# Physics question

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• #612558
000646
Participant

I am trying to learn basic Physics and am learning about applying Newton’s Laws in 2 and 3 dimensional motion. I really know anyone who knows this stuff and I know that a lot of smart people read the stuff here sometimes.. I am hoping maybe someone can help me out:

Did I do this correctly? If not where am I wrong?

Thanks!!

#1011372
Epis
Participant

What is the mass of you car? That is a major factor. According to netons 3rd law, every thing has opposite and equal force. The reason why you don’t fall through the floor is because of this opposite and equal force or better known as the NORMAL FORCE. To fid your normal force, you multiply your mass*gravity (mg). So that is what you are missing the mass of the car.

#1011373
000646
Participant

epis,

Right?

#1011374
Participant

I got 27.3 MPH but that’s because I started out by converting everything to SI units. I think your error is in approximation too early in the equation.

#1011375
Epis
Participant

Your righ I over thought the example.

#1011376
000646
Participant

Dash,

I’m not sure I understand what you mean…

#1011377
Epis
Participant

A 4.20*10^3-kg rocket takes off from the moon where the acceleration due to gravity is 1.60 m/s^2 by exhausting 8.20 kg of gas per second at a speed of 2.20 103 m/s. Determine the acceleration of the rocket.

#1011378
000646
Participant

If you mean it went from 2.2 m/s to 103 m/s while it burned gas for 1 second. Then it’s accelerating at 100.8 m/s. I’m not sure I understood your question though

#1011379
Participant

I’m not sure I understand what you mean…

I went back and looked at what you did and came up with the following

#1011380
Epis
Participant

000646, it is supose to be 2.20*10^3 m/s

#1011381
Avram in MD
Participant

Dash is correct – you have to make sure all of your units line up, which is best done by showing all work.

Newton’s Second Law: F = ma

The gravitational acceleration is g = 9.8 m/s

Centripetal acceleration is a = v^2/r

So to keep that car on track, you want to balance the centripetal acceleration with gravitational acceleration, so: 9.8 = v^2/r

Your r is known: 50ft, which is 15.24m, so v^2 = 9.8 * 15.24, and v = sqrt(9.8 * 15.24) = 12.22 m/s.

Converting 12.22 m/s into mph and you get 27.3.

#1011382
Avram in MD
Participant

Whoops, in my above post, the units for acceleration should be m/s^2, not m/s ðŸ™‚

#1011383
000646
Participant

Dash and Avram in Md

Thanks!

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