1,000,000 Question Thread :D

[kenhung123]

Ok so just to confirm, all those gravitation formulae including Kepler's don't apply when the object is not in a circular orbit?

[appianway]

Kepler's laws also apply for elliptical orbits. The gravitational force is still going to be given by the formula that you've already seen, but the angular velocity won't be constant due to the conservation of angular momentum.

[kenhung123]

Oh thats interesting..Thanks for that

[kenhung123]

Excuse the crappy diagram but I have a question:
1a.The speed of the ball when the pitcher threw it was 10m/s, which direction shown would the ball go when struck by the batter?

b. If the ball was 250g, what is the impulse on the bat?

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A bicycle accelerates from rest, covering 16m in 4s. THe total mass of the bicycle and its rider is 90kg. What is the average acceleration?

[kenhung123]

A cyclist pushes on her pedals with a force F. She doubles her cadence (the rate at which the pedals rotate). Due to a tailwind, she does not need to change her force on the pedals. What has happened to her power output?

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A truck of mass 3t collides with a stationary car of mass 1t. They lock together as they move off. Immediately after the collision the car speed was 7m/s

Use calculations to show whether the collision is elastic or inelastic

[kenhung123]

In a lab class at school, Lee is given a spring with a stiffness of 20N/m and an unstretched length of 0.4m. He hands it vertically and attaches a mass of 0.4kg to it. THe new length of the spring is 0.6m

Lee pulls the mass down a further distance of 0,1m By how much has the potential energy stored in the spring changed?

I did 0.5x20x0.3^2=0.9J which is wrong?

[jasopan]

What is an ISOLATED SYSTEM?

[kenhung123]

According to heinemann, it is where only action and reaction (internal) forces apply to the system, no external forces. So on earth we always have Fg, it isn't possible for any collision to be in isolated system I think.

Can someone please answer my questions?

[Cthulhu]

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A truck of mass 3t collides with a stationary car of mass 1t. They lock together as they move off. Immediately after the collision the car speed was 7m/s

Use calculations to show whether the collision is elastic or inelastic

An inelastic collision occurs when Kinetic energy is NOT conserved i.e
Now assuming the speed of the car after the collision is actually the speed of the car+truck system when they are locked together we can calculate the initial velocity of the truck before the collision

So and thus

Let I assume Tonnes?







Now the kinetic energy of the truck before the collision is:

Kinetic energy after the collision is:

Converting Tonnes to kilograms.
So the collision is inelastic.
I think this is right anyway.

[kenhung123]

Wow, thanks a lot! That helps!

[jasopan]

Calculate the altitude of a satellite in a geo-stationary orbit.
Data G=6.67x10^(-11) Nm^(2)kg^(-2)
Me=6.4x10^(24) kg
Re=6.4x106(6)

[appianway]

The easiest way to do it would be using angular velocity, but since you don't do that in VCE, take the period as being the same as that of the earth. The centripetal acceleration will be the same as the gravitational acceleration. The centripetal acceleration will be 4piR/T^2 or something like that, and the gravitational acceleration will be GM/R^2. Equate

[jasopan]

Thanks I got it before haha (i did normal R^3/T^2 = Gm/4pi62 or something) then equated r to R+Re and solved.

Another one

This is the third part of a big q

Tony has a mass 70 descending at 30 to the slop, the a downwards is 4.
After 6 seconds tony runs onto some soft snow and comes to a rest over a distance of 45m.
Calculate the magnitude of the average retarding force that brings Tony to a rest. in N.

[Cthulhu]

uhm... Units?

[cohen]

I've been trying to figure this out for a while, and have no idea (unfortunately :/ )

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A 160g cricket ball is dropped from a height of 2.0m onto a hard surface. Assume that 32% of the kinetic energy of a 160g cricken ball is stored in it as it bounces on a hard surface.
Calculate:
[b]a) The kinetic energy of the ball as it hits the ground[/b]
which i've calculated ~3.1J.
[b]b) The maximum ammount of elastic potential energy stored in the ball.[/b]

I have no idea how to do b, and my teacher has barely explained energy to us. (he only tells us the formulas and the unit)
Do any of you guys have any idea of how to do it?