FORCES ACT IN PAIRS (2.3)
· The “Big Idea”:
- Forces change the motion of objects in predictable ways.
· Now we will learn:
- How Newton’s third law relates action/reaction forces.
- How Newton’s laws work together.
Newton’s third law
· Every time one object exerts a force on another object the second object exerts a force that is equal in size and opposite in direction back on the first object.
***Observe what happens during our in-class skateboard demonstration. What do you see?
Both skaters are moving away from each other. They travel at the same speed in opposite directions.
Action and Reaction Pairs
· The force that is exerted on an object (action) and the force that the object exerts back (reaction).
- What is the action force acting on the skaters?
The action force is the first skater pushing off.
- What is the reaction force acting on the skaters?
The reaction force is the second skater pushing back.
- Real-life examples: Space shuttle liftoff, banging your toe on the coffee table, pushing on the table (doesn’t result in motion; still action/reaction).
Action/Reaction vs. Balanced Forces
· Balanced forces exert force on one object.
- Two people pulling on a backpack.
· Action/Reaction forces act on two different things.
- Pulling the backpack across the floor.
Newton’s Three Laws Describe and Predict Motion
· Newton’s laws explain the motion of almost any object.
- Kangaroo illustration, page 60
- Explain how a squid moves forward.
The squid squirts water out from behind. That is the action force. The water propelling the squid forward is the reaction force.
· Winning your canoe race:
1. To get the canoe moving, apply a force to overcome inertia.
2. A less massive canoe is easier to accelerate than a more massive one.
3. Predict the best position for your paddle in the water.
4. If you want to move straight ahead, push backwards on the paddle.