How far should the boat be from the bridge (horizontally) before Bond drops the explosive?

Question

Bond needs to drop a grenade from a 35.0 m high bridge on to a speedboat heading downriver at a constant speed of 32.0 m/s.

How far should the boat be from the bridge (horizontally) before Bond drops the explosive?

and i think the answer is 86.4 meters... but i have no idea how to get it! HELP PLEASE!!

Answer 1

The grenade is falling with a constant acceleration which is 9.81=g. Itsequation is x=x0+v0t+at^2/2 (where x0, v0 are "index zero")
Now, Bond drops the grenade so its initial velocity v0 is =0 (he doesn't throw it towards the water)
If you consider the water level as origin for space, that is 0 then x0 would be +35 m and the eq for the origin point goes
0=35 - 9.81*t^2/2 which gives us t=(70/9.81)^1/2
Thats the time needed for the grenade to drop to the water level.
Now, lets calculate the distance for the boat.
It goes with constant velocity so the eq is x=x0+v0t.
We'll consider this time the origin x0=0 where the boat leaves and we'll calculate the distance x to the bomb drop.
We know the velocity v0=32 m/s and the time t=see above
so you calculate x which is 0+32*(70/9.81)^1/2=85.47999629 m

Answer 2

Ok I'll try
we have to understand that the time at which the bomb will get to the surface has to be equal to the boat will get under the bridge from the distance we are to find First let us find this time
from the equation s=1/2*gt^2(If we consider Bond just simply drops the bomb instead of throwing it and giving it some initial velocity)
.: 35=0.5*9.8*t^2 which gives t=2.673 seconds
Now for the boats motion As it is moving with constant speed ie there is no acceleration so we can simply say that Distance=speed*time which gives us the requisite distance d=93.541m
Sorry but my answer doesn't match with yours

Answer 3

First, we need to find the time taken for the bomb to reach the river.
Using "s=ut+1/2 at^2", 35.0=1/2 gt^2
t = sqrt(70.0/g) = 2.67s
Therefore, the boat has to be of a distance of 32.0t away from the bridge when the bomb is dropped.
Required distance = 32.0(2.67) = 85.5m

Anyway, the value of g used here is 9.81m/s2. If you are using 10m/s2, you'll get 84.7m. I'm not sure what value of g you used, giving you the answer of 86.4m...