Mass calculations

[Maveric]

Hi all,

I am new here so sorry if this falls to a wrong section, please could then moderator kindly let this drop to the appropriate one? Thanks!

I have been trying to solve this problem:

1. I will throw a ball weighing 2 kg up from my hand say 1 meters up the air directly.

2. I want to know how much the ball weighs when it lands on my hand, say the same position I threw it up from, that is 1 m below?

Tried to do this with online calculators, but they ended up the mass being exactly the same.

I did not mean that the ball changes in mass, ofcourse it is 2 kg all the time. But is it possible to calculate and how, what it would weigh in kilograms when it drops back to my hand, because of gravity I do not think it can appear to me like only 2 kg?

Hope you catch my drift =D I am totally clueless...

Cheers,
Mav

 

[Maveric]

you're mixing up mass and force. The ball will always 2 kg assuming it doesn't change.

The force it exerts on another object it comes in contact with will depend also on it's accleration.

Newton's law states that Force = mass x acceleration.

If the ball is falling freely in Earth's gravitational field. It experiences acceleration only due to that gravitational force. Close to the Earth, Earth's gravity causes objects to have a constant acceleration towards the center of the Earth. That acceleration is -9.8 meters/second². Negative because it points down.

The acceleration that gravity causes x the object's mass is known as ...

it's weight.

Thank you very much for this answer Romsek. Yes indeed this makes now sense to me. Let's say however, that this to me would be merely like a question regarding physical exercise. And such questions I would like to try to explain to also someone else. Basically it would perhaps sound logical for someone who does not know about all this well enough, just to think straight up whether the ball dropping back to the hand in weight is actually more than the 2kg ball that was released from the hand upwards with a swinging movement and which in a moment would land back to the hand say that 1 meter below?

So in principle, this then would not be at all as simple calculation I guess?

We would need to know the movement causing the ball to fly upwards, like with what speed it was released and like does it fall to a hand flat down 1 meter or does the receiver let the hand down with it even like 30 cm more to not have a plain impact.

Hmm..how would you say I should start collecting the pieces for this calculation, if the pre-setup would be something like explained and the person would only understand kilograms, and the energy or force or such would not be interesting? Only the fact that the ball would actually cause to feel weighing more than the 2 kg, because of the factors affecting to it's movement.

Best,
Mav

 

[Bob Brown MSEE]

Force

What you feel is force.
More massive objects give more force, that we call weight.

You are asking about the additional force that you feel when STOPPING a falling weight. It seems to have more weight, but what it has is more force on your hand due to STOPPING the ball.

When you have finished catching the ball, the only force is again the weight. If you try to stop the ball instantly, you better have a glove because you will damage the bones in your hand.

If you give the ball some distance to stop by letting your hands move with the ball, you can reduce the pain greatly.

The technical description is the reaction force on your hand to support the weight + force required to decelerate the ball.

====================

There is a similar force in excess of the weight, when the ball was thrown upward. That seems to be much less because the ball was thrown upward in an arc (over a much longer path). The ball was given more distance to reach top speed before being released (than the catch stopping it). The acceleration was MUCH less than the deceleration. Therefore MUCH less additional force was felt when throwing it up.

 

[Maveric]

Thank you Bob aswell now,

I am starting to get the point finally...so it is partly fault of an online calculator that claims that when i first have given the 2 kg as input, the speed in m/s and the force that the calculation to mass from that equals 2 kg. That gives the wrong idea, so what has been now explained to me finally made me understand it basically was right! But that because no other aspects were taken in consideration that we are back to the simple fact that the objects mass is what it is, period = 2.0 kg =)

Thanks
Best,
Mav

What you feel is force.
More massive objects give more force, that we call weight.

You are asking about the additional force that you feel when STOPPING a falling weight. It seems to have more weight, but what it has is more force on your hand due to STOPPING the ball.

When you have finished catching the ball, the only force is again the weight. If you try to stop the ball instantly, you better have a glove because you will damage the bones in your hand.

If you give the ball some distance to stop by letting your hands move with the ball, you can reduce the pain greatly.

The technical description is the reaction force on your hand to support the weight + force required to decelerate the ball.

====================

There is a similar force in excess of the weight, when the ball was thrown upward. That seems to be much less because the ball was thrown upward in an arc (over a much longer path). The ball was given more distance to reach top speed before being released (than the catch stopping it). The acceleration was MUCH less than the deceleration. Therefore MUCH less additional force was felt when throwing it up.

 

[HallsofIvy]

We can say how much energy is required because there is a law of "conservation of energy". We cannot say how much force is because there is no "conservation of force".

If you are holding a 2kg mass in your hand you will feel a weight (force) of 2g Newtons. If you throw it up 1 m (which would be an increase in energy 2 Joules), catching it on its return will also require an energy of 2 Joules. What force that requires depends upon how you catch it. If you catch it "sharply", not allowing your arm to fall back much, the force woulld be much greater than if you catch it "loosely", allowing your arm to fall back a great deal.

 

[Deleted member 4993]

We can say how much energy is required because there is a law of "conservation of energy". We cannot say how much force is because there is no "conservation of force".

If you are holding a 2kg mass in your hand you will feel a weight (force) of 2g Newtons. If you throw it up 1 m (which would be an increase in energy 2 Joules), catching it on its return will also require an energy of 2 Joules. What force that requires depends upon how you catch it. If you catch it "sharply", not allowing your arm to fall back much, the force woulld be much greater than if you catch it "loosely", allowing your arm to fall back a great deal.

We can calculate force by calculating change in momentum - the velocity right before striking the hand and velocity right after striking the hand will need to be calculated.