What is the Velocity-Time Graph for a Ball Thrown Straight Up?

[Wolf5370]

In step2 the acceleration is DOWNWARD and it's positive.
it makes sense because when we throw upward , a is Negative, and then the ball reach a pooint where its velocity comes to zero...before falling free and then the acceleration is Positive...and all this matches C...still convinced :shy:

No - gravity is the accelerator here - and is always in the same direction. It does not get faster when leaving the thrower's hand it decelerates and then when gravity wins, it accelerates - but ALWAYS in the downward direction. The ball has no motor or jet pack, it leaves with the force of the throw and starts to immediately decelerate under gravity, -ve (if you like) acceleartion until its KE is exhausted (now G.PE) and it drops once more.

 

[chawki]

If my method was wrong, i wouldn't find that D can be true too...

 

[Wolf5370]

If your method was correct, you would not find that C can be correct too! - Sorry.

 

[chawki]

If your method was correct, you would not find that C can be correct too! - Sorry.

C can be correct...the ball is thrown upward with an initial speed...at some time it reaches 0, ball change direction (velocity also) and head down to the ground, earning speed again.

 

[Doc Al]

I'm using downward as (+) upward as (-)

Well, you'd better think again. Use upward as (+) and downward as (-). Otherwise you're saying that when you throw a ball up, it's velocity is down. (Note the sign of the velocity on the diagrams!)

OK. Now look at diagram C again. What does it tell you? Does the object ever come down?

 

[Doc Al]

C can be correct...the ball is thrown upward with an initial speed...at some time it reaches 0, ball change direction (velocity also) and head down to the ground, earning speed again.

Yes, the ball does change direction. But is that what C shows? Look at the sign of the velocity in C. It never changes sign--which means it never changes direction. Does that make sense?

 

[chawki]

now it's easy to say that D is the answer :) Thank you to all of you!