Transcript Newton`s Third Law

Newton’s Third
Law
8th Grade Science
Fremont Junior High
It’s the Law!
Newton’s 3rd Law of Motion states…
• For every action, there is
an equal and opposite
reaction.
• So…forces always come
in pairs.
Applies to objects at rest
•An object stands on the
floor, pulled down by the
force of Earth’s gravity.
•However, the object does
not move in that direction,
because the floor stops it.
•The floor is exerting on the
object an equal and opposite
force.
http://www-istp.gsfc.nasa.gov/stargaze/Snewton3.htm
Applies to Objects in Motion
Boat
Oars
•
•
•
•
•
The oars move in a backwards motion
The boat moves forward.
The movement of the oar is the “action.”
The movement of the boat is the “reaction.”
The movements are opposite in direction but
equal in force.
Applies to Objects in Motion
• The swimmers arms
move in a backwards
motion
• The swimmer moves
forward.
• The movement of the arm
is the “action.”
• The movement of the
swimmer’s body is the
“reaction.”
• The movements are
opposite in direction but
equal in force.
swimmer
arm motion
Applies to Objects in Motion
• The hot gases move in a
downward motion
• The rocket moves
upward.
• The movement of the
gases is the “action.”
• The movement of the
rocket is the “reaction.”
• The movements are
opposite in direction
but equal in force.
Rocket
hot gases
Applies to Objects in motion +
objects at rest
•
•
•
•
•
The toe exerts a force on the rock.
The rock exerts a force on the toe.
The movement of the foot is the “action.”
The force of the rock is the “reaction.”
The movements are opposite in direction but equal in
force.
• The greater the force, the greater the impact (pain)
Applies to Objects in motion + objects at rest
• The runner exerts a force on the starting block.
• The starting block exerts a force on the runner.
• The push of the foot against the starting block is the
“action.”
• The force of the starting block is the “reaction.”
• The forces are opposite in direction but equal in force.
• The greater the force, the greater “push off”
Skateboarding
•The foot exerts a force
against the ground.
•The ground exerts a force on
the foot.
•The push of the foot is the
“action.”
•The force of the ground is the
“reaction.”
•The forces are opposite in
direction but equal in force.
•The greater the force, the
faster the rider will go.
Rifle Recoil
• A gunpowder explosion creates hot gases that
expand outward allowing the rifle to push
forward on the bullet.
• The bullet pushes backwards upon the rifle.
The acceleration of the recoiling rifle is ...
– a. greater than the acceleration of the bullet.
– b. smaller than the acceleration of the bullet.
– c. the same size as the acceleration of the
bullet.