# Homework Help: Prjectiles , how do i solve this

1. Oct 15, 2005

### kevinlikesphysics

5. [SFHS99 3.P.38.] 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 25.0 m/s at an angle of 45° to the horizontal.

(a) By how much does the ball clear or fall short of the crossbar? (Give a positive answer if the ball clears the crossbar, or a negative answer if it falls short of the crossbar.) _______ meters

(b) Does the ball approach the crossbar while still rising or while falling? (Type 'rising' or 'falling', or 'neither' if it falls short of the crossbar.)

What is the vertical component of velocity of the ball at this time? (Assume the positive direction is upward.) _______ m/s

2. Oct 15, 2005

### rocketboy

try separating the problem into x and y components. make a chart with the displacement, acceleration, and velocity along the x and y axis. set up delta time as well (it's neither x or y). Now you can use your kinematics equations for the x axis and y axis....essentially it becomes two 2-D problems. Then you can solve for a variable using one, and sub that into the other. Make sure you draw an accurate diagram to help you set up the x and y components correctly.

REMEMBER:

acceleration along the x axis is zero
acceleration along the y axis is the acceleration due to gravity

the magnitudes of the velocity's for x and y components are found using your trig....this one is really easy since it's at 45 degrees.

Try it and post what you have done. Then if you are still stuck we can help point you back in the right direciton.

(I just learned this stuff myself last month so it's great practice for me!)

Last edited: Oct 15, 2005