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 current state of motion

3. Inertia is a relative measurement…
Do these guys have a lot of inertia?
Inertia is a relative measurement…
More inertia = MORE MASS
More inertia = harder to CHANGE VELOCITY

4. Force
A force is a push or pull that may change an 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 placed on a table and is motionless.
What forces are acting on it?
FN = NORMAL FORCE
a reaction force that any
object exerts
when pushed on FN
Fg = WEIGHT
a force pulling any object toward the
CENTER OF THE
EARTH Fg

6. Net Force No NET FORCE on an object = EQUILIBRIUM… either
MOTIONLESS
MOVING WITH CONSTANT VELOCITY
A “NET” or “UNBALANCED” force changes an object’s VELOCITY
This means that SPEED AND/OR DIRECTION change
Also called ACCELERATION

7. Net Force  Acceleration
How much acceleration a net force causes an object to have depends on…
amount of FORCE
greater force = GREATER ACCELERATION
amount of MASS
greater 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 2.0 kilogram box is pushed with a net force of 10. newtons. What is the acceleration experienced by the box?
a = Fnet / m
a = (10 N)/(2.0 kg)
a = 5.0 m/s2

10. Weight
The force with which gravity pulls on an object.

11. Example What is the weight of an object with a mass of 30 kilograms?
Fg = mg
Fg = (30 kg)(9.81 m/s2)
Fg = kg·m/s2 or N

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

13. Examples
A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose?
To the WEST

14. Examples
A man getting out of a rowboat jumps north onto a dock. What happens to the boat?
It moves SOUTH

15. Examples
A 60 kilogram astronaut pushes against a 120 kilogram satellite with a force of 15 newtons. How much force does the satellite exert on the astronaut?
15 newtons

16. End of PRACTICE