Newton’s Third Law of Motion: Action-Reaction

8SCIENCE

Review: what are Newton’s Laws of Motion?

• First law of motion:– If forces acting on an object are balanced the object will

remain at rest or in motion (constant velocity)– Also called:

• Inertia

• Second law of motion:– If forces are unbalanced, an object affected by a force

will accelerate in the direction of the force– Involves:

• Force and acceleration

Newton’s third law of motion

• Law: Forces always act in equal but opposite pairs

• Another way to say this: for every action there is an equal but opposite reaction

• One object cannot provide a force without this object causing a return force

• Forces act in opposite directions– Ex: bumper cars, walking

Newton’s third law of motion:• Sometimes it may be hard

to identify the action-reactionpair because one object is much larger than the other– It has so much inertia

that it stays still or hardly accelerates– Ex. Walking: when you walk forward you push

your foot backwards on the ground. The Earth pushes your shoe forward but because the mass of the Earth is so large, it does move when you push against it

– Ex. Putting your hand out of the window in a moving car – force of the wind pushes your hand back in the opposite direction the car is moving

• Can you think of more examples?