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Newton’s third law
Textbook: pp. 82–85
GoalState Newton’s third law and know the properties of force pairs.
New words
Newton’s third law · Nyutonning uchinchi qonuniaction force · ta’sir kuchireaction force · aks ta’sir kuchiforce pair · kuchlar jufti
Explanation
Newton’s third law: two bodies act on each other with forces equal in magnitude and opposite in direction: F₁₂ = −F₂₁. These forces lie along one line, always appear in pairs and are of the same nature. Most importantly, they act on different bodies, so they do not balance each other. For example, if you push a wall, the wall pushes you back with the same force. If you jump from a boat to the shore, the boat moves backward. The accelerations of the two bodies are inversely proportional to their masses: the lighter body accelerates more.
Worked examples
If a horse pulls a cart with 400 N, the cart pulls back on the horse with 400 N.
Two children on roller skates push each other and both move apart; the lighter one gets the larger acceleration.
Class activity
“Push back”: two pupils press their palms together and push gently; each feels the opposite force. Safety: push softly; avoid slippery floors.
Practice
1
State Newton’s third law.
Bodies act on each other with forces of equal magnitude and opposite directions.
2
Why do action and reaction forces not cancel each other?
Because they act on different bodies.
3
A child pushes a wall with 30 N. With what force does the wall push the child (N)?
30
4
Two children on skates push apart: 40 kg and 20 kg. Which gets the larger acceleration?
The 20 kg child: forces are equal, so a = F/m is larger.