Forces and Motion | Energy and Thermal Physics

Momentum depends on mass and velocity

Teaching Guidance for 14-16 Supporting Physics Teaching

Both mass and velocity

Wrong Track: This ball has the larger mass, so it'll have the larger momentum.

Right Lines: Momentum, a quantity that's a good measure of the motion of a body, depends on mass and on velocity. To work out which has the larger momentum you have to pay attention to both the mass and the velocity: p = m × v.

A focus on compensation

Thinking about the learning

Students often fix on only one factor, when they should consider both factors that contribute to a physical quantity. They select either mass or velocity, and they fail to combine the two quantities into the compound quantity that is momentum. So their comparisons are often in error.

Thinking about the teaching

The key relationship is: p = m × v.

That this kind of pattern (A = B × C) turns up so often, and so often causes troubles is one reason for focusing on the issue of compensation throughout the SPT materials. Because it's an often used pattern, and because students are known to have difficulty with it (through selecting just one of the factors) we'd suggest foregrounding it as a pattern, so that students give it due prominence.

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