Circular Motion / Springs

TheSetMiner

---

A car travels at constant speed around a horizontal circular corner of radius 5 m.

(a) Given that the car just starts to skid if its speed is 12 km/h, find the frictional force acting on the car.

I am stuck on this circular motion question. I feel like I am missing something obvious. I have used F= (mv^2)/r to find an expression in terms of m, and I could then use F=umg to get u, the coefficient of friction but I don't see how I can get the frictional force without first being given the mass.

---

A particle of mass 1 kg is attached to one end of an elastic spring of natural length 1 m and modulus of elasticity 50 N. The other end is fastened to a point on a smooth horizontal table. If the spring and particle describe circles on the table at 60 revolutions
per minute, find the extension of the spring.

Is there a formula for use with this? i had intially thought it would involve equating mrw = kx but it didn't seem to work out so I was a little dumfounded. Can anyone point me in the right direction?

---

If anyone can help me with either of those I will greatly appreciate it!

ray giraffe

You might get a quicker answer on the Leaving Cert forum:
http://www.boards.ie/vbulletin/forumdisplay.php?f=107

TheSetMiner

I reckon this is college material though, unless things have changed on the LC syllabus.

Ciaran

The mass should cancel out in the first one.

Your initial thought looks right for the second one. What values are you putting in for the variables? Look at the units on the modulus of elasticity, maybe.

TheSetMiner

Ciaran wrote: »

The mass should cancel out in the first one.

Your initial thought looks right for the second one. What values are you putting in for the variables? Look at the units on the modulus of elasticity, maybe.

While the masses do cancel out, I don't see how that helps in getting the frictional force when in order to get that I will need to multiply the mass by the acc.

That helps to calculate the coefficient but that isn't what is asked. Is it possible without having been given the mass because cancelling the masses really doesn't seem to help!

---

units are N/ m^2.

I ran into another formula here which might actually help: w = sqrt(elasticity/(mass*length)) where w is not angular velocity but
is equal to sqrt(k/m), I think... but I still don't see how the frequency ties in aswell as this. I am aware f = 1/2pi * w and tried throwing in all the figures but it didn't balance so I am very confused. Anyway I've probably left this too late, my own fault.

If there is anyone there with any ideas, I'd love to hear them! Thanks

Ciaran

Sorry, I didn't read the first question properly . You're right, to get the force here you need the mass. Looks like a mistake in the question, I'd guess that the coefficient of friction is what they really wanted.

Do you have that the radius of the circle is the natural length plus the extension in the second one? I get a reasonable equation, my only problem is that I can't remember how to relate the modulus of elasticity to k. I think it's equal to k*l but I can't remember and googling hasn't given consistent answers.

I hope that's more helpful!

TheSetMiner

thanks for replying. beginning to come to the conclusion that the questions are poorly put together alright! despite not having a clue how to do the second question, I've learned how to do a lot of other similar type of questions just from researching it so I guess it isn't all bad. Anyway thanks for the help!