AS Level maths mechanics question

"In this question use g=9.81. A small stone is projected vertically upwards from the top of a cliff, with speed ums^-1. It hits the ground with a speed of 18.2ms^-2. A second small stone is also projected vertically upwards with speed ums^-2 from a nearby cliff which is 4.5m higher. It takes 2.97s to hit the ground. Find the value of u and the height of the first cliff."

Ok so I'm absolutely clueless as to what to do here. I know that you have to use SUVAT, and that maybe there's a simultaneous equation involved but I'm very stuck. Can anyone help?

Reply 1

{ }

5

with speed ums^-2? you mean u ms^-1. Maybe draw a sketch of the scenario and write SUVAT for each scenario, a SUVAT for the normal cliff and a SUVAT for the higher cliff. Then solve simultaneously. With this, you should get:

SUVAT for shorter cliff:

S = -H (I'm calling upwards positive, the net displacement travelled is -H, and H being the height of the cliff)
U = u (projection speed)
V = -18.2
A = -9.81
T = ? (you don't really need this)

SUVAT for higher cliff:

S = -4.5-H
U= u
V = ?? (don't need this
A = -9.81
T = 2.97

And solve simultaneously to find u and H

(edited 4 years ago)

Reply 2

cbawtusername

OP

6

Ok, which SUVAT equation would you use? Because none of the equations contain the same given values.

Original post by { }

with speed ums^-2? you mean u ms^-1. Maybe draw a sketch of the scenario and write SUVAT for each scenario, a SUVAT for the normal cliff and a SUVAT for the higher cliff. Then solve simultaneously. With this, you should get:

SUVAT for shorter cliff:

S = -H (I'm calling upwards positive, the net displacement travelled is -H, and H being the height of the cliff)
U = u (projection speed)
V = -18.2
A = -9.81
T = ? (you don't really need this)

SUVAT for higher cliff:

S = -4.5-H
U= u
V = ?? (don't need this
A = -9.81
T = 2.97

And solve simultaneously to find u and H