Download Newton`s Third LAw

Newton’s Third Law
of Motion
For every “action” there is a “reaction”
Newton’s Third Law of Motion
Whenever an object exerts
a force on a second
object, the second object
exerts a force of equal
magnitude in the
opposite direction on the
first object.
First Object
(Hammer)
Second Object
(Nail)
Check Yourself
A 2 ton car, going 60 m.p.h. losses control and hits
a wall
The force of impact is greatest on, the car or the
wall?
Force on each is
equal (by Newton’s
3rd Law).
Check Yourself
A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h..
The force of impact is greatest on which vehicle, the car or the truck?
Force on each is equal (by Newton’s 3rd Law).
The change in velocity (the acceleration) is greatest for which vehicle?
For the car, which has less mass.
By what principle of physics?
By Newton’s 2nd Law
F = ma
Check Yourself
Arnold Strongman and Suzie Small pull on opposite
ends of a rope in a tug of war. The greater force exerted
on the rope is by
1. Arnold.
2. Suzie.
3. Neither. The force is the same.
Action and Reaction
Common expression of 3rd Law is,
For every action there’s an equal and opposite
reaction.
What’s an “action”?
A force exerted by one object on second object.
How can reaction be “equal” and “opposite”?
Equal in magnitude but opposite in direction.
Action-Reaction Pairs
Here are some
examples of
action-reaction
pairs.
Think of examples
of an object
exerting a force
on a second
object.
Practice identifying
action-reaction
pairs.
Demo: Mutual Attraction
What happens when:
 Mr. A pulls, Mr. B holds.
 Mr. A holds, Mr. B pulls.
 Mr. A & Mr. B both pull.
Mr. A
Mr. B has more
mass than Mr. A
Mr. B
Demo: Mutual Attraction (cont.)
If only Mr. A pulls on Mr. B then Mr. B accelerates.
Reaction force of equal magnitude so Mr. A also moves.
Who moves faster? Mr. A, Mr. B, or the same?
Mr. A
Reaction
Action
Mr. B
Mr. B has more
mass than Mr. A
Mr. A goes faster (greater acceleration) since his mass is less.
Demo: Mutual Attraction (cont.)
When both guys pull then there are two action forces and two
reaction forces.
If both pull with same force, how much greater is the
acceleration than when only one pulls?
Mr. A
Reaction
Action
Action
Mr. B
Reaction
Twice the force so twice the acceleration (by 2nd Law)
IMPORTANT!!!
Action force & reaction
force NEVER cancel
because they act on
different objects!
Adding Forces
When two forces or more forces
act in different directions, finding
the net force is more
complicated.
Have to consider the angle for each force.
Vector Addition
Forces are vectors, with magnitude & direction.
Net Force
A+B
(25 Newtons)
Force B
(10 N)
Object
Force A
(20 Newtons)
Parallelogram rule