Velocity Time Graphs - HELP

[chuckjefster90]

Hey Guys, Im having troubles with this particular question.

A Tennis ball is thrown up into the air, and caught again at the same height.
A graph of veloicty against time for the ball is shown. (It is attached)
What is the velocity and the acceleration of the ball at the top of the trajectory (path)? 
Explain your answer. (Take acceleration due to gravity g=9.8m/s)

Im not sure about this but, Is the Velocity 0m/s  according to the graph as when its 4 seconds it is a constant velocity??
(Sorry that is confusing, correct me please)

THanks Heaps Again

[|ll|lll|]

your graph isn't pretty accurate, but yes, at 4s, it's zero velocity as there is no acceleration (i.e. the ball stops for a split second in mid-air before turning back down).
So acceleration and velocity are both zero.
does that help?

[kamil9876]

So acceleration and velocity are both zero.

That's not true, acceleration is non-zero at the top, it is 9.8m/s^2 the whole path. It is correct that the velocity is zero. (a quantity can be zero at some point, even though its rate isn't)

(i.e. the ball stops for a split second in mid-air before turning back down).

Yes, it stops for a split second, but the reason it turns around is because gravity is still pushing it, hence acceleration is still non-zero, always 9.8m/s^2.

[superflya]

vertical component velocity at max height is 0. acceleration is constant at 9.8 ms^-2 never 0.

[|ll|lll|]

oh, my bad then. anyway, when you plot the displacement-time graph, the point where the ball stops for the split second, the tangent will be zero. so the speed is zero.
how do you get acceleration then?

[superflya]

acceleration is constant. gravity.

[kamil9876]

how do you get acceleration then?

If you want to think about it in a graph/function kind of way, then the acceleration is the rate of change of velocity, so by analogy the tangent of the velocity-time graph is the acceleration.  But the velocity time graph is a straight line, meaning tangent is a non-zero constant.

[|ll|lll|]

Ah I see, thanks i'll think about it (:
I heard that spesh will help in phys (or was it the other way round i.e. phys will help in spesh?)

[kamil9876]

in general, calculus helps with physics. in vce however you do the physics part in spec after semester 1, so that's not helpful. But you can always learn the basic (ie methods level) calculus (don't even need to learn the computations etc. but rather the conceptual stuff so you can picture the quantities in physics)