# Homework Help: Kinematics: 3 unknowns, find Time.

1. Sep 21, 2009

### Rubix

SOLVED.

1. The problem statement, all variables and given/known data
angle = 10 degrees
delta X = 20
delta Y = -1.5

car goes off a 10 degree ramp and lands at the delta X/delta Y.
what is T?

2. Relevant equations
delta X = VxoT
Vy = Vyo + AyT
delta Y = VyoT + (1/2)AyT^2
Vy^2 = Vyo^2 + 2Ay(delta Ymax)

3. The attempt at a solution
none

Last edited: Sep 22, 2009
2. Sep 21, 2009

### rl.bhat

You know the relevant equations.

3. Sep 21, 2009

### Rubix

Thanks for the help >:(

I found Vxo and Vyo in terms of V using sohcahtoa, then I plugged it into and delta X = VxoT and found out T (t = 20.3/v). I plugged Vyo and T into my third eqn. above and got V=1.

There's my attempt.

4. Sep 21, 2009

### rl.bhat

Now in the third equation, substitute the values of T. Y and Ay = g. In this Y and g are negative. Solve for v. Then you can find T.

5. Sep 21, 2009

### Rubix

figured it out. did this earlier but I didn't see the answer for some reason?

AP physics test tomorrow wish me luck i'll need it!

6. Sep 21, 2009

### rl.bhat

Can you show me the calculations, so that I can check it. I am getting v = 9.624 m/s.
Any way good luck to you.

7. Sep 21, 2009

### Rubix

i got V = 20.04 m/s (correct answer from teacher is 20.05)

calculations:

Vxo = Vcos10
Vyo = Vsin10

20 = (Vcos10)t
t = (20.3/v)

-1.5 = (Vsin10)(20.3/v) + (1/2)(-9.8)￼(20.3/v)^2
-1.5 = (V's cancel out, so do 20.3 times sin10)
-1.5 = (3.52) - 4.9(20.3/v)^2
5.02 = 4.9(20.3/v)^2
1.026 = (20.3/v)^2
root everything
1.013 = 20.3/v
1.013v = 20.3
v = 20.039 m/s

8. Sep 22, 2009

### rl.bhat

Good. I think I missed vsin10.