static friction + acceleration

[pizza1994]

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can someone explain how exactly does the fact that static friciton can cause acceleration work? This was something I read in the BR books but did not understand.

 

[Czarcasm]

can someone explain how exactly does the fact that static friciton can cause acceleration work? This was something I read in the BR books but did not understand.

I'm not entirely sure, but I believe sliding friction is kinetic whereas rolling friction is static and so, for a ball rolling down an incline for instance, with a net force in the form of static friction acting on the ball (F=ma), it imposes an increase in velocity on the object (due to its net acceleration).

 

[pizza1994]

I'm not entirely sure, but I believe sliding friction is kinetic whereas rolling friction is static and so, for a ball rolling down an incline for instance, with a net force in the form of static friction acting on the ball (F=ma), it imposes an increase in velocity on the object (due to its net acceleration).

im not sure myself but I was reading that when you are walking its the static friction that accerlates you forward and it has to do with action-reaction forces. not sure! I dont see how static friction can accelerate an object.....also how is rolling friction the same as static friction?

 

[DrknoSDN]

im not sure myself but I was reading that when you are walking its the static friction that accerlates you forward and it has to do with action-reaction forces. not sure! I dont see how static friction can accelerate an object.....also how is rolling friction the same as static friction?

Rolling friction (tires) is static because the surfaces are not sliding along one another. The purpose of tires is to prevent sliding. When you slam on the breaks the tires stop rotating but start sliding along the ground (kinetic friction).

 

[pizza1994]

Rolling friction (tires) is static because the surfaces are not sliding along one another. The purpose of tires is to prevent sliding. When you slam on the breaks the tires stop rotating but start sliding along the ground (kinetic friction).

I still dont get it. when you slam the brakes then its kinetic friction which makes sense. But when the tires are rolling on the ground then how is that static friction...cause in order for you to roll then dont you have to overcome the static friction at rest? so i dont get how rolling means static

 

[orangetea]

I still dont get it. when you slam the brakes then its kinetic friction which makes sense. But when the tires are rolling on the ground then how is that static friction...cause in order for you to roll then dont you have to overcome the static friction at rest? so i dont get how rolling means static

Just think of it this way

Static friction- the friction you need to get over to start moving(walking for example is overcoming static friction)
Kinetic friction- while you are moving the friction that makes you stop(so when a ball is rolling down a hill and it encounters a rough patch or something).

🙂

 

[DrknoSDN]

Just think of it this way
Static friction- the friction you need to get over to start moving(walking for example is overcoming static friction)
Kinetic friction- while you are moving the friction that makes you stop(so when a ball is rolling down a hill and it encounters a rough patch or something).
🙂

Sorta,
Static vs Kinetic does not mean moving or stationary so a rubber ball rolling down a hill is still static most of the time.

I still dont get it. when you slam the brakes then its kinetic friction which makes sense. But when the tires are rolling on the ground then how is that static friction...cause in order for you to roll then dont you have to overcome the static friction at rest? so i dont get how rolling means static

Static:
Take a an example of rubber on the soles of your shoes when you start running. If you looked super closely at the molecules you would see that each molecule of the shoe is experiencing van der walls bonding to the same molecule as force is applied and you start running forward. (The molecules are not sliding, they are locked together holding strongly)

Kinetic:
An example of a car slamming on the breaks. The tires lock up and each molecule of the tire tries to van der walls bond with molecules of the road, but the intertia/momentum keeps pulling the molecules of the tire away from the pavement molecule it was bonded to as the car continues forward.

Comparison:
If a car slows down gradually then as the tire is rotating, once a rubber tire molecule establishes intermolecular bonding with the pavement, that bonding is constant until the tire has rotated enough (15-30 degrees depending on tire air pressure), so that it lifts directly up and away from the pavement (no sliding).
It might almost be easier to think of a tire that is low on air in slow motion... For static friction If there was wet white paint on the tire, once it touched the ground it would mark the ground leaving a single spot on the ground where that part of the tire touched only that single part of pavement.... In Kinetic friction the white paint on the tire would slide across the ground leaving a white smear because the tire is sliding along the ground.. With a normal non skidding car, the tire is not sliding along the ground at all, merely rotating with each tire molecule forming a single intermolecular bond with a single pavement molecule for each rotation.

Oversimplification obviously but that's the jist. Not sliding along one another = static... Sliding along one another = kinetic.
Most importantly rolling is not equal to sliding.

If TLDR,,, and you enjoy khan academy. Video:

 

[pizza1994]

Sorta,
Static vs Kinetic does not mean moving or stationary so a rubber ball rolling down a hill is still static most of the time.



Static:
Take a an example of rubber on the soles of your shoes when you start running. If you looked super closely at the molecules you would see that each molecule of the shoe is experiencing van der walls bonding to the same molecule as force is applied and you start running forward. (The molecules are not sliding, they are locked together holding strongly)

Kinetic:
An example of a car slamming on the breaks. The tires lock up and each molecule of the tire tries to van der walls bond with molecules of the road, but the intertia/momentum keeps pulling the molecules of the tire away from the pavement molecule it was bonded to as the car continues forward.

Comparison:
If a car slows down gradually then as the tire is rotating, once a rubber tire molecule establishes intermolecular bonding with the pavement, that bonding is constant until the tire has rotated enough (15-30 degrees depending on tire air pressure), so that it lifts directly up and away from the pavement (no sliding).
It might almost be easier to think of a tire that is low on air in slow motion... For static friction If there was wet white paint on the tire, once it touched the ground it would mark the ground leaving a single spot on the ground where that part of the tire touched only that single part of pavement.... In Kinetic friction the white paint on the tire would slide across the ground leaving a white smear because the tire is sliding along the ground.. With a normal non skidding car, the tire is not sliding along the ground at all, merely rotating with each tire molecule forming a single intermolecular bond with a single pavement molecule for each rotation.

Oversimplification obviously but that's the jist. Not sliding along one another = static... Sliding along one another = kinetic.
Most importantly rolling is not equal to sliding.

If TLDR,,, and you enjoy khan academy. Video:

Oh my!!! That was absoultely amazing!!!!! I get it now!!! Brilliant!!!! Thanks so much for your help...🙂

 

[TactFR44]

Rolling friction (tires) is static because the surfaces are not sliding along one another. The purpose of tires is to prevent sliding. When you slam on the breaks the tires stop rotating but start sliding along the ground (kinetic friction).

Thank you! Excellent explanation.