TrueTears question thread

[vce08]

Calculate how far an astronaut would need to be away above the Earth in order for his weight to be 0.01 his weight on the Earth's surface.

we need the value of g to be 0.098

since g = GM/r^2
and the value of M is already known then r can be found out very easily. (this is 5.98 x 10^24 kg)
and G = 6.67 x 10^-11

then the value of r required is






Pretty much 10 times the radius of the earth which can simply be seen by looking at the formula where g needs to be reduced by 100 times, hence the radius needs to increase by a factor of 10.

[Mao]

Also this question:

A 50kg passenger in a car is held firmly by her seatbelt during a collision. The seatbelt, in stopping her, puts an average force of 12000N on her for s. Calculate the impulse of the force if the collision time was 0.01s.

So... Isn't

I = = 120Ns.

But my book says its = 12Ns, Impulse doesn't change? But the time changed... So shouldn't the impulse also change?

the seatbelt only exerts the force for 0.001s, as the question states.

last one: Three moons around planet X have masses M, 4M and 9M
If the distances of these moons from the planet centre are R, 4R and 9R respectively, calculate the ratio of their orbital speeds.

(notice how the masses of the moons don't actually account for anything

[TrueTears]

thanks mao and table,

and mao for that seatbelt question, how do you work out the impulse? If you don't know the force exerted for the 0.01s...?

[Mao]

thanks mao and table,

and mao for that seatbelt question, how do you work out the impulse? If you don't know the force exerted for the 0.01s...?

impulse is related to force, rather than measuring momentum [the overall motion of bodies], it is a measure of how a force contributes to motion.

In this case, the force exerted by the seatbelt is only applied for 0.001 seconds, hence the impulse of this force is 12Ns. It does not concern with what happens to the passenger for the duration of the collision, but only this force itself. The sum of impulse of all forces is the net change in momentum, an impulse is specific to a force, much like how work is specific to a force also. [An example of this is pushing someone down a hill, you do work on that person, gravity also does work on that person, both you and gravity contribute to the overall motion, but neither are concerned with what the other force is doing, or what the final motion will be]

[TrueTears]

ah k thanks mao, so when doing impulse the time is how long the FORCE is applied and it has nothing to do with how long the COLLISION was, so the collision could have been 10 seconds, but the force was only applied for , then impulse would still be 12Ns?

[Mao]

ah k thanks mao, so when doing impulse the time is how long the FORCE is applied and it has nothing to do with how long the COLLISION was, so the collision could have been 10 seconds, but the force was only applied for , then impulse would still be 12Ns?

correct, the impulse of the force would still be 12Ns.

[TrueTears]

nice thanks mao XD

[Over9000]

good thread you got going truetears, so pr0

[MattPritchard]

May sound like a stupid question but how did you get from the 2nd line to the 3rd?
Also what does the symbol mean?

[TrueTears]

"proportional to"

in this case because its 1 on something, it will be inversely proportional to

[Flaming_Arrow]

May sound like a stupid question but how did you get from the 2nd line to the 3rd?
Also what does the symbol mean?

means proportional, in this case it means inversly proportional

[Flaming_Arrow]

"proportional to"

in this case because its 1 on something, it will be inversely proportional to

damit beaten

[MattPritchard]

thanks guys for the speedy answers

[TrueTears]

Which one of the following characteristics is common to both projectiles and circular motion

A: the acceleration is always constant and in the same direction
B: the acceleration is at right angles to the direction of motion
C: The magnitude of the acceleration is constant
D: the acceleration is proportional to the square of the instantaneous speed

I said the answer was A, is that right?

Thanks

[kamil9876]

C.

A is wrong because the acceleration direction does change! remember that the acceleration is in the opposite direction of and obviously position is not constant in circular motion.