2024 A/L Physics Past Paper · Question 04 (English Medium)

Answer: (1) only (A) is true.

What the question asks

We want: which of the statements (A), (B) and (C) are true for a pair of action-reaction forces.

What each letter means:

$F_1$ ($\text{N}$): the force that the first object exerts on the second.

$F_2$ ($\text{N}$): the force that the second object exerts on the first.

Relevant theory

When the objects interact (collide), they exert equal and oppsite forces on one another.

The wall pushes back on you with an equal and opposite force.

Suppose that a body of mass m is raised from position A to a position B at a height h. The force of gravitational attraction acting on the body is $F = mg$ vertically downwards.

Reading the options

The first quote is Newton's third law: when two objects interact they exert equal and opposite forces on one another. The second quote gives the example of a wall that pushes back on you. The third quote says that gravity pulls on a body that is lifted to a height. We use these to test the three statements.

The options are combinations of (A), (B) and (C).

We test each statement on its own.

For example, two skaters stand face to face on smooth ice. One skater pushes the other. Both slide apart.

The skater who pushed was pushed back too. There were two forces, one on each skater.

Reasoning and calculation

They are equal in magnitude but opposite in direction.

Newton's third law says that the objects exert equal and opposite forces on one another. So the two forces have the same size and opposite directions.

A

They act only on objects touching each other.

A body lifted to a height is pulled down by gravity, $F = mg$, though it touches nothing. Its pair is the pull of the body on the Earth. So the forces of a pair do not need contact.

B

They act on the same object.

When you push the wall, your push acts on the wall and the wall pushes back on you. One force is on the wall and the other is on you, so they act on two different objects.

C

Statement (A) is true, because the third law gives equal and opposite forces.

Statement (B) is false, because gravity pulls without touching.

Statement (C) is false, because the two forces are on two different objects.

So just (A) is true.

Why the other options fail

(B) is false. Gravity pulls a lifted body without touching it, so a pair of forces does not need contact.

(C) is false. Your push is on the wall and the push back is on you, two different objects.

(A) is true by the third law, and (B) and (C) are false. This option has them the wrong way round.

(B) and (C) are false, so the three are not true together. Gravity needs no contact, and the two forces are on two objects.

Conclusion

For a pair of action-reaction forces, (A) is true. (B) and (C) are false.

The answer is (1).

The two forces of an action-reaction pair are equal and opposite, and they act on two different objects.

They do not need contact, so gravity and magnets also make pairs.

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