Transcript PPT-Newton`s 3rd Law

DEMO
• You have a heavy medicine ball. You throw it
off the back of the cart.
• What do you think will happen?
• Why do you think this will happen?
AFTER DEMO
• What has the greater force acting on it - you or the ball?
• Who has the greater acceleration you or the ball? Why?
• How could you increase the acceleration of the cart?
(HINT - there is more than one way!)
• In what ways is this similar to how a rocket works? (HINT
- think of the hot gas as molecular balls of gas)
• Does a rocket need ground to push off of in order to
launch?
• Did the balls you threw off the back the rolling chair need
something to push off of?
Newton’s 3rd law of Motion
• forces exist in pairs
• every action has an equal
and opposite reaction
paired & opposite forces
Mr. Cimbak’s crew
connection
The crew team uses Newton's third law of motion to move
its boat. When an oar is put into the water, the water exerts
an equal force on both sides of the oars, the surface of the
flat side of the oars pushes against the water. The water
pushes back on the oars with an equal and opposite force.
The boat moves in the opposite direction of the oars with a
force that is equal to that of the oars as they push against
the water. The boat moves because the forces against it
are unbalanced.
The force on a bullet and the force on the gun is same when the
gun is fired, yet the bullet and the gun have very different
accelerations? Why is this?
• Remember Force = mass x acceleration
Force gun= Force bullet
Mgun x
= M x Abullet
• The bullet has a much greater acceleration due to its
small mass/inertia
• Q – Why aren’t bullets as massive as guns?
• Q- What would happen if you fired a gun on a
skateboard?
Agun
bullet
diving boards
ice skating
• What will happen to these 2 ice skaters when
they push off each other?
How does a rocket work?
• Rockets shoot molecular balls of gas
• The rocket pushes on the gas molecules and
the gas molecules push back with the same
force.
• Because there is a small mass to each gas
molecule you need to push them out with a
great acceleration which requires a great
force (explosion)
• Because the gas has a large force on it, the gas
pushes the rocket back with the same large
force
• By increasing the amount of gas shot out per
second, you increase the mass of gas
• To accelerate a larger mass of gas requires a
greater force and thus a larger reaction force
A rocket is a reaction
engine! Rocket pushes
gas out and gas pushes
rocket
Assessment Question
• Joe the fisherman attempts to leap from his
small fishing boat to the dock his boat is next
to, but ends up in the water between his boat
and the dock, why?
• You bring your boat close to the dock. Your
boat is very light and is on a frictionless body
of water. You want to get off the boat but the
dock is just far enough away so you have to
jump off, however it looks close enough where
you could easily do it. You jump… what
happens? Will you move forward? Will you
make it?