Homework Help: Need some help with acceleration.

vandorin · May 17, 2008

1. The problem statement, all variables and given/known data

1.) A brick has a mass of 2kg, how much (In Newtons) does it weigh?
If you drop it, what is its acceleration? (ignoring air resistance)?

2. Relevant equations
Im not sure

3. The attempt at a solution
I found that 1kg = 10N, so 2kg =20 newtons, and I think that if you dropped it, since gravity is 10N (rounded up, which is what our teacher told us to do) then its acceleration would be 10N, but im not sure that's right.

tiny-tim · May 17, 2008

Welcome to PF!

Hi vandorin! Welcome to PF!

Hint: Do you know Newton's second law?

If not, do you know how mass affects the way things drop?

vandorin · May 17, 2008

tiny-tim said: ↑

Hi vandorin! Welcome to PF!

Hint: Do you know Newton's second law?

If not, do you know how mass affects the way things drop?

My teacher thinks it's not that important to teach the formula's, but I do remember that F=ma , but i don't see how that applies here? Since I know that dropping the brick it would accelerate 10m/s every second.

phyzmatix · May 17, 2008

Hi Vandorin!

You're basically there:

Weight = Mass x Gravitational Acceleration

so your weight in N is approximately correct (your teacher told you to round, so you should be okay)

However, re-think your answer for the acceleration

[Hint: Acceleration isn't measured in Newton]

EDIT: By the time I finished typing this, tiny-tim was already here...
I obviously need to speed up a bit

vandorin · May 17, 2008

phyzmatix said: ↑

Hi Vandorin!

You're basically there:

Weight = Mass x Gravitational Acceleration

so your weight in N is approximately correct (your teacher told you to round, so you should be okay)

However, re-think your answer for the acceleration

[Hint: Acceleration isn't measured in Newton]

sorry for putting it in N, im still getting used to putting the right things beside the numbers. Would it be 10m/s?

tiny-tim · May 17, 2008

vandorin said: ↑

Would it be 10m/s?

Ah … you knew the answer, you just wrote it wrong!

Acceleration has dimensions of speed/time, so it's m/s².

vandorin · May 17, 2008

I've actually got one more question if you guys don't mind helping me out again :D

1.) What is the impact speed when a car moving 100km/h Bumps into the rear of another car traveling in the same direction at 98km/h

Now here I would just think that you would take 100 + 98 and then divide them by 2, giving you an answer of 99km/h?

tiny-tim · May 17, 2008

vandorin said: ↑

Now here I would just think that you would take 100 + 98 and then divide them by 2, giving you an answer of 99km/h?

No … the impact speed is a measure of how much impact you'd feel if you were in one of the cars. So it must be … ?

vandorin · May 17, 2008

tiny-tim said: ↑

No … the impact speed is a measure of how much impact you'd feel if you were in one of the cars. So it must be … ?

2? since your already going 98kg/h, the other car is going 100kg/h so when it bumps you, the impact speed is two more than your already going?

tiny-tim · May 17, 2008

Hi vandorin!

That's right … the impact velocity is simply the difference in velocities.

The importance of this is that relativity applies just as much in classical mechanics as in Einsteinian mechanics … an observer in either car is entitled to regard himself as at rest, and all the ordinary Newtonian equations will still work!

vandorin · May 17, 2008

Awesome! Thanks for the help!

tiny-tim · May 17, 2008

You're very welcome!

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