# Calculating the speed problem!

1. Jun 25, 2013

### beginner33

1. The problem statement, all variables and given/known data

the top of the pool table is 0.710m from the floor. the placement of the tape is such at 0m aligns to the edge of the table. the winner of the competition wants to know if he has broken the world record for the break shot of 32mph ( about 14.3m/s) the ball landed a distance of 3.65m from the table edge.

1. calculate his break shot speed
2. at what speed did his pool ball hit the ground?

2. Relevant equations
speed=distance/time m/s

3. The attempt at a solution
givens are 32mph, and the distance which is 3.65m.... the height(im not even really sure if this relates to the problem) is 0.710m. im not sure what the 0m has to do with this either. i'm pretty sure im suppose to find the time now since i have the distance 3.65m.
sorry if this information sucks but i just started physics 3 days ago and the class pace is obviously too fast for me. it would help if u provide a step by step explanation process. thanks!

2. Jun 25, 2013

### Ampere

I think the 0m is given just so that you know the distance from the table's edge is 3.65-0 or 3.65m.

I would begin this one by looking at the table's height. How long does it take an object to fall 0.71m? Its initial vertical speed is 0.

3. Jun 26, 2013

### thecommexokid

In projectile motion problems (such as this one) you can always consider the motion in the horizontal direction and the motion in the vertical direction as two separate problems and tackle them one at a time.

Let's tackle question 1 first. You want to know the horizontal speed of the ball. As you pointed out in your relevant equations, to calculate the horizontal speed of the ball you need to know the distance it traveled and the time it took to do so. You know the distance already—it's given as 3.65 m. But you need to know how long it takes to travel that distance. Hm.

To figure out the time the ball spends in the air, we'll switch over to thinking about its vertical motion for a bit. We'll completely ignore what it's doing horizontally now. Vertically, our situation is that a ball starts at rest 0.71 m in the air. Due to gravity, it will accelerate toward the floor at 9.8 m/s2. We want to know how long it will take to travel the 0.71 m to the floor. Do you know an equation that relates time, distance, and acceleration?

Once you figure out which equation you can use to find the time in the air, then you can go back and plug that time in to the horizontal problem, to find the horizontal speed.