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Question components of velocity ( Physics Help and Math Help - Physics Forums Introductory Physics )
Updated: 2008-05-16 05:55:37 (18)
components of velocity

A ball is thrown toward a cliff of height h with a speed of 30m/s and an angle of 60 degrees above horizontal. It lands on the edge of the cliff 4.0s later.

How high is the cliff?
do you just use the equation h=.5gt^2 ? and put it in -9.80=g and 4=t ?

Answers: components of velocity ( Physics Help and Math Help - Physics Forums Introductory Physics )
components of velocity

What is the ball's impact speed?

i thought what you should do is find the final velocity in the x component and y component. and i thought that vfx= 15 since there is no acc in the x direction and vfy= -26 b/c of neg acc. and then i thought you should take the magnitude, but this does not work out to be right. do you know what i am doing wrong

wait i think i did this wrong, look at my next post please

klm

components of velocity

are the equations i used alright?

klm

components of velocity

um i think the vertical speed should be 0

klm

components of velocity

so would it be okay to use the equation vfy= viy +ayT so 0= 26+9.8t so t= 2.65 and then cut then stick that time in that first equation you gave me?

klm

components of velocity

so y= 26(2.65) -.5(9.8)(2.65)^2 =34 .4m ?

klm

components of velocity

No, you can't assume that the peak is at half the time. After all, it lands up on a cliff, so it spent more time rising than falling. (If it fell back down to the original height, then you'd be correct.)

Instead, use a velocity equation for the y-direction to figure out the time when it reaches maximum height. Hint: At the maximum height, what's the vertical speed?

Doc Al

components of velocity

Good!

Doc Al

components of velocity

thanks! can you help me with the next part too..! What was the maximum height of the ball?
i think the equation is just the same as the one you wrote, but just cut t=4 in half to get the peak height so t=2 ..so y= 26(2)-.5(9.8)(2)^2 = 32.4 m ?

klm

components of velocity

ohh sorry, so y=0 + 26(4) - .5(9.8)(4)^2 = 25.6 m

klm

components of velocity

thank you Doc Al! do you mind one more question, it will be the last one i promise! =)

klm

components of velocity

Originally Posted by klm
would this be correct: y0=26 , v0=30, g=-9.8 , t=4 ... so y=26+(30x4)- .5(-9.8)(4)^2 = 224.4 m ?

i got vy by doing v0sintheta= 26
No. In the formula I gave:
y0 is your initial position, which I presume is on the ground at height = 0
v0 (which I'll change to v0y) is the vertical component of the initial velocity, what you call v0sin(theta)
g = 9.8 m/s^2

To make it less confusing, I'll relabel v0 to be v0y in my equation.

Doc Al

components of velocity

You also need to consider the initial speed in the y-direction; here's the complete equation:
Click to see the LaTeX code for this image

Doc Al

components of velocity

would this be correct: y0=26 , v0=30, g=-9.8 , t=4 ... so y=26+(30x4)- .5(-9.8)(4)^2 = 224.4 m ?

i got vy by doing v0sintheta= 26

klm

components of velocity

Originally Posted by klm
oh no actually should vfy= 65.2 because i tried the equation vfy=viy+ay x T so 26+(9.8x4)= 65.2
Careful here. ay = -9.8 m/s^2.

Doc Al

components of velocity

thank you so much doc al!!

klm

components of velocity

ohhh so should it be vfy= -13.2 and then do the sqaure root (13.2^2 +15^2 ) =19.98

klm

components of velocity

Good!

Doc Al

components of velocity

Very good!

Doc Al

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