# Homework Help: Kinematics of a thrown rock. Calculate speed

1. Jan 17, 2004

### DR33

A rock is thrown at 25 m/s in a 50 degree angle (horizontal). At what time will the speed reach an angle of + / - 30 degree (horizontal) ??

thx u

2. Jan 17, 2004

### himanshu121

Hint:Write vector Equation for motion

3. Jan 17, 2004

### Generic

lol...

4. Jan 17, 2004

### DR33

lol.. posted at wrong place

my try:

Vx = Vxcos Theta
= 25cos50
= 16.07 m/s

Vy = Vysin(theta)t
= (25sin50)t - 4.9t²
= 19.15t - 4.9t²

Vy / Vx = tan30

therefore.. 4.9t² - 19.15t + 9.28 = 0.... doesnt give the right answer :( what Do I do wrong??

5. Jan 17, 2004

### Generic

is that equation possible to-do without a Calculator ?

6. Jan 17, 2004

### DR33

obvs. not.. look at the answers.. 1.01 & 2.91 s..
If u can get those numbers without a calculator, good luck

7. Jan 17, 2004

### Generic

Gimme a min on that one ;)...

8. Jan 17, 2004

### himanshu121

You have applied wrong formula for vy

9. Jan 17, 2004

### DR33

I really appreciate your help.. but would you mind explaining instaed of giving hints?

10. Jan 17, 2004

### Generic

dang (Should have payed attention to my math teacher...)LOL

well i know theres gravity 9.8m/s, air friction,

so vector is 25m/s at 50(Deg)angle
how long it would take for 30(Deg)angle hmmm....

11. Jan 17, 2004

### DR33

and the formula I applied is actually Vy = Vysin(theta)t - gt²/2

I just forgot the last part

12. Jan 17, 2004

### Generic

didnt your mom tell you not to use Calculators in math ?

13. Jan 17, 2004

### Generic

gimme 10mins tops and all figure out the Anwser without a Calculator ;)

14. Jan 17, 2004

### himanshu121

Here u have given your try i appreciate and on this network which is an interactive one.

we try to inculcate the habit of problem solving by the poster itself
giving u the answer would defy the process of learning

Still you are welcomed and Pls wait while i give u complete detail

15. Jan 17, 2004

### DR33

k.. anyone help?

16. Jan 17, 2004

### DR33

ok thx a bunch

17. Jan 17, 2004

### Generic

18. Jan 17, 2004

### himanshu121

$$v_x=25\cos{\theta}=25\cos50$$
$$v_y=25\sin50 - 9.8t$$

Now from gien pro
$$\frac{v_y}{v_x}=\pm tan30$$

Last edited: Jan 17, 2004
19. Jan 17, 2004

### Generic

i got rejected in a Physics class, seriouly i would not comment on it but, its not helping someone who is dedicating his time on this forum for help.... :|

20. Jan 17, 2004

### DR33

ok so therefore

Vx = 16,07 m/s
Vy = 19.15 - 4.9t

Vx / Vy = tan30... solve for t..

doesnt give the right answer :((