M1 q.

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Original post by interlanken-fall

then why in the
b)The speed with which the balls hits the ground
this means the balls falls down so the gravity should be =9.8 but they use -9.8? why is that?

I've just explained that!

"Upwards" is positive so gravity, which always acts downwards. is negative.

You seem to be seriously over-thinking this :smile:

Let's try again.

Forget about the ball completely.

Suppose you have a set of (x, y) axes and y is the vertical axis. If the positive y axis is upwards, gravity is always acting downwards so it has a negative sign. Now suppose you start throwing balls or bricks or particles around. It doesn't matter whether these balls, bricks, particles or whatever are going up, down or sideways, gravity is still acting downwards relative to the y-axis so it has a negative sign.

Reply 21

interlanken-fall

Original post by davros

No!

They choose upwards as positive so acceleration (due to gravity) is always negative.

If you choose downwards as positive, then acceleration is positive.

The sign attached to the force is dependent on your choice of absolute direction, it doesn't flip over just because the ball is moving in a different direction - gravity is always acting in the same direction throughout the motion :smile:

and since we take up as positive and were trying to find the speed it gets once its hits the ground
it should be
s=-4
u= -18 (going down)
v=?
a= 9.8?
t

Reply 22

mqb2766

Original post by interlanken-fall

and since we take up as positive and were trying to find the speed it gets once its hits the ground
it should be
s=-4
u= -18 (going down)
v=?
a= 9.8?
t

Those values are not consistent. If its positive down, then s=4.

Note you;ve not sketched and uploaded both the parabolic trajectory and the basic diagram with your chosen positive direction. It really helps to do it.

Reply 23

davros

Original post by interlanken-fall

and since we take up as positive and were trying to find the speed it gets once its hits the ground
it should be
s=-4
u= -18 (going down)
v=?
a= 9.8?
t

why have you written a = 9.8? We've agreed that upwards is positive and we know that gravity acts downwards, so we know that a = -9.8.

Original post by interlanken-fall

Is that the position verses time?
Upwards positive? If so, acceleration of -9.8 is 9.8 downwards.
4 downwards displacement from the start point would be s=-4.

(edited 2 years ago)

Reply 26

interlanken-fall

Original post by mqb2766

Is that the position verses time?
Upwards positive? If so, acceleration of -9.8 is 9.8 downwards.
4 downwards displacement from the start point would be s=-4.