Action and Reaction Act on Different Objects

Lab Brief #2

🎯 Your mission

Set it up. Test it. Be amazed.

⚡ The twist

Speed isn't the same as force. Don't mix them up.

What you'll learn

The two forces in Newton's third law are equal in size and opposite in direction, but they never act on the same object, which is exactly why they never cancel. A rifle and its bullet feel forces of identical size; the bullet accelerates enormously more because a = F/m and its mass is thousands of times smaller. Two forces cancelling on one body is the first law, not the third — confusing the two is the commonest error in the topic.

Key words

third law pair
Two forces of equal size and opposite direction that act on two different objects.
balanced forces
Two or more forces acting on the same object that add to zero net force.

Make a hypothesis first

Guess: where does the energy go?

Say your guess out loud — then find out if you were right.

Get ready

🧰 What you need

skateboard or wheeled chair
medicine ball
light ball
tape measure

Get ready

⚠️ Safety first

Clear a wide space, work on a flat floor, and have a spotter standing by.

Step 1 of 4 · Read

Think about what you just learned: say what "third law pair" and "balanced forces" mean in your own words before you start.

Step 2 of 4 · Do

Stand on a skateboard holding a heavy medicine ball and throw it forward to a partner. Measure how far you roll backwards, then repeat throwing a light ball the same way. Explain the difference using equal forces and unequal masses.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

A rifle fires a bullet. Compared with the force on the bullet, the force on the rifle is:

How sure are you?😐 maybe

Question 2 of 3

Why do a third-law pair of forces never cancel?

How sure are you?😐 maybe

Question 3 of 3

The bullet accelerates far more than the rifle because:

How sure are you?😐 maybe

Mind = blown

🔥 A bolt of lightning is 5 times hotter than the surface of the sun.

Nice work!

You got 0 of 3 right.

Next: Momentum Is Conserved Even When Energy Is Not All lessons

🎯

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✨ More to explore

🌍 Where you see this in real life: This is why airplanes can fly and why rockets reach space.

🏆 Stretch challenge (try it this week): This week, find one thing in your house that turns one kind of energy into another.

👨‍👩‍👧 For the dinner table: “Show me something at home that uses this kind of energy.”

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Action and Reaction Act on Different Objects

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