P1. Motion
P1.0 – Motion
1. Define speed and calculate average speed from: total distance / total time.
Speed is the rate at which something moves. It is calculated by dividing the distance by the time.
Therefore to calculate the average speed you take the total of both values (time and distance) and use
the same formula. You then get the formula, total distance / total time.
2. Distinguish between speed and velocity.
Imagine a race. If you measured speed and not velocity the runner could be running backwards and
according to you, he is in the first place. With velocity, you have speed and direction so you can tell at
what speed he is travelling toward the finish line. velocity contains direction and magnitude so it is a
vector quantity, speed only contains magnitude so it is a scalar quantity.
3. Plot and interpret a speed time graph and a distance-time graph.
In maths you should’ve learnt how to plot a graph, this is where the question, “where will I actually
use this?”, is answered. On the x – axis we put time and speed on the y – axis. We receive marks for the
accuracy of our points and our line. Here is an example:
With this graph we can tell three things; its speed, the distance it has travelled and its acceleration
(the rate at which the speed changes). You can read the speed directly off of the graph. You can find
out the distance by calculating the area under the graph. The acceleration is found by calculating the
gradient of the line.
With a distance-time graph you can find out the distance by reading it off of the graph, and speed can
be found by calculating the gradient of the line.
4. Recognise from the shape of a speed-time graph when a body is:• at rest
• moving with constant speed
• moving with changing speed.
When a speed time graph is at rest the line meets the x – axis. It moves with a constant speed when
the line is parallel to the x – axis. If the line slopes, it moves with a changing speed.
5. Recognise linear motion for which the acceleration is constant and calculate the
acceleration.
Linear motion is motion across a straight line. on a speed time graph, you can calculate the
acceleration by calculating the gradient of the line. Acceleration on a speed time graph is constant
when the line is straight. The equation for acceleration is:
Change in speed/change in time.
or
(speed 2 – speed 1)/(time 2 – time 1)
They are both the same equation but the second one is calculated using solid values, it is the same
formula for the gradient.
6. Recognise motion for which the acceleration is not constant.
If on the speed time graph the line is bent or curved it does not have a constant acceleration, because
the gradient of the line changes.
7. Calculate the area under a speed-time graph to work out the distance travelled for
motion with constant acceleration.
This point (discluding the maths) explains itself and was explained in earlier points.
8. Demonstrate a qualitative understanding that acceleration is related to changing speed.
Earlier we have discussed two formulas to calculate acceleration:
Change in speed/change in time.
and
(speed 2 – speed 1)/(time 2 – time 1)
Both of these formulas contain speed as a variable, therefore, we can deduce that acceleration changes
when speed changes. From this, we can further deduce the change in acceleration is directly
proportional to the change speed. After all, acceleration is the rate at which speed changes.