OCR A Level Physics

Revision Notes

3.2.2 Investigating Motion & Collisions

Investigating Motion & Collisions

  • There are a number of techniques and procedures that can be used to investigate the motion of objects
  • A few examples of experiments that could be carried out are:
    • Measurement of speed and acceleration of trolleys down a ramp
    • Collisions between trolleys on an air track to investigate momentum and energy changes
    • Determination of acceleration due to gravity, g
    • The terminal velocity of a falling object
  • Typically, these experiments require the use of one or more of the following:
    • Trolleys
    • Air-Track Gliders
    • Ticker Timers
    • Light Gates
    • Data Loggers
    • Video Techniques

Trolleys

  • Trolleys are essential when studying motion and collisions
  • They can be used to investigate speed, acceleration, and momentum
  • They are composed of
    • A light block of wood or plastic
    • Ball-bearing wheels (to reduce friction)
    • A spring-loaded plunger (for collisions)
    • A flat top surface to allow stacking or additional masses to be added

Air-Track Gliders

  • This technique can also be used to investigate the conservation of momentum resulting from a collision
  • Air tracks are the optimum equipment to use for this as they reduce the friction
  • Hence the energy lost to overcoming friction is minimised and the collision is kept as elastic as possible

Ticker Timers

  • Another way of analysing motion in a physics lab is to use ticker tape
  • A long tape is attached to a moving trolley and threaded through a device that places a tick upon the tape at regular intervals of time
  • The ticker timer will produce a certain number of dots per second on the tape, which will travel at the same speed as the trolley
  • The distance between dots and the time can then be used to determine the velocity

Light Gates

  • Light gates provide the most accurate way of measuring the time taken for a trolley to move through a set distance
  • A card is attached to the top of the trolley as this is will interrupt the light beams on the gates
  • The trolley is released from the top of the ramp, with one light gate just in front of the release point and the other at the bottom of the ramp
  • The time taken to travel between the light gates, t, can be used to work out the initial speed, u, and final speed, v
  • The acceleration, a, can then be calculated using the equation:

v = u + at

Data Loggers

  • A data logger is an electronic device that records data and stores it for further analysis
  • It can be set to record at regular time intervals or when triggered by a sensor, such as a light gate
  • Data loggers are often used as a more accurate way of measuring time and eliminate the error from the human reflex speed needed to stop and start a stopwatch

Video Techniques

  • Taking videos or successive photographs of objects in motion is a useful method for determining
    • The acceleration of freefall
    • Projectile motion
    • Terminal velocity
  • This technique has two requirements:
    • The frames per second must be known, as this can be used to determine the time taken
    • The distance must be known, usually from placing a ruler in the shot with the object
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