Name: ________________________________________
Core: ______
Date: ________________
FORCES AND NEWTON’S LAWS OF MOTION – February 8, 2016
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.
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Forces occur in pairs and they can be either balanced or unbalanced.
Balanced forces do not cause change in motion or
acceleration.
•
They are equal in size and opposite in
direction.
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.
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.
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
•
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.
GRAVITY AS A FORCE
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We encounter the force of gravity every day.
•
Gravity = the force that acts to pull objects straight towards the center of the earth.
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As an object falls, its velocity changes at a rate of 9.8 m/s. What is this an example of? __________________
•
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
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 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 AND 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).
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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.
NEWTON’S FIRST LAW OF MOTION = 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.

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???
o
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.
o
A book sliding across a table slows down because of the force of friction.
o
If you throw a ball upwards it will eventually slow down because of the force of gravity.

What about in space?
o
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 OF MOTION = Force equals mass times acceleration (F = ma).

Acceleration: a change in velocity (changing speed or changing direction).

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.

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? _______________

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 OF MOTION = For every action there is an equal and opposite reaction.

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.

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.

Example #3: The baseball forces the bat to the left (an action); the
bat forces the ball to the right (the reaction).