# Grade 11 Physics - Dynamics

1. Apr 18, 2005

### 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.

2. Apr 18, 2005

### 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.

3. Apr 18, 2005

### 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

4. Apr 18, 2005

### 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

5. Apr 18, 2005

### OlderDan

Yes, that would be the correct conclusion

6. Apr 18, 2005

### Slats88

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

7. Apr 18, 2005

### 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?

8. Apr 18, 2005

### futb0l

I think that's exactly right.

9. Apr 18, 2005

### Slats88

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

10. Apr 18, 2005

### 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.

11. Apr 18, 2005

### Werg22

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.

12. Nov 23, 2009

### igforce

Im actually surprised why you guys cant answers this, the answers are simple

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

Thats because in order to increase the speed of an object you need to add more force to change its momentum, but you will need less force to keep the momentum at a constant.

2. The space arm works in weightless conditions (actually called microgravity conditions). Why must it still be made strong?
it must be made strong because since there is no gravity and the space arm moves in one direction, it will need a lot more force to stop itself moving or to move in the opposite direction, if it isn't strong then it will break or bend.

3. Albert Einstein, in his special theory of relativity, predicted 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.
In his theory, it is an idea that light has no mass, or in other words has very little mass that the lightest small push, will make it move really fast, and everything else is heavier than light or has more mass which tend to become slower. However i dont belive in this i actually belive that you can reach any speeds as long as its not infinity, or teleportation, i belive its possible for humans to reach a speed of light in a spacecraft that would accelarate to the speed of light with less than 50Gs cause thatch the amount of G a human can survive on.

4. Why does oiling the contact areas between two surfaces reduce the amount of friction between those surfaces?
Oil is a liquid that has the ability to flow, this is why it is "slippery" because it moves around instead of staying in one place, which would create friction, now it still has friction because there is always viscosity, but it definitely is better than contact between 2 solid 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.
Friction increases not when you use a sanding machine to make a surface flat, this will never happen. a normal surface that appears to be flat will actually look like the rocky mountains if you look through an electron microscope. this type of surface will actually reduce the amount of resistance or friction. But if you ever manage to get a surface so flat that you can see just a plain flat surface in an electron microscope, this is when the friction increases. The reason to why this happens is because the atoms between the two contact surfaces will actually bond together when this happens. This is currently being used in fiber optics. They do this by using a special type of cutter that cuts the fiber optics into an ultrafine particle line that when you just push the 2 ends of the fiber optics together, they will stick together as if they had never even been cut.