Explosions and recoil revision notes

Edexcel International GCSE Physics (4PH1), chapter 4: Momentum, specification point 1.27P, 1.29P. Free lesson.

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Explosions and recoil change how objects move, but total momentum is conserved. If parts start at rest, they move apart with equal and opposite momentum, so the lighter part has the greater speed.

Why momentum is conserved in an explosion

An explosion is like a collision run backwards: parts that were together are pushed apart. If everything starts at rest, the total momentum before the explosion is zero. The parts then gain equal and opposite momentum, so their momenta still add to zero afterwards. The explosion exerts equal and opposite forces on the parts. This is Newton’s third law. The momenta can be written as +p and −p. They add to zero, so the total momentum stays unchanged.

How to calculate recoil velocity

When an object fires something forwards, it recoils in the opposite direction. Use conservation of momentum: the total momentum before firing equals the total momentum afterwards. A negative answer means the object moves opposite to the chosen positive direction.

Why the lighter part moves faster

Parts pushed apart have equal-sized momentum in opposite directions. Since momentum depends on both mass and velocity, the smaller mass must have the greater speed to have the same momentum. Equal forces do not mean equal speeds. For the skaters, A’s momentum is 40 × 3 = +120 kg m/s. B’s momentum is 60 × (−2) = −120 kg m/s. The momenta are equal in size and opposite in direction. The lighter skater, A, moves faster.

How a rocket moves in space

A rocket does not need air to push against. Its engine pushes exhaust gas backwards at high speed. By Newton’s third law, the gas pushes the rocket forwards with an equal and opposite force. The gas gains momentum backwards, so the rocket gains equal momentum forwards and the total momentum stays the same. For a three-mark explanation, state the force on the gas, the equal and opposite force on the rocket, and the momentum link.

Applying momentum to a thrown tool

An astronaut and a tool start at rest, so their total momentum is zero. When she throws the tool, their momenta are equal in size and opposite in direction. The astronaut’s momentum gives the tool’s momentum, which can then be used to find its mass.