Projectile motion (1 Viewer)

[denoz]

Could someone give me a hand with these questions (from fitzpatrick):

17. A stone is thrown so that it will hit a bird at the top of a pole. However at the instant the stone is thrown, the bird flies away in a horizontal straight line at a speed of 10m/s. The stone reaches a height double that of the pole and, in its descent, touches the bird. Find the horizontal component of the velocity of the stone. (12.1m/s)

24. Find the speed and direction of a particle which, when projected from a point 15m above the horizontal ground, just clears the top of a wall 26.25m high and 30m away. (25m/s and 36 degrees 52 minutes)

Question 24 just seems as though its missing a value or something (obviously not) and 17 I dont really have much of an idea.

So some guidance or working would be appreciated.

 

[tichondrius]

Was just doing this exact question. I need help too, I've gotten two answers after solving a quadratric and one is REALLY close.

My two answers are 11.715 and 68.28 - The books answer is 12.1 - Got them from solving a quadratic

would also appreciate some help on this.

 

[denoz]

Anyone? I'm sure a lot of you have done these questions before.

 

[malady]

24. V(y)= 0 when x=30 and t = VsinA/10
which gives 300 = V*V sinA cosA ......(1)

Also subst y = 26.25 and t=VsinA/10 into equation y= -5t*t + VtsinA +15
have finally 225 = V*V sinA*sinA ..... (2)

from (1) and (2) tan A = 225/300
and so sin A = 3/5
The results follow


17. let equation be y= -x(x-a). When y = 2h, x=a/2 ==> a= sqrt(8h)
solve h= -x(x-a), so dist bird flies = sqrt(4h) and horiz dist stone travels = sqrt(2h) + sqrt(h)

therefore sqrt(4h)/10 = horiz. dist stone travels/V(x)
so V(x) = 10 (sqrt(2h) + sqrt(h))/sqrt(4h)
= 5(sqrt(2) +1)