Question

A tennis player hits a ball 2.0 m above the ground. The ball leaves his racquet with a speed of 16.0m/s at an angle 4.5? above the horizontal. The horizontal distance to the net is 7.0 m, and the net is 1.0 m high.

By how much?

Answer 1

The solution provided by MangoMan is incorrect. Try it yourself. Specifically the step where they calculate s:

s = (1.25)(0.44) + (1/2)(-9.81)(0.44^2) = 1.49m

You'll see that the answer here is NOT 1.49m. I get -0.40m!

The signs used are correct, just the arithmetic is not. If you go with -0.40m from this step you would add 2.0 m for the initial height of the tennis ball and then substract 1.0m for the height of the net to get the clearance? This is done just as MangoMan did but your final answer, using -0.40m instead of 1.49m, should be .60m which is what I calculated in my solution.

It seems as though they just didn't take the negative sign into account when doing the arithmetic, otherwise the method should work.

The difference of 0.60m and my 0.61m just comes from me carrying more digits in each step. Neither of us followed the rules for significant figures but in physics this is generally only an issue when it comes to lab reports, not homework or exams.

The equation I used is one of the kinematic equations (constant acceleration) which all first semester physics students are expected to know.

Good luck!