Transcript Newton`s Third Law - K

SPH3U – Unit 2
Newton’s Laws
First… a quick video! 

http://www.youtube.com/watch?v=NWE_
aGqfUDs
Newton & Galileo

Newton and Galileo were very important to the
development of physics. Together they
discovered some very important principles of
motion. In this course we will study:
 Newton’s first law of Motion
 Newton’s second law of Motion
 Newton’s third law of Motion
Inertia

During the 1600’s Galileo performed
experiments with objects in motion and
discovered that an object in motion would
“like” to resist changes in its state of motion.
He called this Inertia.
Inertia

Inertia  the property of an object that resists
change in its state of rest or motion.
“Objects at rest tend to stay at rest”
“Objects in motion tend to stay in motion”
Newton’s First Law

Later, Newton expanded on this idea and it
became the first law:

Newton’s first Law of Motion  If the net
force acting on an object is zero, the object will
remain at rest or move at a constant velocity.
Question

Ex. Which object is in motion?

F = 2N

F
= 4N

F
= 2N

F
= 2N
Question

Ex. Which object is in motion?

F = 2N

F
= 4N
The external net force
is 2N right. Therefore
it is actually
accelerating.

F
= 2N

F
= 2N
The external net force
is 0. Therefore it could
either be not moving
or moving with a
constant velocity
Newton’s First Law
Implications

There must be an external force in order to
change motion.
(You can’t change the motion of a car by
pushing on the seat while inside it – that’s an
internal force, but you can change its motion
by pushing on the outside of the car – that’s an
external force).
Newton’s First Law
Implications

Moving objects will continue to move in a
straight line at a constant speed unless acted
on by an external net force greater than zero.
So… what happens if the external net
force is not zero?
Newton’s Second Law

Newton’s Second Law  If the external net
force on an object is not zero, the object
accelerates in the direction of the net force.
The magnitude of the acceleration is
proportional to the magnitude of the net force
and is inversely proportional to the object’s
mass.
Newton’s Second Law

Basically:


Fnet  ma
or

 Fnet
a
m
Newton’s Second Law

Note: Many people believe that Force is
related to velocity. They will say things like,
“The more force you add, the faster the object
will go”. Be careful about this! It is not
accurate. What the person means to say is
that, “The more force you add the greater the
rate at which the object speeds up.” Force is
related to acceleration. Zero net force is
related to constant velocity!
Newton’s Third Law

Newton’s Third Law  For every action force
there is a reaction force equal in magnitude,
but opposite in direction.
Newton’s Third Law
We can restate the Third Law as this:
 Every force is one of a pair of interaction
forces.
 The two members of the interaction force pair
act on different objects.
 The two members of the interaction force pair
are equal in magnitude but opposite in
direction.
Newton’s Third Law

That is 

FAonB  FBonA
Example 1 – Ball hits a bat
Action Force: The ball pushes
against bat (exerts a force on the
bat)
Reaction Force:
Example 1 – Ball hits a bat
Action Force: The ball pushes
against bat (exerts a force on the
bat)
Reaction Force: The bat pushes
against the ball (exerts a force on
the ball)
Together the Action Force and
The Reaction force make an
‘interaction force pair’.
Example 1 – Ball hits a bat
Notice:
 There are two forces
 Both act on different objects
(one on the bat, one on the
ball)
 The two forces are actual
equal in magnitude (same
size) but opposite in
direction.
Example 2 – Push against wall
Action Force: You push against a wall (exert
a force against the wall)
Reaction Force:
Example 2 – Push against wall
Action Force: You push against a wall (exert
a force against the wall)
Reaction Force: Wall pushes against you
(exerts a force on you).
Example 2 – Push against wall
Action Force: You push against a wall (exert
a force against the wall)
Reaction Force: Wall pushes against you
(exerts a force on you).

Do you believe that the wall is exerting a force
on you?
Example 2 – Push against wall
Consider this: Sit in a wheeled chair and
push against the wall. What happens?
Example 2 – Push against wall
Consider this: Sit in a wheeled chair and
push against the wall. What happens?
 You move backwards! (Why do you move?)
 As we’ve already discussed, you can’t have
motion without a force. Therefore there must
be a force pushing you back in the direction
you are moving. This force is the Reaction
force exerted by the wall!
Try These

Action Force:
Ball exerts force on pin

Reaction Force:
Try These

Action Force:
Ball exerts force on pin

Reaction Force:
Pin exerts force on ball.
Try These

Action Force:
Ball exerts force on glove

Reaction Force:
Try These

Action Force:
Ball exerts force on glove

Reaction Force:
Glove exerts force on ball.
Discussion of Misconceptions
Homework
1)
Complete the online investigation of Newton’s
Laws and self-quiz at:
http://science.discovery.com/games-andinteractives/newtons-laws-of-motioninteractive.htm
You are expected to do the full investigation,
read about Newton’s bio, and complete the
quiz. (It will be fun - trust me! You get to play
with a cute little worm!)
2)
Complete the following practice questions:
pg. 129 # 2,4,12
pg. 136 # 1-4
pg. 141 #1,2,5