# Difference between static friction and kinetic friction -help please

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#1
I don't understand the difference between static friction and kinetic friction.
I understand that the force of frictional force acting on a stationary object is the static frictional force and we use the coefficient of static friction to determine the minimum force needed to start the object moving, but once the object is moving, the frictional force can be static and kinetic and I don't understand how to differentiate between the two cases.
For example:
When car accelerates gently to a higher speed, the frictional force acting on it is static friction, why ? Why not kinetic ?
If a car "floors it" and peels out of a stop light, the friction is kinetic, why ?
A car takes a turn gently. Static. Again, how and why ?
0
9 months ago
#2
so is the block/car with hand-brake on that you are pulling with that big rope moving yet?

No, then that is static friction, so pull harder!!!

if it starts to slide, then it is kinetic friction

(there are bound to be a few more subtleties!)
0
9 months ago
#3
acceleration, that's an frame of reference problem, so dv/dt & friction was constant, but with a sudden impulse extra energy input, either the car starts to move faster or the wheels spin (if it is wet, old tyres etc)
0
9 months ago
#4
(Original post by Leah.J)
I don't understand the difference between static friction and kinetic friction.
I understand that the force of frictional force acting on a stationary object is the static frictional force and we use the coefficient of static friction to determine the minimum force needed to start the object moving, but once the object is moving, the frictional force can be static and kinetic and I don't understand how to differentiate between the two cases.
For example:
When car accelerates gently to a higher speed, the frictional force acting on it is static friction, why ? Why not kinetic ?
If a car "floors it" and peels out of a stop light, the friction is kinetic, why ?
A car takes a turn gently. Static. Again, how and why ?
Its about how the objects interact when in contact, in stationary condition more chemical bonds form the the objects becomes more 'fitted' together meaning an increased force is needed to create seperation
0
9 months ago
#5
there MUST be youTube explanation somewhere from a geek on this....
0
9 months ago
#6
Put an object on a ramp then slowly lift it up to the point that it just starts to slide by gravity. Mark the height.

Then raise the ramp (without the object on) to set height and place the object on once at a set height, the object will accelerate downwards at a lower angle when you just place it on before lifting the ramp, this is due to the time the objects have had to interact at level where small geometrical changes to the objects surfaces can interact and hence more bond energy needs to be broken to overcome the force of friction.

There are also more forces at play in more complex models but its this principle.
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