# Lightning, struck me down

1. Apr 5, 2005

### Sidearm

i have a physics assignment and cant do a few questions, normally i would not resort to asking, but i am really stuck

1. 2 lightning strikes occur simultaneously 10km apart if an observer was 2km from one and 8km from another
a) find the time delay between the first flash and the second reaching him
b) the time delay between the sound of the closer strike and the distant strike reaching him,

2. if i dropped a 40kg rock over the edge of a 60m high cliff on the moon, where g = 1.64ms -1
a) how long would the rock take to reach the ground
b) what would the rocks weight be

Any help would be greatly appreciated, also if you could mention how to do this for future reference that would be great

2. Apr 5, 2005

### whozum

1a) Do you know the speed of light? Its 3 x 10^8m/s
With this speed, and the distances, 8km, and 2km, find the time it takes to get him.

The equation for this is $t = \frac{d}{v}$

The time delay between two events is $t_2-t_1$

Now do the same with sound, sound has a speed of 340m/s, One million times less than the speed of light.

The numbers for a) will be pretty small, the numbers of b) will be a few seconds each.

2. You know the position equation for an object in free fall is:

$$x(t)-x_0 = v_0t + \frac{gt^2}{2}$$

Since there was no initial position (x-x_0) or initial velocity (v_0), the equation simplifies to:

$$h = \frac{gt^2}{2}$$

You can solve this for t by rearranging and taking the square root, from there a) should be fairly easy.

Weight is a measure of the force of an object due to gravity, what is the equation for force? You are given a mass, and an acceleration. Find the force.

3. Apr 6, 2005