Kinematics - describing motion

Kinematics - describing motion

1. speed

   • you can calculate the avrage speed of something moving if you know the distance it moves and the time it takes. For this you use the equation, average speed = distance / time. In symbols this is written as V = X / t
   • If the object is moving at a constant speed this equation will give us the speed during the time taken. If the speed is changing then the equation gives us average speed.
   • If you look at the speedometer in a car, it doesnt tell you the average speed. It tells you the insantaneous speed, this is the speed at the instant you look at it.

2. units

   • Systéme Internationale d'Unités (SI system), distance is measured in metres (m) and time in seconds (s). Therefore, speed is in metres per second ms-1 (or as m/s)

3. laboratory measurments of speed

   • two light gates
     • use a trolley with a section of card attached to it and accelerate it forward, the edge of the card breaks the first light beam which starts the timer. when the second beam is broken the thimer stops. from this the computer works out the speed from the time interval and the distance between the two gates
   • one light gate
     • similar to using two light gates but insead of the dictance between two light gates, the length of the csrd is used. The computer uses the time taken from the beam being broken to the beam rejoining and the length of the card to work out the speed
   • ticker timer
     • a ticker timer marks dots on a tape at regular intervals. Attach the tape to the trolley and push it. The spacing of the dots will indercate the movement: even spaces show constant speed and increasing spaces show increasing speed. Measure the distance of every fith dot from the start, this will give you the trolleys distance at intervals of 0.1 seconds
   • motion sensor
     • A motion sensor transmits regular pulses of ultrasound at a moving trolley, the computor then records the time it took for the waves to reach the tralley and get back. From this the computor can work out the speed at which the trolley was travelling
   • which is the best method?
     • Does the method give and average speed or can it be used to give the speed at different points?
     • how percisely does the method measure time?
     • how simple and convenient is the method to set up in the lab?

4. distance and displacement, scalar and vector

   • distance
     • how far that was actually travelled
   • displacement
     • distance moved in a particular direction
   • scalar
     • has just magnitude e.g. distance
   • vector
     • has both magnitude and direction e.g. displacement

5. speed and velocity

   • Its important to know the speed of an object and its direction, these two quantities can be combined to form another quantity called velocity
   • Velocity can be thought of as speed in a particular direction, this makes it a vector quantity.
   • since velocity is a vector quantity it is defined in terms of displacement so the equation is: velocity = change in displacement / time taken
   • the definition of velocity is: the rate of change of an objets displacement