Forces and Newton’s Laws
Video Warm Up
What is a force?
• A force is a push or pull that causes an object to
move, stop, or change direction.
• In physics, a force is anything that makes an object
accelerate.
Balanced and Unbalanced Forces
• Forces occur in pairs and they can be either
balanced or unbalanced.
Balanced Forces
• Balanced forces do not cause change in motion or
acceleration.
• They are equal in size and opposite in direction.
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Balanced Forces
• Example 1: The floor pushes upward
and gravity pulls downwards on the
person.
• Example 2: The table pushes
upward on the book and gravity
pulls downward on the book.
Unbalanced Forces
• An unbalanced force acting on an object will
change the object’s motion.
• An unbalanced force can cause an object to
start moving, stop moving, or change direction.
• The object will accelerate in the direction of the
greater force.
Unbalanced Force Examples
Example 1: Gravity is a
constant force. The
parachute is working
against the force of gravity.
Example 2: The person’s
finger pushes the toy truck
because it is using a greater
force than the toy truck
pushing against the finger.
Calculating the Net Force
• Net force is the overall force acting on an
object. It is a combination of the magnitude
and direction.
- Magnitude = the difference between 2 forces
- Direction = direction of the largest force
Calculating the Net Force
• If there is a net force acting on the object, the
force is unbalanced.
• To calculate the net force, you subtract the weaker
force from the greater force.
• For example, in this tug of war, the net force is 3N
(Newtons) to the left, because 5N-2N = 3N.
What is the net force in these examples?
125N – 75N = 50N 
If the object isn’t moving
or changing direction,
the net force acting on it
is 0 N. (F – F = 0N)
10N – 7N = 3N 
What is the net force in this example?
To find the net force,
subtract opposing
sides and add same
sides.
5N up and 5N down
cancel each other.
10 N – 5 N = 5 N
The greater force is
going left.
What is the net force in this example?
To find the net force,
subtract opposing
sides and add same
sides.
10 N – 5 N = 5 N up
10 N – 5 N = 5 N right
Draw an arrow for the direction of
movement in these examples.
Gravity As a Force
• We encounter the force of gravity every day.
• Gravity = the force that acts to pull objects
straight towards the center of the earth.
• As an object falls, its velocity changes at a rate
of 9.8 m/s2. What is this an example of?
ACCELERATION! (It accelerates as it falls.)
Make a Prediction
• Does the weight of an object affect how fast it
accelerates as it falls?
• What will hit the ground first?
A) bowling ball
B) golf ball
C) They will hit at the same time
https://PBS Video - Experiment
Friction As a Force
• Friction is the force that one surface exerts on
another when the two rub against each other.
• Friction acts in a direction opposite to the
object’s direction of motion.
• Without friction an object would continue to
move at a constant speed forever.
Friction As a Force
• Friction is increased by the amount of surface
area the object has in contact with the surface.
• For example, tires with more surface area have
better traction on the road.
• Friction causes objects to slow down or stop.
Sir Isaac Newton &
Newtons of Force (N)
• The unit for measuring
force is called the
Newton. It was named
after the famous
scientist, Sir Isaac
Newton (1643-1727).
• Discovered the law of
gravity.
• Discovered the 3 Laws
of Motion.
Newton’s Laws of Motion
= Laws that describe the motion of all objects on
the scale we experience in our everyday lives.
1) An object in motion tends to stay in motion and
an object at rest tends to stay at rest unless
acted upon by an unbalanced force.
2) Force equals mass times acceleration (F = m*a)
3) For every action there is an equal and opposite
reaction.
Newton’s First Law of Motion
An object at rest tends to stay at rest and an
object in motion tends to stay in motion unless
acted upon by an unbalanced force.
Newton’s First Law of Motion
Basically, an object will “keep doing what it was
doing” unless acted on by an unbalanced force.
If the object was sitting still, it will remain
stationary. If it was moving at a constant velocity, it
will keep moving.
It takes force to change the motion of an object.
Then why don’t moving objects
keep moving forever???
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
A book sliding across a table slows down
because of the force of friction.
If you throw a ball upwards it will eventually
slow down because of the force of gravity.
What about in space?
In outer space, away from gravity and any sources of
friction, a rocket ship launched with a certain speed and
direction would keep going in that same direction and at
that same speed forever.
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a change in velocity (changing speed or
changing direction).
What does F = m*a mean?
Force is directly proportional to mass and acceleration. Imagine a ball
of a certain mass moving at a certain acceleration. This ball has a
certain force.
Now imagine we make the ball twice as big (double the mass) but
keep the acceleration constant. F = ma says that this new ball has
twice the force of the old ball.
More about F = ma
If you double the mass, you double the force. If you double the
acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples the force.
So . . . what if you decrease the mass by half? How much force
would the object have now?
What does F = ma say?
F = ma basically means that the force of an object comes from
its mass and its acceleration.
Something very massive (high mass) that’s
changing speed very slowly (low
acceleration), like a glacier, can still have great
force.
Something very small (low mass) that’s changing
speed very quickly (high acceleration), like a
bullet, can still have a great force. Something
very small changing speed very slowly will have
a very weak force.
Newton’s Third Law
For every action there is an equal and opposite
reaction.
What does this mean?
For every force acting on an object, there is an equal force
acting in the opposite direction.
Example #1: Right now, gravity is pulling you down in your seat,
but Newton’s Third Law says your seat is pushing up against you
with equal force. This is why you are not moving. There is a
balanced force acting on you– gravity pulling down, your seat
pushing up.
3rd Law
Example #2: Flying gracefully through the air, birds
depend on Newton’s third law of motion. As the
birds push down on the air with their wings, the air
pushes their wings up and gives them lift.
Other examples of Newton’s Third Law
Example #3: The baseball
forces the bat to the left
(an action); the bat forces
the ball to the right (the
reaction).
3rd Law
The reaction of a rocket is an
application of the third law of
motion. Various fuels are burned in
the engine, producing hot gases.
The hot gases push against the
inside tube of the rocket and
escape out the bottom of the tube.
As the gases move downward, the
rocket moves in the opposite
direction.
Think about it . . .
What happens if you are standing on a skateboard or a
slippery floor and push against a wall? You slide in the
opposite direction (away from the wall), because you
pushed on the wall but the wall pushed back on you
with equal and opposite force.