Kinematics of a thrown rock. Calculate speed

[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) ??

answer is 1.01s and 2.91s

thx u

 

[himanshu121]

Pls Post your try
Hint:Write vector Equation for motion

 

[Generic]

lol...

 

[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... doesn't give the right answer :( what Do I do wrong??

 

[Generic]

is that equation possible to-do without a Calculator ?

 

[DR33]

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

 

[Generic]

Gimme a min on that one ;)...

 

[himanshu121]

You have applied wrong formula for v_y

 

[DR33]

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

 

[Generic]

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


well i know there's 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...

 

[DR33]

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

I just forgot the last part

 

[Generic]

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

 

[Generic]

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

 

[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

 

[DR33]

k.. anyone help?

 

[DR33]

ok thanks a bunch

 

[Generic]

Sorry DR33 but i think he means your on your own...:(

 

[himanshu121]

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

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

 

[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... :|

 

[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 :((

answers are 1.01 and 2.91

 

[DR33]

n/m, i used 4.9..

wow, thanks man
I appreciate

l8er

 

[himanshu121]

Originally posted by 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 :((

answers are 1.01 and 2.91

What look at it again it is not 4.9 its 9.8

 

[chroot]

Please post further homework help questions here, in the homework help forum -- not in the general physics forum. Thank you.

- Warren

 

[Generic]

um were did my perpetual motion thread go ? (Sorry i am new)

 

[chroot]

It went into the Theory Development forum, where it belongs.

- Warren

 

[Generic]

ok thx, but its not a theory it works ;)

 

[himanshu121]

Dont worry u will enjoy that section more than anything