Download Newton`s Third Law

Group Work
1. A hockey puck of mass 0.25 kg slides eastward
across the ice at 25 m/s.
a. What is its momentum p1 (magnitude and direction)?
b. The puck collides with a hockey stick that was lying
motionless on the ice while its owner fights. The
puck rebounds in the exact opposite direction of its
approach, moving at a speed of 10 m/s. Now what
is its momentum p2 (magnitude and direction)?
c. What was the momentum change Dp = p2 – p1 of the
puck from before to after the collision (magnitude
and direction)?
Exam ½ Retake
• should be available for you Thursday
• Finish the Equations of Motion exercise
Newton’s Third Law
actually stems from conservation
of momentum
What’s the Point?
• Where do forces come from?
• Nothing changes its motion on its own!
• Conservation of momentum is one if the
biggest ideas in physics.
Objectives
• Given the force exerted by one object on
another, determine the reaction force.
• Use the conservation of momentum to
analyze the motion of interacting objects.
Poll Question
If a 0.25-g insect collides with a 1250-kg
compact car, which experiences the greatest
(magnitude of) force in the collision?
A.
B.
C.
D.
The insect.
The car.
It’s a tie.
Insufficient information to answer.
Newton’s Third Law
• To every action there is an equal and
opposite reaction.
• If object A exerts force F on object B,
object B exerts force –F on object A, along
the same line of interaction.
• FAB = –FBA
Bug + Windshield
Small car: 1250 kg
Large insect: 0.00025 kg
From the same force, the
bug accelerates a lot
more!
Poll Question
Your educated mule argues that there is no
point in pulling a cart, because the cart will
pull back on him as hard as he pulls on it.
What should you tell him?
A. Oh, sorry, you’re right.
B. It won’t, trust me.
C. The cart’s pull isn’t the only force on you.
D. It has to work. Newton must be wrong.
Interaction Forces
All forces are interaction forces!
– gravity
– wind
– jumping
– everything!
• This means: whenever something
accelerates, something else accelerates in
the opposite direction! Whoa!
Poll Question
If a 0.25-g insect collides with a 1250-kg
compact car, which experiences the greatest
(magnitude of) impulse in the collision?
A.
B.
C.
D.
The insect.
The car.
It’s a tie.
Insufficient information to answer.
All-Class Work
2. Show that when two otherwise isolated
objects interact, their total change in
momentum is zero.
Dp1 + Dp2 = 0
or
Dp1 = –Dp2
Hint: When force F1 is applied to the first object for
time Dt, what is its momentum change? What
happens to the second object during this time?
Poll Question
When a bug hits a car windshield, whose
momentum changes the most? (Assume
there are no external forces.)
A.
B.
C.
D.
The bug’s.
The car’s.
It’s a tie.
Need more information to know.
Conservation of Momentum
Conservation of Momentum
The total momentum of an isolated
system never changes.
Conservation of Momentum
• Newton’s first law:
– no outside force
– no change in v
– thus no change in p
• So an isolated object’s momentum never
changes.
Conservation of Momentum
• Newton’s third law:
– interacting objects apply equal and opposite
impulses to each other
– they experience equal and opposite
momentum changes
• So their total momentum remains the
same.
Group Work
3. Continue the hockey scenario from
problem 1. (puck Dp = 8.75 kg m/s W)
a. What was the momentum change of the
hockey stick during the collision?
(Momentum is conserved.)
b. If the hockey stick has a mass of 1.0 kg.
What is its velocity (magnitude and
direction) after the collision?
Elastic Collisions
• Objects bounce apart after collision
– same relative speeds as before
• Total momentum is conserved
– Some momentum is transferred from one
object to another
• Kinetic energy (more on that later) is also
conserved
Totally Inelastic Collisions
• Objects cling together after collision
– same final velocity
• Total momentum is conserved in the
coupled mass
Inelastic Collisions
• Objects bounce apart after collision
– relative speed less than initial
• Total momentum is conserved
• Kinetic energy less than initial
Group Work
4. If, instead of bouncing apart, the puck
and stick clung together when they
collided (totally inelastic collision), what
would their velocity (magnitude and
direction) be after the collision?
Hint: What is their momentum after the collision?
Group Work
5. Don’t calculate, just think and answer: If
the puck were initially moving eastward,
but had rebounded off the stick so that it
moved northward after the collision,
which direction would the stick have
been moving after the collision?
Stick and puck have opposite momentum changes.
Reading for Next Time
• Work and Energy
• Power
• Important ideas
– Work and energy are scalars
– Kinetic and potential energy