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# Synoptic-ish explain questions watch

1. As asked, I'll start off with a few "explain" qs to get us thinking. Best and most understandable answers will get rep

A man commits suicide by jumping off a cliff. Explain how momentum is conserved and where the man's initial momentum before the collision with the ground "goes"?

A ball is thrown up into the air and is momentarily at rest at the greatest height of its flight. Explain whether it is in equilibrium or not.

(taken from a thread in the maths forum: )
A cosmonaut is walking in space next to a space station orbitting the Earth. She throws the lens cap of her movie camera directly towards the Earth. Where does it go? Justify your answer.
2. (I'm not great with things like this so these are my best guesses)

A man commits suicide by jumping off a cliff. Explain how momentum is conserved and where the man's initial momentum before the collision with the ground "goes"?
The law of momentum conservation states that the total momentum of a system remains constant unless acted upon by an external force. When the man jumps off the cliff he experiences a force (due to gravity) and accelerates. As such his momentum increases (since his velocity increases). The man will have the most, the instant before the (messy) collision with the ground. When he does hit the ground, there is a large change in momentum, due to his body being stopped by the ground.Due Newton's second law, his body experiences a large force (Large change in momentum, over a short time). If we assume none of the man's mass escaped (no ligaments break off) then his momentum will return to the initial (if he stepped off the cliff).

A ball is thrown up into the air and is momentarily at rest at the greatest height of its flight. Explain whether it is in equilibrium or not.For an object to be in equalibrium the forces acting on it must be balanced. At the top of the flight, the ball's weight is acting downwards and the ball's momentum (Note no upward force exists on the ball since the actual throw) is acting upwards. Since the ball isn't moving these must be balanced and therefore the ball must be in equalibrium

A cosmonaut is walking in space next to a space station orbitting the Earth. She throws the lens cap of her movie camera directly towards the Earth. Where does it go? Justify your answer.
Since the cap already had a force acting parallel to the earth (due to the fact the cosmonaut is in orbit), the cap will spiral in towards the earth, increasing it's velocity as it does so, until it burns up in the atmosphere or hits the earth.
3. Endeavour, could you add this onto the initial post?:

I'm gonna try these now
4. A man commits suicide by jumping off a cliff. Explain how momentum is conserved and where the man's initial momentum before the collision with the ground "goes"?
As the man jumps from the cliff, he is acted on by a force, g, which causes his momentum to change and hence he accelerates (Newton's second law of motion). The principle of the conservation of momentum states that within a closed system, the total momentum in any direction remains constant. Newton's third law states that when two bodies interact, the forces they exert on each other are equal and opposite. Since the mass of the Earth is pulling the man towards it, the mass of the man exerts an equal and oppsite force on the Earth, hence both the Earth and the man has equal but opposite forces acting on them, thus as the man is falling down to Earth, the Earth is 'falling' up towards the man. The the momentum of both the Earth and the man must be equal but in opposite direction, so as they collide the momentum 'cancels' out (assuming both the mass of the Earth and the man to be constant). Since the total momentum before the collision is zero, and the total momentum after the collision is zero, momentum is conserved.
The man's initial momentum BEFORE the collision with the ground goes nowhere as nothing is there to stop it's velocity.
(Really confused about the second part of the Q )

A ball is thrown up into the air and is momentarily at rest at the greatest height of its flight. Explain whether it is in equilibrium or not.
Newton's first law of motion states that an object will remain at rest or in a state of uniform motion unless it is acted on by an external force. The ball is given an initial force to make it go up, after that there is no upwards force acting on the ball, but there is a constant force of gravity acting downwards towards the ground. The forces are not balanced as there is no force to balance the force of gravity, assuming the velocity going upwards to be positive, at the greatest height where the velocity is zero, it's velocity is still changing, it does not have a constant velocity and hence it is not in equalibrium. Imagin a number line representing the velocity of the ball:
5, 4, 3, 2, 1, 0, -1, -2, -3, ....; the velocity is moving from positive to negative, it has to pass zero at one stage, and hence at an instant the ball's velocity is 0ms-1, it doesn't mean it has a constant velocity of 0ms-1 and hence it is at rest, similarly at an instant the ball's velocity is 2ms-1, it doesn't mean the ball is travelling at a constant speed of 2ms-1. A change in velocity means a change of momentum (assuming the mass of the ball to be constant) as stated in Newton's second law of motion, the force acting on the ball and therefore producing the change is proportional to the rate of change, and so one may conclude that the ball is not in a state of equilibrium.
(I would really like to know the real answer to this question)

A cosmonaut is walking in space next to a space station orbitting the Earth. She throws the lens cap of her movie camera directly towards the Earth. Where does it go? Justify your answer.
Depending on the size of the mass of the space station and the distance between the lens cap betweent the Earth and the space station, the gravitaional field strength of both the Earth and the space station may be calculated and the lens cap could to the space station or to Earth depending on which gravitational field strength is the strongest. The gravitational field strength may be calculated using the following formula:
g=-GM/r^2 where g is the gravitational field strength, G is the gravitational constant, M is the mass, r is the distance between the two centre of masses in question.

EDIT: Also depends on the initial force on exerted on the lens cap.
5. (Original post by bilb)
At the top of the flight, the ball's weight is acting downwards and the ball's momentum (Note no upward force exists on the ball since the actual throw) is acting upwards. Since the ball isn't moving these must be balanced and therefore the ball must be in equalibrium.
I thought forces only balance out other forces, momentum isn't a force and hence one of it's unit is Ns, a force has a SI unit of N.
6. (Original post by MRLX69)
I thought forces only balance out other forces, momentum isn't a force and hence one of it's unit is Ns, a force has a SI unit of N.
Force is the rate of change of momentum. Since the ball is slowing down, it's momentum is changing. That balances it's weight at the instance of stopping.
7. I agree with MRLX on this- momentum isn't the same as a force. And besides, the momentum when it is momentarily at rest is 0, as v=0.
8. ah true... I guess an instant before the ball stops it has the force upwards, but at the instant it has stopped there is only a force downwards.
9. Surely it only has the force downwards (gravitational force of Earth on ball), and the movement upwards is caused by the original force of the person throwing the ball up? Therefore, although it starts with a positive velocity (taking up as positive) it has a constant (or as good as constant) deceleration, and the forces are always unbalanced.
10. (Original post by endeavour)
A cosmonaut is walking in space next to a space station orbitting the Earth. She throws the lens cap of her movie camera directly towards the Earth. Where does it go? Justify your answer.
Wouldn't the cosmonaut move away from the earth?
11. (Original post by Mikemania)
Surely it only has the force downwards (gravitational force of Earth on ball), and the movement upwards is caused by the original force of the person throwing the ball up? Therefore, although it starts with a positive velocity (taking up as positive) it has a constant (or as good as constant) deceleration, and the forces are always unbalanced.
Yes this is true- gravity decelerates the ball as it is the only force acting on it (ignoring wind etc).

(Original post by Laffo)
Wouldn't the cosmonaut move away from the earth?
Yes she would, forever, until another force acts on her.
12. 3.00am, the morning before my Forces, Fields and Energy and also the infamous synoptic paper........ ahhhhh! .

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