Question
A 0.26 kg rock is thrown vertically upward from the top of a cliff that is 27 m high. When it hits the ground at the base of the cliff the rock has a speed of 24 m/s.
(a) Assuming that air resistance can be ignored, find the initial speed of the rock.
(b) Find the greatest height of the rock as measured from the base of the cliff.
I know I have to use projectile motion somehow. I don't know how to without knowing the initial velocity. I can't find that without solving the first part. I'm having trouble picking an equation. Please help.
(a) Assuming that air resistance can be ignored, find the initial speed of the rock.
(b) Find the greatest height of the rock as measured from the base of the cliff.
I know I have to use projectile motion somehow. I don't know how to without knowing the initial velocity. I can't find that without solving the first part. I'm having trouble picking an equation. Please help.
Answers
bobpursley
Initial KE+ initialPE=final KE
I suspect you can solve it from that.
Greatest height? Final KE= greatest PE
solve for greatest height.
I suspect you can solve it from that.
Greatest height? Final KE= greatest PE
solve for greatest height.
KE initial = 0
PE initial = mgh = 68.8662 = KE final
KE final = 1/2mv2
v = 23.01
That isn't right. What have I done wrong?
PE initial = mgh = 68.8662 = KE final
KE final = 1/2mv2
v = 23.01
That isn't right. What have I done wrong?
bobpursley
Where did you get initial KE is zero? That is what you are solving for. You need a tutor, pronto.
I am just so confused by this problem. I have done other problems with ease and I'm having so much trouble with other ones.
you do:
1/2mv(initial)^2+mgh=1/2mv(final)^2
so simplify that to : v(inital)= sqrt(v(final)^2-2gh)
For b): 1/2mv(initial)^2=mgh
solve for h : h=v^2/2g
then add your original height to the new height for your answer
1/2mv(initial)^2+mgh=1/2mv(final)^2
so simplify that to : v(inital)= sqrt(v(final)^2-2gh)
For b): 1/2mv(initial)^2=mgh
solve for h : h=v^2/2g
then add your original height to the new height for your answer