Projectile Problems -M1

Two types I can't work out, please can you show me how to do them.
1.Azmat throws a ball to susan who is 80m away and who catches it at the same height as it was thrown. The ball is in the air for 5 seconds, find the initial speed and the angle the ball was thrown.
2.tim is standing 12m away from the net on a tennis court, the net is 1m high he hits the tennis ball horizontally with speed U m/s from a height of 1.75m
given that the ball just clears the top of the net find the time taken for the ball to reach the net
thank-you

Reply 1

Plummie

if you wouldn't mind, it would be very helpful i have so far
1. 80= 5Ucosx
5Usinx - 122.5
v= 5Ucosx
5Usinx - 49
after that I go really wrong
not sure how to start with 2

Reply 2

Plummie

thank-you for trying but i'm afraid your working doesn't help me much as the equations you have used aren't the ones which we need to know for the exam, also I'm afraid your answer doesn't match the one in the book this is probably because you worked out v when i need to find u and i know i'm supposed to find it by some combination of what i typed up earlier on but i can't figure out what to do from that starting point.

Reply 3

Plummie

thank-you once again for your help, though this time the same problems occur ie. different equations to one i need to use/different method to one i need to use. though your answer was correct. Thank-you all the same for your efforts.

Reply 4

silent ninja

using the equations of motion

(1)
horizontally,
s=80 t=5 a=0

s= ut + 0.5at²
80= 5u + 0 => u= 16

vertically,
s=0 (since the vertical displacement is zero) a= -9.8 t=5

s= ut + 0.5at²
0 = 5u-4.9*25 => u= 24.5

So now you have the two components you can work out the speed (pythagoras) and angle using basic trig/vectors.

Reply 5

silent ninja

(2)

vertically, distance travelled to net is 0.75m since this is how far the ball falls. Then
s=0.75 a=9.8 (i am taking down as positive) u=0 (because you are told it is hit horizontally i.e. there is no vertical component of velocity)

s= ut + 0.5at²
0.75= 4.9t² => t= 0.391 s

Reply 6

Plummie

sadly the answer is actually u=29.3, rather annoying as you both got 24.5, thankyou for the second answer though, very helpful my issue was in fact not reading the question correctly and hence messing about with sine's and cosines' bluergh, not fun