Grade 11 Physics - Dynamics

[Slats88]

If anyone could answer even just one of the questions below I would really appreciate it.

1. Why do cars get better gas mileage at constant speed than they do with a lot of accelerating?

2. The space arm works in weightless conditions (actually called microgravity conditions). Why must it still be made strong?

3. Albert Einstein, in his special theory of relativity, prediced that nothing with mass will ever be able to reach the speed of light. Explain how Newton's 2nd law (F=ma) supports Einstein's prediction.

4. Why does oiling the contact areas between two surfaces reduce the amount of friction between those surfaces?

5. Polishing the contact area between two surfaces initially decreases friction but when the surfaces are extremely smooth and flat, friction increases. Explain why.


I have a rough idea for numbers 3 4 and 5, but I want to hear some suggestions on this. Thanks in advance to anyone who helps.

[Slats88]

(sorry, forgot to post my attempts at the problems first)

For number 1 I'm assuming that it takes more gas for the car to accelerate than it does to keep a constant speed.

2. I really have no idea on this one, I barely know what the space arm is.

3. Ditto to number 2, the best idea I can come up with is that the force must be incredibly large, while the mass is incredibly small.

4. I'm assuming oil simply changes the coefficient of friction as the two surfaces are no longer rubbing against each other as the oil is between them.

5. Here I think the polishing creates more ridges and depressions in the surfaces than they initially had, increasing friction.

[OlderDan]

OK.. I'll say something about number 3. Newton's second law in the form written does not support Einstein's theory. Newton had no reason to believe mass was anything but constant for a given object, and no reason to think a large force applied to a small mass could not result in unlimited velocities. Relativity suggests that as the speed of an object increases, so does its mass, with the mass becoming infinite at the speed of light. Only with this modification does Newton's second law support Einstein's theory

[Slats88]

So would it be appropriate to say that since the mass would become infinitely large, there could never be a force large enough to accelerate the mass to the speed of light as a = F/m?

Damnit I hate these questions :cry:

[OlderDan]

Yes, that would be the correct conclusion

[Slats88]

Any thoughts on number 5? It is the last one that I haven't come up with a half-decent answer to.

[matheson]

im only a year 12 but wouldnt there be more friction simply because theres more surface contact? and the polish just creates a rougher bumpy surface maybe?

[futb0l]

5. Here I think the polishing creates more ridges and depressions in the surfaces than they initially had, increasing friction.

I think that's exactly right.

[Slats88]

Yeah I dont' think surface area is the answer (considering the formula for friction is
(Fn)u = Ff). Thanks for the help guys.

[OlderDan]

The equation you are quoting suggests that the frictional force is proportional to the normal force. Your "u" is really the Greek letter "mu" and is usually considered constant for a material of one composition sliding against another. In other words, two objects of the same weight made of the same material sliding on a horizontal surface will have the same frictional force even if the contact areas are different. Your question is aking why mu is affected by the smoothness of the surface.

For rough surfaces, there are a lot of ridges on the surfaces. There are places where the surfaces fit together, sort of like gears meshing. To slide sideaways, an object has to move up and down on a microscopic level to get over the ridges. If you knock off the ridges, sliding gets easier, but surface roughness still prevents many regions of the two surfaces from coming into contact. Very smooth surfaces have more contact. The molecules of one surface attract the molecules of the other surface, and the more surface contact, the greater the force, i.e., more friction.

[Werg22]

If anyone could answer even just one of the questions below I would really appreciate it.

1. Why do cars get better gas mileage at constant speed than they do with a lot of accelerating?

2. The space arm works in weightless conditions (actually called microgravity conditions). Why must it still be made strong?

3. Albert Einstein, in his special theory of relativity, prediced that nothing with mass will ever be able to reach the speed of light. Explain how Newton's 2nd law (F=ma) supports Einstein's prediction.

4. Why does oiling the contact areas between two surfaces reduce the amount of friction between those surfaces?

5. Polishing the contact area between two surfaces initially decreases friction but when the surfaces are extremely smooth and flat, friction increases. Explain why.


I have a rough idea for numbers 3 4 and 5, but I want to hear some suggestions on this. Thanks in advance to anyone who helps.

1.Acceleration means force. Force needs to be provided.

2.Because it needs a great force to opperate with heavy materials... Real mass do not change dependently from location, either on earth or space, if the mass is important, it needs greater force.

3.Only because of E=v^2m. Since energy with mass implies acceleration, thus it cannot reach the speed of light wich is constant, or it would be paradoxal.

4.Because oil has a little coefficiency of friction.

5.This I cannot tell why.