# Need some help with acceleration.

• vandorin
In summary, a car moving 100km/h bumps into the rear of another car traveling in the same direction at 98km/h, and the impact speed is two more than the already going speed.f

## Homework Statement

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)?

Im not sure

## 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 I am not sure that's right.

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? 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.

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)

[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 Last edited:
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)

[Hint: Acceleration isn't measured in Newton]

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

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. 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?

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 … ? 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?

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! Awesome! Thanks for the help!

You're very welcome! ( … if aok now, click on "Thread Tools" to mark the thread [SOLVED] … )