.
nd
2
Law
F__
m a
An object of mass 3kg falls off a
cliff. How much force is acting on
the object?
Answer:
F_______
3kg 9.8 m/s2
29.4N
Free fall without air
resistance
Both the elephant and
the feather land at
the same time!
Free fall with air
resistance
The feather takes
lots longer to hit
the ground!
Newton’s Third Law

For every action, there is an equal and
opposite reaction
For every action, there is an equal
and opposite reaction

In rocketry……
The gases
being expelled
is the action.
The rocket
propelling
forward is the
reaction.
How do Newton’s laws describe the
motorcycle rider?
 1st:
Inertia keeps the rider moving forward.
 2nd: The force with which the rider hits the
ground will be equal to his mass times his
deceleration.
 3rd: The action of hitting the ground will
equal the reaction of the dirt.
Projectile motion
Projectile motion

An object moves forward due to
inertia and accelerates downward
due to gravity.
Projectile motion of a ball fired
vertically
The motion
of the truck
causes the
ball to project
forward.
What would
happen if
the truck
stopped?
In this video, the ball is dropped
straight down and moves forward
due to inertia
Package dropped from plane
The forward motion
of the plane causes
the package to land in front
of where it’s dropped.
Satellites