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

• 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

OarsBoat

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

• 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

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.