Forces and Motion | Energy and Thermal Physics

Changing momentum

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Changing direction of motion needs a change in momentum.

Things that are already moving require a force acting on them for a certain duration. You can't change the momentum without the accumulation of the action of force over time.

Here students change the motion of rolling balls of different masses and velocities by applying impulses.

What to Prepare

  • a straw per student, to blow through
  • three balls of varying masses, ideally all of the same size (For example, marble, ball bearing, plastic ball.)
  • a marked ramp to ensure that the balls are launched at a repeatable velocity
  • a series of routes marked out on paper, perhaps with momentum arrows

Safety note: Students should not swap straws.

What Happens During this Activity

The ball is launched so that it heads out along the desired inbound path. Students are to direct it to along the outbound route.

Discussions should focus around the momentum of the inbound ball and the impulse that has to be applied so that the momentum alters by the desired amount. Sets of three challenges work well, and students could profitably work so as to explore these systematically.

Note that the impulse is a compensated quantity, and students should be encouraged to explore varying both the force and the duration.

appears in the relation p=mv F=dp/dt λ=h/p ΔxΔp>ℏ/2
has the special case Angular Momentum
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