Kinematics, is this a poorly worded question?

[shirozack]

Homework Statement

tina threw a ball to the height of a flagpole 25m high. how long will it take the ball to reach that height and drop back down if it accelerates at 0.5 m/s2

Relevant Equations

na

Like, i don't even understand what it means by the ball accelerating at 0.5m/s2. is it accelerating upwards constantly at 0.5m/s2? what about gravity?

the answer given is weird.
initial speed u = 5m/s and t = 10s
it says time taken to go up is = time taken to go down. (why?)
so total time is 20s.

i can't help but feel this is wrong?

 

[PeroK]

Homework Statement: tina threw a ball to the height of a flagpole 25m high. how long will it take the ball to reach that height and drop back down if it accelerates at 0.5 m/s2
Relevant Equations: na

Like, i don't even understand what it means by the ball accelerating at 0.5m/s2. is it accelerating upwards constantly at 0.5m/s2? what about gravity?

the answer given is weird.
initial speed u = 5m/s and t = 10s
it says time taken to go up is = time taken to go down. (why?)
so total time is 20s.

i can't help but feel this is wrong?

That would be right if you are on a dwarf planet with a surface gravity of $0.5m/s^2$.

 

[kuruman]

It is a poorly worded question.
1. When the ball leaves Tina's hand it has a constant acceleration. Unless the ball has a tiny rocket engine to propel it, it cannot accelerate "up" because . . .
2. what defines "up"? The conventional definition is that "up" is opposite to "down". In other words we are to understand that if Tina just let go off the ball from rest, the ball will accelerate opposite to up in some sort of force field. Therefore . . .
3. the fact that there is a force field, probably gravitational, is supported by the statement that the ball drops back down.

So one may safely conclude (although the problem does not specify it) that Tina is on an asteroid or dwarf planet with an acceleration of gravity of 0.5 m/s² as @PeroK suggested.

Now that I think of it, here is a better problem to consider along these lines.

Tina releases a ball from rest in gravity-free space. The ball is equipped with a rocket thruster that accelerates it at a constant $0.5~\text{m/s}^2$. When the ball is 25 m from Tina, the thruster (but not the ball) abruptly reverses direction and the ball eventually returns to Tina.
(a) Find the time of flight.
(Find the total distance traveled by the ball.

 

[Orodruin]

Is this the question as stated verbatim? Something makes me doubt that.

Still, I would probably interpret the problem as constantly accelerating in the down direction, ie, the seemingly intended interpretation.

Now that I think of it, here is a better problem to consider along these lines.

Tina releases a ball from rest in gravity-free space. The ball is equipped with a rocket thruster that accelerates it at a constant 0.5 m/s2. When the ball is 25 m from Tina, the thruster (but not the ball) abruptly reverses direction and the ball eventually returns to Tina.
(a) Find the time of flight.
(Find the total distance traveled by the ball.

That is a significantly more involved problem than the given one, which is just motion at constant acceleration. It also does not demonstrate the same time reversal symmetry as the original.

 

[kuruman]

It also does not demonstrate the same time reversal symmetry as the original.

The original can be found everywhere. This one requires additional thought to put together, not just looking for the right SUVAT equation to plug in. Time reversal symmetry is still there. The ball is at the thruster reversal point at two different times moving with the same speed in opposite directions.

 

[DaveC426913]

I know they don't usally throw in trick questions but maybe this is one of the those where the correct response is "Setting aside for the moment why the ball might only be accelerating at 0.5m/s^2 instead of 9.8m/s^2, the question is incomplete since it does not specify the direction of acceleration. In a more realistic scenario, one might assume that acceleration is downwards, but this is obviously not a realistic scenario.

Here are both answers, one wth a=+ and the other with a=-. In the future, please be more careful writing your questions."

That's sure to get a hearty chortle out of your prof, and perhaps even a "You've got moxie, kid. Have an A+."

All seriousness aside, solve it as you see fit, but be sure to state your assumptions explicitly.

 

[Orodruin]

The original can be found everywhere.

Exactly. But there is also a reason for that. It is an instructive problem for introductory kinematics. Complicating it in the beginning is only going to lead to confusion.

This one requires additional thought to put together, not just looking for the right SUVAT equation to plug in.

Exactly, which makes it a worse introductory problem rather than a better one. The OP here is struggling with the basics, so piling on top of that is not a good idea imo.

Time reversal symmetry is still there.

That was not clear from your problem statement. You would need to specify that the thruster changes direction every time it passes the reversal point. If not it can be interpreted differently.

The ball is at the thruster reversal point at two different times moving with the same speed in opposite directions.

Yes, but your statement did not make it clear that thrusters should reverse both times.

 

[erobz]

What are the most probable interpretations? I assume it was just supposed to have an initial velocity such that it reached $25 \text{m}$ at the peak of the trajectory.

EDIT: I will note that Tina has a heck of an arm in this interpretation...