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# Calling all A-Level Physicists!!! watch

1. I've been having trouble on these questions and I can't seem to work out the answers, I appreciate the help you can give me.
6. A ball is thrown vertically upwards with speed 28m/s from a point which is 1 metre above ground level. Find (a) the speed the ball will have when it returns to the level from it was projected, (b) the height above ground level of the highest point reached.
7. A ball is thrown vertically upwards from a point A, with initial speed of 21m/s, and is later caught again at A. Find the length of time for which the ball was in the air.
8. A ball is kicked vertically upwards from ground level with an initial speed of 14m/s. Find the height above ground level of the highest point reached and the total time for which the ball is in the air.

Acceleration due to gravity= 9.81m/s^2
2. It's just a case of applying the suvat equations
3. How would you describe s(displacement)?
4. By 'describe', do you mean what actually is it or how would it be defined mathematically?
5. Imagine a set of graph axes, x and y.

If you started at (0,0) - the origin - and moved 4 spaces right on the x-axis, to (4,0), your displacement would be 4. i.e. you have moved 4 units from your starting point.

Let's go back to (0,0). If you moved upwards instead, on the y-axis, 4 spaces, to (0,4), your displacement is also 4, because you've moved 4 units from where you started, even if it was in a different direction.

If you now move back to the origin from your spot on the y-axis, so you move backward 4 spaces from (0,4) to (0,0) then your displacement is back to 0 - you are back in the place you started and hence you are not DISPLACED from where you started. However, you did move up 4 and back down 4, so you've actually travelled a "distance" of 4+4 = 8 units, but overall you haven't moved. Resultant displacement is 0.

If as part of this movement you finally decided to move down to -1 on the y-axis, at (0, -1) then your final displacement is one unit from the centre, even though overall you've travelled 9 units.

Similarly, if you started at the origin and moved 3 right, then 4 up, you'd be 5 units overall displacement from the origin because if you drew a line from your final position (3,4) to the origin, it'd be 5 units long; the hypotenuse of a triangle of base 3 height 4, pythagoras giving

h^2 = 3^2 + 4^2
h^2 = 25
hypotenuse = 5

Displacement is how far you've travelled relative to a starting point at a given time.
6. (Original post by Thorsas)
Imagine a set of graph axes, x and y.

If you started at (0,0) - the origin - and moved 4 spaces right on the x-axis, to (4,0), your displacement would be 4. i.e. you have moved 4 units from your starting point.

Let's go back to (0,0). If you moved upwards instead, on the y-axis, 4 spaces, to (0,4), your displacement is also 4, because you've moved 4 units from where you started, even if it was in a different direction.

If you now move back to the origin from your spot on the y-axis, so you move backward 4 spaces from (0,4) to (0,0) then your displacement is back to 0 - you are back in the place you started and hence you are not DISPLACED from where you started. However, you did move up 4 and back down 4, so you've actually travelled a "distance" of 4+4 = 8 units, but overall you haven't moved. Resultant displacement is 0.

If as part of this movement you finally decided to move down to -1 on the y-axis, at (0, -1) then your final displacement is one unit from the centre, even though overall you've travelled 9 units.

Similarly, if you started at the origin and moved 3 right, then 4 up, you'd be 5 units overall displacement from the origin because if you drew a line from your final position (3,4) to the origin, it'd be 5 units long; the hypotenuse of a triangle of base 3 height 4, pythagoras giving

h^2 = 3^2 + 4^2
h^2 = 25
hypotenuse = 5

Displacement is how far you've travelled relative to a starting point at a given time.
Thank you very much, my teacher didn't really explain this to us but now I understand it thanks

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Updated: September 23, 2014
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