Newton’s Third Law  For every force, there is an equal and opposite force.

Newton’s Third Law  Forces naturally balance. If you place a force on something it automatically places an equal force back on you.

Demo  Spring Scale – tool used to measure force.  Gently use one spring scale to pull on another with 15N of force.  How hard is the other Spring scale pulling back?  Why?

If you push the wall, it will push you back!

This is why when you throw something you experience a backward force on your body.

Equal and Opposite Forces For every action force, there is an equal and opposite reaction force. 1 st Balloon pushes air  2 nd Air Pushes Balloon 

If for every force there is an equal and opposite force, why does anything ever accelerate? 8

Net Force If two equal and opposite forces are acting so that they cancel out (net force = zero) the object will not accelerate. 9

Net Force If the equal and opposite forces are acting so that the forces on each object are not being canceled, they will cause acceleration. 10

Will Acceleration Occur? 11

Which force is the equal and opposite force to the horse pulling on the wagon? Will the system accelerate? A CB

Which force is the equal and opposite force to the force of Earth’s gravity on the bike? A C B

If the bike and boy weigh 200 N, how hard is the ground pushing up on the bike? A. 50 N B. 100 N C. 200 N

Limitation of Force  This also means that you can only push or pull with up to the amount of force that the other body can match.

Limitation of Force  What do you think would happen to the force if we let the huge guy pull on the wall?

The man places a force of 30,000 Newtons  on the diving board. How much force will the board place on the man? A. 0 Newtons B. 300 N  C. 30,000 N  D. 30,000 N 

Thought Experiment  Imagine you are an astronaut repairing the space station.  An unexpected gas leak just set you adrift away from the station; your tether cord ripped.  All you have is your space suit and a bag full of heavy tools.  How will you get back to the station and keep from falling into the Earth’s atmosphere? Hint: Use the third law.

Thought Experiment If you were floating weightless in an infinitely big room with no walls in sight, what could you determine? A. If you are still B. If you are moving at a constant rate C. Your direction of movement D. If you are accelerating