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Further Mechanics

Investigating momentum and analysing collisions on video

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Overview

Recall questions on the core practical linking force to change of momentum, the graph and its gradient, the systematic error, and video analysis of colliding spheres.

What you need to know

Answer each one before you reveal it.

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Evaluation

Why is momentum still conserved in a real collision between ball bearings, and what systematic error affects the practical?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    In the core practical linking force to change of momentum, how is a steady accelerating force applied to a trolley?

    Model answer

    A hanging mass on a string passing over a pulley at the end of the runway provides a constant pulling force on the trolley.

  2. 2

    In that practical, how is the trolley's change of momentum measured?

    Model answer

    Its velocity is measured using light gates with an interrupt card, or a motion sensor, and the change of momentum is the trolley's mass multiplied by its change of velocity.

  3. 3

    In the core practical linking force to change of momentum, what graph would you produce, and what should its gradient equal?

    Model answer

    Plotting change of momentum against the time for which the force acted should give a straight line through the origin whose gradient is the accelerating force.

  4. 4

    In the core practical linking force to change of momentum, what systematic error affects the results, and how can it be reduced?

    Model answer

    Friction in the wheels and pulley makes the measured momentum change too small; it is reduced by tilting the runway slightly so the trolley moves at constant velocity when given a gentle push.

  5. 5

    How is ICT used to analyse a collision between small spheres such as ball bearings on a table top?

    Model answer

    The collision is filmed at a high frame rate and video-analysis software tracks the position of each sphere frame by frame, so velocity components before and after can be calculated from the known time between frames.

  6. 6

    Why must a metre rule or similar scale appear in the video frame during that analysis?

    Model answer

    It calibrates the recording, allowing distances measured on screen in pixels to be converted into real distances in the plane of motion.

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