What causes motion?
Forces
 A push or a pull is a force.
 Some forces cause movement of objects.
 Just because something isn’t moving, it doesn’t mean
that forces are not at work.
Forces
 When multiple forces are at work on an object, the
combined forces are called the net force.
 If the net forces cancel each other out we say that they
are balanced forces.
Net Forces and Balances Forces
 In tug of war, two groups of people




pull on either end of a rope.
Team A pulls in one direction and
Team B in the opposite direction.
Both teams are exerting a force on
the rope.
The net force will determine who
wins.
If the forces are balanced, no one
will win because the net force will be
zero and the rope will not move.
Inertia
 Inertia is an objects tendency to resist a change of
motion.
 Inertia is directly related to an objects mass. (It takes
more force to change the motion of a bowling ball than
it does a ping pong ball).
Sir Isaac Newton
 Sir Isaac Newton was a British scientist (1642-1727)
who is known for his laws of motion.
 These are known as Newton’s Laws of Motion.
Newton’s
st
1
Law of Motion
 Objects at rest tend to stay at rest unless acted upon by
an outside force.
 Objects in motion tend to stay in motion unless acted
upon by an outside force
Newton’s
st
1
Law of Motion
 Newton’s 1st Law is also known as the Law of Inertia.
 Remember: Inertia is an objects tendency to resist a
change in motion.
 In other words, if something is moving it will continue
to move at the same rate in the same direction unless
some other force comes along and changes it’s motion.
Likewise, if something is not moving, it’s not going to
move unless a force comes along and puts it into
motion.
Wait a minute…
 If I slide across the hardwood floor in my socks, why




don’t I continue to slide forever?
Friction!
If an apple comes loose from a tree, why does it fall to
the earth?
Gravity!
Not all forces are as obvious as physically pushing or
pulling things with our hands.
Newton’s 1st Law of Motion in the
car…
 Think about when you are riding in a car. Pretend
you’re sitting in a car with your eyes shut.
 The car begins at rest. You are also at rest.
 The car accelerates. What do you feel?
 The car moves forward, so you feel a force on your
back, pushing you into motion.
Newton’s 1st Law of Motion in the
car…
 Once the car is traveling at a constant speed, you no
longer feel yourself being pushed by the car. This is
because you are also traveling at this constant speed.
 The car turns to the left. What do you feel?
 You are pushed to your right, why? Because the car is
essentially ‘cutting you off.’ You would continue to
move in a straight line, but the car pushed you to the
left.
Newton’s 1st Law of Motion in the
car…
 The car comes to a sudden stop. What happens?
 You are thrown forward. This is because your body
was traveling at the same rate of the car when it
stopped. You were pulled back by your seatbelt.
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