A ball is thrown upwards and another dropped

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In summary: It falls to the ground, the ball is moving faster?In summary, the ball that was thrown upwards will move faster than the ball that was dropped from rest.
  • #1
Nubcake
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Simultaneously , a ball is thrown upwards and another dropped from rest. Which one hits the ground moving faster?
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The ball thrown upwards because during it's descent as it starts to fall back down it's velocity is increasing due to acceleration from gravity hence the greater the distance the greater velocity it will achieve before hitting the ground

I'm not confident with my anwser ; is it safe to say its velocity is increasing by 9.8 m s-1 every second?

Can someone please give me a detailed explanation on this?
 
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  • #2
well, really its velocity is changing by 9.8m/s every second. so the velocity is not necessarily increasing.

Also, about the problem generally, we are not supposed to just give you the answer on this forum. But we can help. My advice is to think about energy. Start thinking of things you know about energy in this situation.
 
  • #3
Changing is what I meant , how does energy relate to this question? It is to do with projectiles , are you referring to kinetic energy of the ball?
 
  • #4
Nubcake said:
Simultaneously , a ball is thrown upwards and another dropped from rest. Which one hits the ground moving faster?



-



The ball thrown upwards because during it's descent as it starts to fall back down it's velocity is increasing due to acceleration from gravity hence the greater the distance the greater velocity it will achieve before hitting the ground

I'm not confident with my anwser ; is it safe to say its velocity is increasing by 9.8 m s-1 every second?

Can someone please give me a detailed explanation on this?

Nubcake said:
Changing is what I meant , how does energy relate to this question? It is to do with projectiles , are you referring to kinetic energy of the ball?

In addition to thinking about the energies involved, you can also ask yourself what the velocity of the thrown ball will be when it passes you on the way down. Is it moving faster at that point than the ball that you dropped?

As for the equation, use the kinematic equation of motion for constant acceleration (gravity's acceleration is constant for this problem) -- use the one that relates the final velocity to initial velocity and acceleration, and the one that relates final position to initial position, velocity and acceleration...
 
  • #5
Yes, the ball has kinetic energy and one other kind of energy. I was hinting about energy, because I think your teacher will be more happy if you give an explanation using the concept of energy. Your answer: "The ball thrown upwards because during it's descent as it starts to fall back down it's velocity is increasing due to acceleration from gravity hence the greater the distance the greater velocity it will achieve before hitting the ground" Is true, but not very qualitative. A better answer would be to say something definite about the difference in kinetic energy of the two balls when they hit the ground.

Edit: I meant to say quantitative, not qualitative. Those two are easy to get mixed up :)
 
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  • #6
I'm positive that the velocity of the ball that was thrown up on its way down will be greater than the ball that was dropped from rest but another person who I posed this question says otherwise that both balls would hit the ground with the same velocity since acceleration is constant for both balls regardless of the height it was dropped from or thrown up

So it is the ball thrown up then? Could anyone explain why relating to projectiles because we haven't covered energy in my class yet so It must be to do with velocity and acceleration
 
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  • #7
Nubcake said:
So it is the ball thrown up then? Could anyone explain why relating to projectiles because we haven't covered energy in my class yet so It must be to do with velocity and acceleration

If you haven't covered energy yet in class, you really need to think in terms of velocity, acceleration and time. You are right, the thrown ball will hit the ground with greater speed than the dropped one, but why?
Think: the ball thrown up will rise , loses its speed, and starts to fall back from a higher position than the other ball which was dropped. From higher position, the ball falls for longer time. How is the time and speed of falling bodies related?
You can calculate the final height of the ball if it was thrown up with speed Vo from a tower of height H. And then, how long does it fall to the ground. And what will be its final speed. And what is the speed of the other ball which was dropped from height H.

ehild
 
  • #8
ehild has the correct explanation as far as avoiding energy explanations. Here's an alternative, though:

Isn't it true that if you throw a ball in the air that, by the time it falls to the same height you threw it from, it has the same speed (just in a different direction)? You should see that this is true just from the symmetry of the parabolic trajectory.

Now, consider the limiting case of the balls being released from a small distance above the ground.
 
  • #9
Nubcake said:
So it is the ball thrown up then? Could anyone explain why relating to projectiles because we haven't covered energy in my class yet so It must be to do with velocity and acceleration
Other people have given some good advice. Your friend is right that the acceleration is constant for both, but as you observed, the change in velocity per second is equal to the acceleration. and you are right that the greater distance means greater velocity will build up.
 

What is the difference between throwing a ball upwards and dropping a ball?

When throwing a ball upwards, you are providing an initial velocity to the ball, while dropping a ball means releasing it from rest. In both cases, the ball will experience a downward acceleration due to gravity, but the initial velocity in the upward throw will cause the ball to travel higher before falling back down.

What factors affect the height reached by a thrown ball?

The height reached by a thrown ball is affected by the initial velocity, air resistance, and the force applied to the ball by the thrower. The angle of the throw and the mass and size of the ball can also play a role.

Why does a ball eventually fall back down after being thrown upwards?

This is due to the force of gravity pulling the ball back towards the ground. As the ball rises, it experiences a decreasing upward velocity until it reaches its maximum height, where the velocity becomes zero. Then, gravity takes over and causes the ball to accelerate downwards.

How does air resistance affect the motion of a thrown ball?

Air resistance, also known as drag, can slow down the motion of a thrown ball. The higher the velocity of the ball, the greater the drag force it experiences. This can cause a thrown ball to not reach as high of a height as it would in a vacuum.

What is the relationship between the time of flight and the height reached by a thrown ball?

The time of flight, or the time it takes for a thrown ball to reach its maximum height and fall back down, is directly proportional to the height reached by the ball. This means that the longer the ball stays in the air, the higher it will travel before falling back down.

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