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Newton’s Laws
Physics 11
Humour….this one is…..bad!
Summary: Newton’s Laws
1.
2.
3.
Inertia
Fnet=MA
F=-F
Newton’s First Law of Motion
“An object in motion will
remain in motion or an object
at rest will remain at rest until
acted upon by an external
force.”
Newton’s First Law
These pumpkins will not move unless acted on
by an unbalanced force.
Newton’s First Law continued…
•
o
Changes in motion do not happen
spontaneously.
So an object at rest stays at rest and an
object at a constant velocity stays at that
velocity until……

A force acts to change its state of motion!
•
Which leads to Newton’s 2nd law…….
What causes a change a state of
motion?
Newton’s 2nd Law:
Fnet=ma
Net force = the total amount of force and direction of the
force acting upon an object.
Ex: Two people are pushing a box. Person1 pushes with 50
N right. Person 2 pushes with 50 N right as well.
Fnet =100 N [right]
Remember: Acceleration =
Change in speed … or…
Direction
What is the Net Force?
What is the direction of Fnet?
Will the car speed up or slow down?
The difference is 20 N, to the left.
The car will speed up.
If there is no net force…
Fnet = ma
0 = ma
No Acceleration=Constant Speed!
Which can accelerate easily?
Small masses
change motion
easily.
It is easy to accelerate small masses.
Inertia = resistance to changes in motion.
Large masses
resist net forces
and do not
accelerate much.
A large mass resists changing
its velocity.
What if we put rockets on each creature?
Small masses change
motion easily.
Large masses resist
changing motion.
Consider how mass and acceleration are
related……
For a given Net Force: Mass and Acceleration are
inversely proportional.
F=MxA
10 = 10 x 1
10 = 5 x 2
10 = 1 x 10
As mass decreases,
acceleration increases
Consider how mass and acceleration are
related……
•
•
Double the force - double the acceleration.
Double the mass - halve the acceleration.
Which car wins the race assuming drag,
friction and Fnet are the same for both?
Fnet=ma
Newton’s Third Law of Motion
•
“To every action (force) there is always
opposed an equal reaction (force)”
•
Every object that exerts a force on another
object is always acted upon by a reaction
force.
Newton’s Third Law of Motion
Examples:
1.
2.
3.
4.
5.
6.
Recoil of a gun.
Thrust of a rocket.
Rebound of a hammer from a struck nail.
Book sitting on a table.
Walking.
Lumberjack running on a log in a lake.
Example:
If a Mack truck
and a Volkswagen
have a head-on
collision, which
vehicle will
experience the
a) the Mack truck
greater impact
b) the Volkswagen
force?
c) both the same
d) ... it depends on other factors
Example:
If a Mack truck and
a Volkswagen
have a head-on
collision, which
vehicle will
experience the
Answer: (c)
greater impact
Newton’s Third Law ~ action and
force?
reaction
Today’s practice:
Draw FBD’s for ques 2, 4, 5!
• Page 163 #1, 2, 3
• Page 168 # 4, 5
•
DO # 2 together?????