Math Is Fun Forum

Toast

Real Member

Registered: 2006-10-08

Posts: 1,321

Moar Physics!

1. If a force is applied to a 2kg mass, which is initially at rest on the frictionless surface, for 3 seconds and then removed, what is the speed of the object after 6 seconds?

Do I have to just calculate for the first 3 seconds? For the remaining 3 seconds does the speed remain constant?

2. The brakes of a 1.2 tonne car exert a stopping force of 2500 N. How long would it take the car to stop if the brakes were applied when the car was travelling at 15 ms-¹.

My working:

Ok, I think that's correct, but... I have a question:
1. If the friction is 0, will the car even be able to break?
2. If the answer to 1. is no, then what is the correct level of friction that will result in a stopping time of 7.2s?

3. A trolley pulled down a friction-compensated slope by a cerain force has an acceleration of 1 ms-². When the mass of the trolley is doubled and the pulling force trebled, what is the new acceleration, assuming that an adjustment is made to friction-compensate the slope again?

, hence,

right?

Thanks, I have a physics test on monday and I want to clear up a few things

Last edited by Toast (2007-03-03 00:28:18)

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: Moar Physics!

1. I think the magnitude of the force needs to be given in order to get the answer (unless you want the answer to be in terms of the force). But yes, once the external force is removed the object will no longer accelerate.

2. I am not an expert on physics but I’ll take a swipe at it here.

1. No.
2. The friction has to be 2500 N. This is equal to μR, where μ is the coefficient of kinetic friction of the road surface and R is the reaction of the road on the car, which is equal to the weight of the car, which is 1200g N, where g is the acceleration due to gravity. Hence 2500 = μ(1200g) ⇒ μ ≈ 0.21 (taking g to be 9.81 ms[sup]−2[/sup]).

3. Looks fine to me. (But then, as I said, I’m no physics expert.)

Last edited by JaneFairfax (2007-03-03 04:20:02)

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Moar Physics!

I agree with Jane.

You could technically say that although friction is 0, the car would still be able to stop in other ways. For example, if it had a parachute then it could stop using air resistance in the same way as a drag race. Or it could aim towards a hill and lose kinetic energy that way (although the lack of friction would make it slide down again). But yes, for all normal situations, you can't stop on a frictionless surface.

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Why did the vector cross the road?
It wanted to be normal.