1. Push and Pull
Newton’s Laws

2. Newton’s First Law An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity (that is, constant speed in a straight line)
unless it experiences a net external force.
Also known as the “Law of Inertia”
Inertia
Tendency of an object to maintain its STATE OF MOTION
Proportional to MASS

3. Do these guys have a lot of inertia?
MORE MASS
means
MORE INERTIA
LOTS OF INERTIA
hard to…
GET MOVING or
STOP

4. Force A push or pull on an object
May change the object’s STATE OF MOTION
CONTACT FORCE – requires touching
Normal, Tension, Friction
FIELD FORCE – no contact required
“Action over a distance”
Gravity (Weight)

5. FN Fg A block of wood is sitting motionless on a table.
What forces are acting on it?
Normal Force is a
REACTION
force that any
object exerts
when pushed on
Normal FN
Weight is the force of gravity
pulling an object toward the
CENTER OF THE
EARTH Fg
Weight

6. Net Force
If there is no NET FORCE on an object it is at equilibrium and either:
MOTIONLESS OR MOVING WITH CONSTANT VELOCITY
So a “net” or “unbalanced” force will
CHANGE AN OBJECT’S SPEED
Changing speed means ACCELERATION

7. Force  Acceleration How much acceleration? Depends on:
AMOUNT OF FORCE
MORE FORCE = MORE ACCELERATION
MASS OF OBJECT
MORE MASS = LESS ACCELERATION

8. Newton’s Second Law
“The acceleration of an object is directly proportional to
the net external force acting on the object and inversely
proportional to the mass of the object.”
Unit of force is the NEWTON (N)

9. Example
A 2 kilogram box is pushed with a net, unbalanced force of 10 newtons.
What is the acceleration experienced by the box?
a = Fnet / m
a = (10 N)/(2 kg)
a = 5 m/s2

10. “For every action, there is an equal and
Newton’s Third Law
“For every action, there is an equal and
opposite reaction” FT FN Fg Fg

11. Third Law Examples
A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose?
A man getting out of a rowboat jumps north onto the dock. What happens to the boat?
There will be a force on the hose to the WEST
The boat will move to the SOUTH