Newton's Third Law Physics Example 2

Follow the full solution, then compare it with the other examples linked below.

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70 \text{ kg}

skater pushes a

50 \text{ kg}

skater with a force of

100 \text{ N}

. What is the acceleration of each skater?

1
By Newton's third law, the $50 \text{ kg}$ skater pushes back on the $70 \text{ kg}$ skater with $100 \text{ N}$ .
2
Acceleration of the $50 \text{ kg}$ skater: $a_1 = \frac{100}{50} = 2 \text{ m/s}^2$
3
Acceleration of the $70 \text{ kg}$ skater: $a_2 = \frac{100}{70} \approx 1.43 \text{ m/s}^2$

Answer

a_{50} = 2 \text{ m/s}^2, \quad a_{70} \approx 1.43 \text{ m/s}^2

The forces are equal and opposite (third law), but the accelerations differ because the skaters have different masses (second law).

About Newton's Third Law

For every action force, there is an equal in magnitude and opposite in direction reaction force.

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