Newton's third law – interaction pairs

What it shows

Newton's third law says that when body A pushes or pulls body B, B pushes or pulls A with a force of the same size and type, in the opposite direction, at the same moment. The two forces act on different bodies, so they never cancel each other; each body accelerates according to the resultant force on it, a = F/m. That is why a light skater moves off faster than a heavy one, and why the Earth hardly moves when it pulls an apple. Ice, wheels and the balloon's string are frictionless, spring balances are light and g = 9.81 m/s².

How to use

Choose a Situation and press Start. The two force meters at the top show the force on each body. Tap a body to see every force on it, then tap an arrow to light up its partner. Each run adds a row to the table. For the tasks, use Book on a table and Two skaters with Skater B of unknown mass, then press Check.

Parameters you can change

• Situation Person pushing a wall, Two skaters, Book on a table, Two spring balances, Cannon recoil, Balloon rocket, Earth and apple
• Mass of skater A (also the person at the wall) 20–100 kg
• Mass of skater B 20–100 kg
• Largest push 20–200 N
• Skater B has an unknown mass
• Mass of the cannon 2–20 kg
• Mass of the ball 0.2–2 kg
• Pull on the spring balances 1–20 N
• Mass of the book 0.2–3 kg
• Hand pressing on the book 0–20 N
• Mass of the apple 0.05–0.5 kg

Questions to explore

1. Are the weight of a book and the table's push on it a third-law pair?
2. When two skaters push each other apart, which one moves off faster, and why?
3. How big is the force with which an apple pulls the Earth, and why does the Earth hardly move?