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Learning that forces always come in pairs and how they interact between different objects.
Imagine you are floating in the middle of a perfectly frictionless frozen lake. You can't walk or crawl. How could you get to the shore using only the heavy backpack you're wearing?
1. You stand still and push against a wall with a force of to the right. 2. According to Newton's Third Law, the wall immediately pushes back on your hands with to the left. 3. The 'Action' is your push on the wall; the 'Reaction' is the wall's push on you.
Quick Check
If a giant truck hits a tiny ladybug, which object experiences the greater force?
Answer
They both experience the exact same amount of force.
A common point of confusion is: If action and reaction forces are equal and opposite, why don't they just cancel out and result in zero motion? The secret lies in the target of the force. To cancel out (result in a net force of zero), forces must act on the same object. In an action-reaction pair, the forces act on different objects. For example, when you walk, your foot pushes the ground backward (Force 1 on Ground), and the ground pushes your foot forward (Force 2 on You). Because the forward force is acting on you and not the ground, it causes you to accelerate forward. You cannot 'cancel' a force acting on yourself with a force you are exerting on something else!
1. A swimmer wants to move forward in a pool. 2. The swimmer uses their hands to push the water backward (Action). 3. The water pushes the swimmer's hands forward (Reaction). 4. Because the reaction force acts on the swimmer (a single object), the swimmer accelerates forward according to .
Quick Check
Why don't the action and reaction forces cancel each other out?
Answer
Because they act on two different objects, not the same one.
Many people mistakenly believe that rockets move by 'pushing off' the air or the ground. If this were true, rockets wouldn't work in the vacuum of space! In reality, rockets rely entirely on Newton's Third Law. Inside the engine, the rocket exerts a massive downward force on the exhaust gases (Action). Simultaneously, those exhaust gases exert an equal upward force on the rocket (Reaction). This upward force, called thrust, is what carries the rocket into orbit. The rocket doesn't need air to push against; it carries its own 'stuff' (fuel) to throw backward so it can be pushed forward.
1. Imagine a cannon firing a ball. 2. The cannon exerts a force of on the ball. 3. The ball exerts a reaction force of back on the cannon. 4. While the forces are equal, the accelerations are not! Using : - Ball acceleration: - Cannon acceleration: 5. This explains why the small ball flies away while the heavy cannon only 'recoils' slightly.
An archer shoots an arrow. If the action force is the bowstring pushing the arrow, what is the reaction force?
A astronaut pushes a satellite with a force of . What is the magnitude of the force the satellite exerts on the astronaut?
Rockets cannot travel in space because there is no air for them to push against.
Review Tomorrow
Tomorrow morning, try to identify three 'interaction pairs' you experience while getting ready (e.g., your feet on the floor, your hand on a toothbrush).
Practice Activity
Blow up a balloon and let it go without tying it. Observe the direction the air travels versus the direction the balloon travels. Can you label the action and reaction forces?