Transcript Action Reaction

Chapter 5
Newton’s Third Law
of Motion
For every “action” there is a “reaction”
29-Mar-16
Physics 1 (Garcia) SJSU
Newton’s Third Law of Motion
Whenever an object exerts First Object
(Hammer)
a force on a second
object, the second object Second Object
(Nail)
exerts a force of equal
magnitude in the
opposite direction on the
first object.
29-Mar-16
Physics 1 (Garcia) SJSU
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?
The change in velocity (the acceleration) is greatest for which vehicle?
By what principle of physics?
29-Mar-16
Physics 1 (Garcia) SJSU
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.
29-Mar-16
Physics 1 (Garcia) SJSU
Action and Reaction
Common expression of 3rd Law is,
To every action there’s an equal and opposite
reaction.
What’s an “action”?
How can reaction be “equal” and “opposite”?
29-Mar-16
Physics 1 (Garcia) SJSU
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.
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Physics 1 (Garcia) SJSU
Walking, Running & Jumping
What forces accelerate us into motion when
we walk, run, or jump?
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Physics 1 (Garcia) SJSU
Forces when Walking or Jumping
For a person walking, running, or jumping,
the three main forces on the person are:
• Gravity (Downward)
• Support of the floor (Upward)
• Frictional force of the floor (Horizontal)
Only these forces can accelerate the person.
Gravity is constant but the force exerted by
the floor can increase in reaction to the
person exerting a force on the floor.
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Physics 1 (Garcia) SJSU
Walking Forward
When weight is on back foot it acts by pushing
back on the floor. Reaction is the friction of the
floor, which pushes your body forward
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If there were no friction
then dancer would fall
Reaction straight down and
Action
could not walk forward
Physics 1 (Garcia) SJSU
Jumping
Jumping is done by pushing
downward on the ground
(action) so the ground
pushes upward on you
(reaction).
How high you jump depends
on the force and on the
distance over which you
apply that force.
29-Mar-16
Physics 1 (Garcia) SJSU
Can only push while in contact
with the ground so squatting
helps by increasing distance.
IMPORTANT!!!
Action force & reaction
force NEVER cancel
because they act on
different objects!
Repeat this to yourself over and over again
29-Mar-16
Physics 1 (Garcia) SJSU
Check Yourself
Miss A pushes the car (action);
car pushes back on her
(reaction). Do these forces
cancel?
Miss A
Action
Reaction
Force on Miss A is to the left;
how can she move forward
(to the right)?
What if floor had zero friction?
29-Mar-16
Reaction
Action
Physics 1 (Garcia) SJSU
ActionReaction
Pairs
Check Yourself
Miss B also pushes the car;
can she move the car by
herself?
ActionReaction
Pairs
In terms of Newton’s laws, why
is this not possible?
What other force does Miss B
exert on the car besides her
hands?
29-Mar-16
Physics 1 (Garcia) SJSU
Miss B
Action
Reaction
Action
Reaction
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.
29-Mar-16
Physics 1 (Garcia) SJSU
Vector Addition
Forces are vectors, with magnitude & direction.
Net Force
A+B
(25 Newtons)
Force B
(10 N)
Object
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Force A
(20 Newtons)
Physics 1 (Garcia) SJSU
Parallelogram rule
Check Yourself
Nellie Newton hangs motionless by one hand from a
clothesline as shown—which is on the verge of
breaking. Which side of the line is
most likely to break?
Two upward forces
must add together
to balance Nellie’s
weight.
29-Mar-16
1. Left side
2. Right side
3. 50/50 chance
of either side
breaking
Physics 1 (Garcia) SJSU
Lab: Force Table
Practice addition of
forces as vectors in
the Physics 1L lab
using “force tables.”
Hang weights and
adjust angles until
forces balance.
29-Mar-16
Physics 1 (Garcia) SJSU