some confirming plz

[Ice-cream]

here's a question i want some confirming on:

 

Alice, Bob, Charlie and Daniel are standing on the edge of a canyon which is 100m deep and 20m wide. On the opposite side of the canyon, there is a 10m high cave, 15m below the top of the cliff. They want to see who can throw a rock horizontally and get it into the cave. They each throw a rock with the following initial velocites: Alice 5m/s, Bob 10m/s, Charlie 15m/s and Daniel 20m/s. Whose rock makes it into the cave?

 

I got an initial velocity of 5.12 m/s so therefore alice's rock will make it. Did any1 else get this answer as well? if not, can you tell me the answer you got instead? thanx

[J.C.MacSwell]

here's a question i want some confirming on:

 

Alice' date=' Bob, Charlie and Daniel are standing on the edge of a canyon which is 100m deep and 20m wide. On the opposite side of the canyon, there is a 10m high cave, 15m below the top of the cliff. They want to see who can throw a rock horizontally and get it into the cave. They each throw a rock with the following initial velocites: Alice 5m/s, Bob 10m/s, Charlie 15m/s and Daniel 20m/s. Whose rock makes it into the cave?

 

I got an initial velocity of 5.12 m/s so therefore alice's rock will make it. Did any1 else get this answer as well? if not, can you tell me the answer you got instead? thanx[/quote']

 

20 metres wide. Alice's rock will cross it in 4 seconds. (ignoring aerodynamics) In that time it will develop a vertical component of 39.2 m/s but will have averaged half of that, 19.6 m/s, for the 4 seconds.

It will have dropped 78.4 metres.

 

I'm not sure if the top, bottom or midheight of the cave is at 15 m from the top but Alice's rock won't make it in.

[swansont]

The stones will drop s= 1/2 gt² in a time t = d/v, or thrown at v=d/t

 

You want s to be between 15 m and 5 m, so 1.01s > t > 1.75s

 

19.8 m/s > v > 11.4 m/s

 

Charlie makes it, Daniel just misses high and Bob a little low. Alice is way off - as JC notes, 78.4 m, so her throw almost hits bottom instead of hitting the other side. Sexism? "Throws like a girl?"

[Ice-cream]

ahh i c wat i did wrong...i took the height from the bottom to top, instead of top to bottom lolz. thanx guys.