Physics questions

[appianway]

Here's a good example of the directions of static friction.

Imagine a particle undergoing uniform circular motion whilst on an incline (ie it spins around a cone). Resolving forces in the direction of the radius gives a component of the normal force and a component of the static frictional force (as the object does not change its radius, there must be a static frictional force acting parallel to the plane) acting as the centripetal forces. However, what direction is the frictional force in?

Consider the scenario where the centripetal force is too small. The particle would move outwards (to increase r), and as the static frictional force would oppose this motion, it would be directed inwards. This would correspond to a large angular velocity as the N force would be too small. However, imagine the opposite. If the centripetal force was too large, the particle would move inwards, meaning that the frictional force would be directed outwards! This would correspond to the particle having a small angular velocity. In other words, the static frictional force could act in either direction, allowing for both a maximum and minimum angular velocity. 

[TrueTears]

STATIC(literally, still...motionless) friction: motion is NOT occuring, being prevented by the static friction.


KINETIC(literally, in motion) friction: motion is occuring but the magnitude of it is lessened by the kinetic friction.

thanks i kinda get it, so like, if the kinetic friction equals the force that causes motion, then does it become the static friction?

Here's a good example of the directions of static friction.

Imagine a particle undergoing uniform circular motion whilst on an incline (ie it spins around a cone). Resolving forces in the direction of the radius gives a component of the normal force and a component of the static frictional force (as the object does not change its radius, there must be a static frictional force acting parallel to the plane) acting as the centripetal forces. However, what direction is the frictional force in?

Consider the scenario where the centripetal force is too small. The particle would move outwards (to increase r), and as the static frictional force would oppose this motion, it would be directed inwards. This would correspond to a large angular velocity as the N force would be too small. However, imagine the opposite. If the centripetal force was too large, the particle would move inwards, meaning that the frictional force would be directed outwards! This would correspond to the particle having a small angular velocity. In other words, the static frictional force could act in either direction, allowing for both a maximum and minimum angular velocity. 

thanks for the help, but i dont really get that at all, my fundamental knowledge for physics is very bad coz i dont rly have much interest in it so i dont understand how things actually work, i just memorise the formulas

[TrueTears]

also in my book it says rolling friction acts in the direction opposite to the motion, but how is this possible? in the diagram attached, the red arrow is the force of the wheel on the ground, and its acting left, and the green arrow is the ground acting on the wheel (newtons third law) but this green arrow points in the direction of motion? ITS NOT OPPOSITE???!

[TrueTears]

any ideas??

[TrueTears]

what i dont get even more is that the f_c on the car's free body diagram is the rolling friction to the left, WHY DOES IT POINT LEFT? i thought friction was the force of a surface on the object, so the tires of the car is pushing the ground to the left so shouldn't the rolling friction be to the RIGHT??

furthermore, for the truck, the book says the f_T is a propulsion force, since the tires of the truck push the ground to the left, the ground pushes the truck to the right? THEN WHERE THE FUK IS THE ROLLING FRICTION?? SHOULDN'T IT BE TO THE LEFT? (according to the car's free body diagram)

OMG SOMEONE PLS HELP IM FKN SOO CONFUSED!

[/0]

You don't really have to understand rolling friction in detail.
Basically if you deform something like a tyre then it will lose more energy than it gains when it reforms again (because it doesn't obey hooke's law completely). The continuous deforming and reforming of the tyre consumes energy.


For the FBD I think they left out the rolling friction from the truck... is the static friction pushing the truck forward.
For the car, the rope and the static friction pushing the car forwards are essentially the same thing, but the rope perhaps the more important reason

[TyErd]

Hey TrueTears what book do you use??

[Gloamglozer]

Hey TrueTears what book do you use??

He's most likely using a uni physics book. 

[TrueTears]

You don't really have to understand rolling friction in detail.
Basically if you deform something like a tyre then it will lose more energy than it gains when it reforms again (because it doesn't obey hooke's law completely). The continuous deforming and reforming of the tyre consumes energy.


For the FBD I think they left out the rolling friction from the truck... is the static friction pushing the truck forward.
For the car, the rope and the static friction pushing the car forwards are essentially the same thing, but the rope perhaps the more important reason

oh okay, so they left out the static friction pushing the car forwards for the first FBD and they left out the rolling friction for the truck FBD?

[/0]

Yeah they left out the rolling friction for the truck

But the 'rope' is like the 'driving force' for the car. Instead of getting the engines to turn the wheels, the rope is doing it.

Here are the two scenarios:

Engine -> turns wheels -> static friction propels car
Rope -> propels car -> static friction turns wheels

The point is that the rope is the thing moving the car. The wheels just turn as a consequence of the rope pulling.

[TrueTears]

thanks, also u know how E_system = E_mechanical + E_thermal for the conservation of energy of an isolated system, would E_mechanical = Kinetic+Potential + W_external

where W_external is the work done on the system by external forces? Or is that not a mechanical energy?

[/0]

I think technically



Since it describes an isolated system.

[TrueTears]

Then how come in my book it says "another way of putting the conservation of energy is E_i + U_i + W_external = E_f + U_f + change in E_thermal"

lolz

[/0]

I'm not really sure what that's all about. How can that be conservation of energy if you're got an external force?

Conservation of energy is defined for isolated systems, and it's for total energy, not just mechanical energy.

The law of conservation of energy is an empirical law of physics. It states that the total amount of energy in an isolated system remains constant over time (is said to be conserved over time).

[TrueTears]

hmmm okay, ill just memorise both formulas then... and use them whenever haha