FORCE PAIRS
According to Newton’s Third Law, forces always come in what?
Pairs (equal and opposite
You push a door open with your hand. Name the two forces in the pair.
Hand pushes door forward; door pushes hand backward
You push a shopping cart forward.
The cart pushes you backward
A 10 N force pushes right on a box. What is the size and direction of the reaction force?
10 N to the left
If Object A pushes on Object B with 20 N, how hard does Object B push back on Object A?
20 N in the opposite direction
A soccer player kicks a ball. Name the force pair between the foot and the ball.
Foot pushes ball forward; ball pushes foot backward
A baseball bat hits a ball to the right.
The ball hits the bat to the left (with equal force)
A 60 kg student stands on the floor. Approximate the force the student exerts on the floor and the force the floor exerts on the student. (Use g ≈ 10 m/s²)
Student on floor: 600 N down; Floor on student: 600 N up
True or False: The action and reaction forces act on the same object.
False (they act on different objects)
A person stands on the ground. Identify the force pair between the person and Earth.
Earth pulls person downward (gravity); person pulls Earth upward with equal force
A person jumps up from the ground.
The ground pushes the person upward
A rocket expels gas backward with a force of 5000 N. The rocket’s mass is 1000 kg. What is the force of the gas on the rocket?
5000 N forward
Why don’t action-reaction force pairs cancel each other out?
Because they act on different objects
A hammer hits a nail into wood. Identify the force pair between the hammer and the nail.
Hammer pushes nail downward; nail pushes hammer upward
A helicopter’s blades push air downward.
The air pushes the helicopter upward
Two ice skaters push off each other. Skater A has a mass of 50 kg and Skater B has a mass of 70 kg. Who accelerates more, and why?
Skater A accelerates more because the forces are equal but Skater A has less mass (a = F/m)
A book rests on a table. Identify the force pair involving the book and the table.
Book pushes down on table with its weight; table pushes up on book with equal normal force
A fish swims forward by pushing water backward with its fins. Identify both forces in the interaction.
Fish pushes water backward; water pushes fish forward
A swimmer pushes water backward with their hands.
The water pushes the swimmer forward
A 4 kg ball is thrown and collides with a wall, exerting a force of 80 N on the wall. What force does the wall exert on the ball? If the ball’s acceleration during the collision is 20 m/s², does this match Newton’s Second Law? Show your reasoning.
Wall exerts 80 N on the ball (opposite direction). Yes — F = ma → 4 kg × 20 m/s² = 80 N