revising (1 Viewer)

[shantu1992]

was just revising and i dno ive just gone blank on two questions on projectile motion..

3. a ball is projected vetrically upwards with a velocity of 24.5ms-1 from a position 50m high. How long will it be before it reaches the ground below and what will be its velocity just before it hits the ground?

9. a stone is dropped from a cliff .. wait TALL cliff and 1 second later another stone is projected vertically downwards with a velocity of 6 ms-1. if the ball raeches the sea in 8 seconds, what is the height of the cliff?

if anyone could answer any of these it would be great
thanks heapps

 

[JasonNg1025]

3) Okk what we do is choose projection point as the origin, and this is easier because it's only got one dimension. What we're told to solve is the t and v at s = -50 then. So we use -50 = ut - 0.5at², and solving for t we get t = 6.56 or -30.5. Since the origin is 50m above the ground, we would expect one negative time as the ball has already been lifted up to 50m (the "past" time is when the ball was still sitting at our -50m altitude). So we ignore that and get 6.56s.

This bit, I'm not toooo sure. But I think we use v = u + at, so you get -39.8 which makes sense because we chose upwards direction as positive, and it hits the ground in a downwards direction.

9) There's 2 stones and a ball.. which one hits the sea in 8 seconds?