# Projectile Motion

#### chriszollman

Can someone work show me how to work this? I am very confused.
1. A place kicker must kick a football from a point 36.0 m (about 40.0 yd) from the goal, and the ball must clear the crossbar, which is 3.05 m high. When kicked, the ball leaves the ground with a speed of 31.0 m/s at an angle of 50° to the horizontal.What is the vertical component of velocity of the ball at this time? (Assume the positive direction is upward.)

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#### Hurkyl

Staff Emeritus
Gold Member
Don't multiple post.

I have a better idea -- why don't you tell us how you worked the problem, and we can tell you where you went wrong, if anywhere?

#### chriszollman

Sorry. First time in the threads

I got 13.7 for the m for the change in X. I dont no where to go past that. What should I do.

#### chriszollman

Vi= 31
Angle-50
X=36
Y=13.7? I think. Where do I go from there? Thanks

#### chriszollman

Is it Vx=Vxi=Vxf so is it 31?

#### Tom Mattson

Staff Emeritus
Gold Member
chriszollman said:
What is the vertical component of velocity of the ball at this time? (Assume the positive direction is upward.)
At what time? Initially? At the crossbar? Some other time?

If it's the first one, then this problem is a piece of cake. You can ignore all the stuff about the distances and just use the info on the initial velocity.

#### chriszisabiiitch

this is a toughy!

yeah i tried really hard on this problem and i cant work it out. i think it has to do with the cosin or sin of the angle because it makes a right triangle, let me know if anyone figures anything out.

#### chriszollman

Its the one on the cross bar

#### Tom Mattson

Staff Emeritus
Gold Member
OK, so among other things you've got the following equation to work with:

$$x=v_i\cos(\theta)t$$.

This expresses the physical fact that there is no acceleration in the x-direction (neglecting air resistance). You can use this equation to figure out when the ball reaches the crossbar.

Then you've been asked for the vertical component of the velocity. That would be the y-component. You have an equation for the $v_y$ as a function of time. You will need to use that to answer the question.

Give that a shot, and if you are still having trouble post your steps so that we can see them.

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