- #1
Martyn Arthur
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- TL;DR Summary
- I am studying Questions In Science at the Open University. Is a ball projected vertically upwards stationer at maximum altitude, does the concept of unbalanced gravity contradict Newton? (I’m not bothered about the mark I got, just the question).
When a ball projected vertically upright reaches its maximum height is it instantaneously stationery (calculus provides for instantaneous events) before it starts its downwards journey?
is gravity acting on the object at that instant?
Newton provides that an object at rest must be subject to balanced forces.
Is gravity is acting does it thus follow that an equal opposite force.
The OU perspective from my tutor, in its marking notes on this says the following (I’m not bothered about the mark I got, just the question).
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Hi Martyn,
Thanks for your mail. As a non-physicist this answer always makes me ponder as well! Here's the full version from my marking notes:
"At the top of the bounce (maximum height) the only force acting on the ping pong ball is its weight (gravity). At the top of its trajectory the ping pong ball is still accelerating under gravity at 9.8 m s−2. An unbalanced force is acting on the ping pong ball and changing its motion - there is a change in the velocity of the ping pong ball."
The gist of it as I understand it is that you can't ever remove the effects of gravity so even at the top of the bounce the velocity is changing from positive through zero to negative but always in the process of changing so there is an acceleration and hence a force active. The table top isn't moving so no change in velocity.
Hope this makes sense!
Cheers,
'
is gravity acting on the object at that instant?
Newton provides that an object at rest must be subject to balanced forces.
Is gravity is acting does it thus follow that an equal opposite force.
The OU perspective from my tutor, in its marking notes on this says the following (I’m not bothered about the mark I got, just the question).
'
Hi Martyn,
Thanks for your mail. As a non-physicist this answer always makes me ponder as well! Here's the full version from my marking notes:
"At the top of the bounce (maximum height) the only force acting on the ping pong ball is its weight (gravity). At the top of its trajectory the ping pong ball is still accelerating under gravity at 9.8 m s−2. An unbalanced force is acting on the ping pong ball and changing its motion - there is a change in the velocity of the ping pong ball."
The gist of it as I understand it is that you can't ever remove the effects of gravity so even at the top of the bounce the velocity is changing from positive through zero to negative but always in the process of changing so there is an acceleration and hence a force active. The table top isn't moving so no change in velocity.
Hope this makes sense!
Cheers,
'