Download Newton`s Third Law of Motion states, “When one object exerts a

Notes: Newton’s Laws of Motion
Standard 8.P.2 The student will demonstrate an understanding of the effects of
forces on the motion and stability of an object.
Motion occurs when there is a change in position of an object with respect to a
reference point. The final position of an object is determined by measuring the
change in position and direction of the segments along a trip. While the speed of the
object may vary during the total time it is moving, the average speed is the result of
the total distance divided by the total time taken. Forces acting on an object can be
balanced or unbalanced. Varying the amount of force or mass will affect the motion
of an object. Inertia is the tendency of objects to resist any change in motion.
Newton’s First Law states. “The velocity of an abject will remain constant unless a
net force acts on it.” It is often call the Law of Inertia.
 If an object is moving, it will continue moving with a constant velocity (in a
straight line and with a constant speed) unless a net force acts on it.
 If an object is at rest, it will stay at rest unless a net force acts on it.
 Inertia is the tendency of the motion of an object to remain constant in terms of
both speed and direction.  The amount of inertia that an object has is dependent on
the object’s mass. The more mass an object has the more inertia it has.
 If an object has a large amount of inertia (due to a large mass)
o It will be hard to slow it down or speed it up of it is moving.
o It will be hard to make it start moving if it is at rest.
o It will be hard to make it change direction.
 Examples of the effects of inertia might include:
o Inertia causes a passenger in a car to continue to move forward even though the
car stops. This is the reason that seat belts are so important for the safety of
passengers.
o Inertia is the reason that it is impossible for vehicles to stop instantaneously.
o Inertia is the reason that it is harder to start pushing a wheelbarrow full of
bricks than to start pushing an empty wheelbarrow. The filled wheelbarrow has
more mass and therefore, more inertia.
o Inertia is also the reason that it is harder to stop a loaded truck going 55 miles
per hour than to stop a car going 55 miles per hour. The truck has more mass
resisting the change of its motion and therefore, more inertia.
II. Newton’s Second Law-Force equals mass times acceleration (F = ma)
Acceleration: a measurement of how quickly an object is changing speed.

If you double the mass, you double the force. If you double the
acceleration, you double the force.

Doubling the mass and the acceleration quadruples the force.

Something very massive (high mass) that’s changing speed very
slowly (low acceleration), like a glacier, can still have great force
III. Newton’s Third Law of Motion states, “When one object exerts a
force on a second object, the second one exerts a force on the first that is
equal in magnitude and opposite in direction.”
○ This law is sometimes called the “Law of Action and Reaction”.
○ Even though the forces are equal in magnitude and opposite in direction,
they do not cancel each other. This law addresses two objects, each with only
one force exerted on it.
■ Each object is exerting one force on the other object.
■ Each object is experiencing only one force.
● The action and reaction forces are reciprocal on an object.
Examples may include:
○ A swimmer swimming forward:
■ The swimmer pushes against the water (action force), the water pushes
back on the swimmer (reaction force) and pushes her forward.
○ A ball is thrown against a wall:
■ The ball puts a force on the wall (action force), and the wall puts a force
on the ball (reaction force) so the ball bounces off.
○ A person is diving off a raft:
■ The person puts a force on the raft (action force) pushing it, and the raft
puts a force on the diver (reaction force) pushing them in the opposite
direction.
○ A person pushes against a wall (action force), and the wall exerts an equal and
opposite force against the person (reaction force).
○ The Space Shuttle engines push out hot gases (action force), and the hot gases
put a force on the shuttle engines (reaction force) so the shuttle lifts (there is no
sling shot doing it!)