projectile motion.. (2 Viewers)

[shantu1992]

hey guys ive liek totally forgotten how to do these
can do it the maths why but for some reason it doesnt work the physics wayy ayy..

can someone please do this question both ways, or if you dont have time or something preferably the physics way pleasee... that would be mu ch appreciated

anyways enough talking and heres the question..


A ball is projected vertically upwards with a velocity of 24ms-1 from a position 50 m high. How long will it be before it reaches the ground below and what will its veloicty be just before it reaches the ground..

thanks heapps

 

[undalay]

hey guys ive liek totally forgotten how to do these
can do it the maths why but for some reason it doesnt work the physics wayy ayy..

can someone please do this question both ways, or if you dont have time or something preferably the physics way pleasee... that would be mu ch appreciated

anyways enough talking and heres the question..


A ball is projected vertically upwards with a velocity of 24ms-1 from a position 50 m high. How long will it be before it reaches the ground below and what will its veloicty be just before it reaches the ground..

thanks heapps

u(y) = 0

s(y) = ut + 1/2 at^2

s = 24t + 4.8t^2

s is the vertical displacement from its initial location.
If you just sub in s=50 here, that will find the time for it to reach 100m.
To find the time it takes to reach the ground, you need to sub in s = -50
I would do the rest but I take that's the part that's giving you trouble.

 

[khorne]

Shouldn't you use a negative value for the acceleration (i.e -g) as gravity is acting against the initial movement of 24m/s (assumed in the positive direction)...

Sorry if this is wrong, I was just under the impression it seemed correct..

 

[Drongoski]

Shouldn't you use a negative value for the acceleration (i.e -g) as gravity is acting against the initial movement of 24m/s (assumed in the positive direction)...

Sorry if this is wrong, I was just under the impression it seemed correct..

I think u are right; if upwards is +ve, then a (gravity g) is -ve.