# Homework Help: Simple Kinematics Question

1. Sep 22, 2006

### siddharthmishra19

A stone (taken as a point mass) is thrown from a cliff of height H = 80m perpendicular to the ground. It falls L = 60m away from the cliff. What is the initial velocity V with which the stone is thrown?

Obviously we have to deduce the formula for velocity using kinematic equations w/ respect to H and L. Help will be very appreciated. Thank you.

2. Sep 22, 2006

Think of the equations of motion for both the x and y direction. In which direction is the velocity constant? IN which direction does gravity act?
Btw, the angle of the throw is not given?

3. Sep 22, 2006

### siddharthmishra19

g acts in y direction. velocity is constand is x direction. angle of throw, as mentioned, is perpendicular to the ground, and sincs cos90 = 1 it can be neglected.

i need help solving it.

4. Sep 22, 2006

### nrqed

Just list what you know
$a_x = 0, a_y = -9.80 m/s^2, x_i=y_i=0, x_f=60 m, y_f =-80 m, v_{yi}=0$ and what you are looking for is $v_x$.

Just write the equations along x and along y that you will need. Notice that you will have to find the time as well. So the idea is to find the time using the y equation and then use that in the equation along x to find the velocity.

5. Sep 22, 2006

### siddharthmishra19

thanks, all done

6. Sep 22, 2006

### siddharthmishra19

In the same problem, what is the time taken by the body to attain a velocity twice its initial velocity?

What is the displacement of the body at that time?