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Newton’s Three Laws of Motion
NEWTON’s FIRST LAW- An object at rest will
remain at rest and an object in motion will continue in
motion UNLESS acted upon by a net force greater
than 0.
This cart can continue moving at any constant velocity
forever with NO force (ZERO Newtons) to keep it
moving!
The first law is also called the LAW of INERTIA.
A
B
Cart B will resist a change in velocity more than cart
A because cart B has more mass. Resistance to a
change of velocity is called INERTIA. Greater mass
means greater INERTIA.
NEWTON’s SECOND LAW- The second law states
three things: 1A force acting on a mass will cause the
mass to accelerate in the direction of the force, 2the
acceleration being proportional to the force and
3inversely proportional to the mass of the object.
The mass ( cart) will always 1accelerate in the direction
of the force. Force and acceleration ALWAYS act in
the same direction.
A
B
Though both carts have the same mass, cart B will have
a greater acceleration than cart A because it has a
greater force acting on it.
(2Acceleration is proportional to force.)
A
B
Cart A and B both have the same force acting on
them. Cart A will have a greater acceleration than B
because it has less mass. (Acceleration is 3inversely
proportional to the mass of the object.)
NEWTON’s THIRD LAW- Every action force produces
an equal and opposite reaction force. ( Forces always come
in equal opposite PAIRS.)
Action force
Exhaust gasses
Reaction force
Thrust
The two forces are equal in magnitude and opposite
in direction. They are called an ACTION,
REACTION PAIR.
Another Action, Reaction pair. The weight of the cart
pushes down on the table and the table pushes up with a
force equal to the cart’s weight.
In this situation you pull on the cart and the
cart pulls back on you with a force that is equal
in magnitude and opposite in direction.
A
B
Here are two astronauts in space. Both have the same
mass. Astronaut A pushes on astronaut B. They BOTH
move apart at the same rate because they BOTH
experience the same force.
Each astronaut experiences the same force, but in
opposite directions. This is Newton’s third law of
motion.