Can Anyone Help Me With These Physics Q'?s

Duffman'05

Ok so physics is mandatory on the new 3rd level course I have just started but I've never done it before:o

Traveling at a speed of 13.0 m/s, the driver of an automobile suddenly locks the wheels by slamming on the brakes. The coefficient of kinetic friction between the tires and the road is 0.620. What is the speed of the automobile after 1.46 s have elapsed? Ignore the effects of air resistance.

Totally lost:o

Three uniform spheres are located at the corners of an equilateral triangle. Each side of the triangle has a length of 1.41 m. Two of the spheres have a mass of 2.26 kg each. The third sphere (mass unknown) is released from rest. Considering only the gravitational forces that the spheres exert on each other, what is the magnitude of the initial acceleration of the third sphere?

I tried to work out the effect of gravity on the 3rd sphere and then get the resultant force using pythag but had no luck..

If anybody can instruct me as to how to approach these questions I would be incredibly grateful! I'm perplexed!

Thank you in advance!

bnt

Looking at the second part first: basic trigonometry should do it, since you know the direction the 3rd sphere will travel (straight between the other two), so you can find the component of one of the forces in that direction. The resultant force will be double that, since it's a symmetrical system. Drawing a diagram with the angles should help.

edit: I first thought you'd need the mass or weight of the vehicle, but you don't. You know the downwards acceleration of the vehicle (gravity = 9.81 m/s²), so multiply that by the friction coefficient to get the deceleration of the vehicle (a negative acceleration), then use the standard equations of motion.

Pamplemousse19

I was stuck on these problems earlier on today. We should really set up something on this for help with Wileyplus problems! Here's a useful link that shows you how to do both of these problems :
http://www.zoklet.net/bbs/showthread.php?p=1187749

bnt

I took a look at that forum, and there's one problem: harry_hardcore_hoedown made a mistake on the gravitation formula - he didn't count both masses. It should be
[latex]\displaystyle F=G\frac{m_1 m_2}{r^2}[/latex].

In this case m1 and m2 are the same, but you still have to count them both. So you use this formula to find the force between the moving sphere and each of the fixed spheres, and then it's just as case of resolving the 2 force vectors in to their vertical and horizontal components.

Pamplemousse19

bnt wrote: »

I took a look at that forum, and there's one problem: harry_hardcore_hoedown made a mistake on the gravitation formula - he didn't count both masses. It should be
[latex]\displaystyle F=G\frac{m_1 m_2}{r^2}[/latex].

In this case m1 and m2 are the same, but you still have to count them both. So you use this formula to find the force between the moving sphere and each of the fixed spheres, and then it's just as case of resolving the 2 force vectors in to their vertical and horizontal components.

I think it's a different formula. I'm new to physics as well but I looked it up in the log tables and the formula he used seemed to be gravitational acceleration which is g=M/d^2. d^2 just seems to be the same as r^2.

bnt

Pamplemousse19 wrote: »

I think it's a different formula. I'm new to physics as well but I looked it up in the log tables and the formula he used seemed to be gravitational acceleration which is g=M/d^2. d^2 just seems to be the same as r^2.

Ah - OK, I see what you mean. It's a valid formula only when one sphere is fixed, as it is in the question, so it'll give the same result. Take the formula I used, divide both sides by the mass of the moving sphere, and you get its acceleration. The typical application for this formula is when M is something huge and relatively immobile, like the Earth. :cool: