# Physics

We're studying projectile motion in my Physics class right now, and last week we went outside and hit tennis balls with a bat. Now we must make graphs showing the position of the ball in both the x and y directions. I must also explain why each line has the shape that it does.

For my explanation for the x-position, I said that the line is increasing on the graph because this is the distance that the ball is covering. It is getting farther and farther away the starting point, or the point from where I hit it at base. For the y-direction, I said that the line is going up and then coming back down b/c this is how the ball acted when in the air. It initially went up when I hit it and continued going up until gravity acted upon it and brought it back to the ground.

Are both of these explanations correct? Is there anything I could do to improve my answers, or make them more complete?

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1. This statement is not correct:
"initially went up when I hit it and continued going up until gravity acted upon it and brought it back to the ground. "

The error is in saying "until gravity acted upon it". Gravity is always acting on the ball, at a constant rate, decreasing the upward velocity component at first, and then accelerating it downwards.

The rest of what you wrote is fine.

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2. Ahh ok I see.
There isn't anything I should add to them? I just want to make sure they are as complete as possible.

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3. You could say that the slope of the y vs t graph is a straight line,
Vy (t) = Vy,o - g t
and the slope of a Vy vs t graph is a constant (-g).

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4. Great, thanks. That helps a lot.

But now I have to do a graph of velocity and the x/y directions. My velocity in the x-direction is a staight line, and the vel. of the y continually decreases. They ask me to explain why each line has the shape that it does in terms of slope and acceleration. For the y, is what you said...that the slope is a constant?

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5. I said the slope of Vy vs t is constant (-g).

For the x direction, Vx is constant and the slope of Vx(t) is zero.

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