Kinematics HMWK Help: Gr 12 Physics Projectile Motion

[humaders99]

Kinematics HMWK help!

hi, I am in gr 12 physics and i need help in a kinematics question about Projectile motion... here is the question:

"a medievil prince is trapped in a castle wraps a message around a rock and throws it from the top of the castlewall with an initial velocity of 12 m/s [42 degrees above the horizontal]. The rock lands just on the far side of the casltes moat, at a level of 9.5 m below the initial level. Determine the (a) time of flight (b) width of moat and (c) velocity at impact"

i really can't figure this out and i need help ASAP

 

[mbrmbrg]

Hey, welcome aboard!
What work have you done on this problem?
What do you know? What are you looking for? Any ideas of what equation(s) you'll need to use?

 

[humaders99]

ok well what i have so far is my vertical and horizontal velocities, which are 8.0 m/s and 8.9 m/s respectively...and i don't know where to go from here... i tried using d=v1t + 1/2 at*2 but when i put it in the quadratic formula i can't get it to work so i don't know where to go from here

 

[mbrmbrg]

ok well what i have so far is my vertical and horizontal velocities, which are 8.0 m/s and 8.9 m/s respectively...and i don't know where to go from here... i tried using d=v1t + 1/2 at*2 but when i put it in the quadratic formula i can't get it to work so i don't know where to go from here

OK, so it sounds like you're solving for x and y components separately, which is very good.

You know more than that: assign a value for the initial x position (I'd use 0m) and y position (which you could set at zero, but since the rock falls below the throw line, you'd have a lot of negatives flying around. I don't like negatives, so I'd set the initial y position as the height of the wall--which is given). You also know that there is no acceleration in the x direction, and the acceleration in the y direction is acceleration due to gravity.
I suggest starting by solving for time, using an equation involving y-components only.

 

[humaders99]

ok ... now i have figured out the time ... it is 2.4 s ... how would i figure out the length of the moat? would i have to use the x components now? and if i do how would i do that?

 

[mbrmbrg]

yes, yes!

I'd use $x=x_{0}+v_{0x}t+\frac{1}{2}at^2$

Remember that x acceleration = 0.

 

[humaders99]

ok i got the length of the moat (22 m) ! thanks for ur help... but i have one more question that i hope u can answer... how would i figure out the velocity at impact?

 

[humaders99]

nvm... i got it ... i just used $V_2^2=V_1^2+2ad$... and i got the answer :) thnx again for ur help

 

[pack60]

hey I am new here and need some help with a few basic questions

 

[pack60]

is anyone online?

 

[pack60]

can anyone in here help me with some questions

 

[mbrmbrg]

it might help if you posted your question...