What is the initial velocity of the projectile given the displacement and angle?

[helpmee]

Homework Statement

i have a problem from an in class lab where i have to find the initial velocity of the ball. given is the horizontal and vertical displacement of the projectile.
Dy(height)=100cm
Dx(range)=200m
0(theta)= 15degrees.
Find the initial velocity.

Homework Equations

not sure what equation to start with

The Attempt at a Solution

x-comp:
Vx1=Vo cos 60

y-comp:
Vy1=Vo sin 60

t=200/Vo cos 60

-100=Vo sin 60(200/Vo cos 60) + 1/2(-9.8)(200/Vo cos 60)^2
Vo= 2 numbers that make no sense

 

[rock.freak667]

Draw the initial velocity v₀ at the angle 15 and then split that into vertical and horizontal components.

Then use your equations of motion.

 

[helpmee]

hey
i haven't learned equations of motion only kinematic equations

 

[Redbelly98]

hey
i haven't learned equations of motion only kinematic equations

That's what he meant, they are the same thing. Use the kinematic equations.

 

[helpmee]

i still don't understand
can u be specific on which ones to use?

 

[rock.freak667]

i still don't understand
can u be specific on which ones to use?

the working in your original post looks fine, what values for v₀ did you get? Also I don't understand how you have sin60 and cos60 when your angle is 15. Also the '-100' should just be 100

 

[helpmee]

sorry that should be 15 not 60 and in class we learned that dy was also negative in this type of question

 

[rock.freak667]

sorry that should be 15 not 60 and in class we learned that dy was also negative in this type of question

it was a lab, so I assume, you all weren't at the ground just shooting it at 15 degrees but did it ona a table at 100cm. In that case, you are right, it is -100.

So forming a quadratic in v₀ what values of v₀ did you get?

 

[helpmee]

i got 2000m/s and 0.00125m/s. i think i might have done the calculations wrong though

 

[rock.freak667]

i got 2000m/s and 0.00125m/s. i think i might have done the calculations wrong though

yes check that back, because your equation shows you will get something like av₀²-b=0

 

[helpmee]

can u help me on how to approach the math on this question?

 

[rock.freak667]

ok so this what you have right?


$$-100=(V_o sin15)*(\frac{200}{V_o cos15}) + \frac{1}{2}(-9.8)(\frac{200}{V_o cos 15})^2$$


So in the first term there are we seeing anything canceling out? Can we simplify it if anything did cancel out?


in the second term, when you square out everything, can you find the number K such that the second term is K/V_o² ?

 

[rl.bhat]

In the problem D(y) is 100 cm. Is it correct?
Mainly point of projection is not given. If it is from the ground, at a given angle of projection and range, D(y) will be attained at different time depending on the initial velocity. So there cannot be a unique value of initial velocity.

 

[helpmee]

Thanks for the help. i got it now