Need help in understanding friction

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In summary, the friction between the object and a surface can have a maximum value, and the magnitude of the friction force is maximum when the pushing force is equal to or greater than the static friction.
  • #1
Clockclocle
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When we push an object on the surface, if I push hard enough such that it reach the maxium static friction of the object then it start moving with a constant speed and F_push = F_friction. But when I realease the object F_push immediately become zero remain only F_friction. Does the friction get smaller to static friction when the object velocity become 0 and stop moving? Why it getting smaller?, it don't behave like f_push while f_push is executed immediately after I release the object. Is there a case where the friction decrease the velocity of the object and make it moving backward?
 
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  • #2
Clockclocle said:
Is there a case where the friction decrease the velocity of the object and make it moving backward?
No, because friction is a resisting force and there is friction in both directions!
 
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  • #3
Clockclocle said:
When we push an object on the surface, if I push hard enough such that it reach the maxium static friction of the object then it start moving with a constant speed and F_push = F_friction. But when I realease the object F_push immediately become zero remain only F_friction.
The magnitude and direction of the force of static friction are a reaction to (both of them mirror) the magnitude and direction of your pushing force, according one of Newton's laws of motion.

Please, see:
https://en.wikipedia.org/wiki/Newton's_laws_of_motion#Third

In the case of friction (always a reaction), the static type can be forced to grow from zero to a maximum value (μN).
If the reactive friction is forced to grow above that limit value, movement begins to occur (reason to be called kinetic).
 
  • #4
Be aware that the massive block, when in motion with some velocity, will not stop instantly when the pushing force is removed. The will be a finite interval where the friction forces (we then call it kinetic friction) will decelerate the block until its velocity is zero.
(Kinetic friction)≤(static friction) and so once in motion the block will usually continue for a little distance. The nature of kinetic friction, because it involves complicated motions of many particles, is usually to take away energy from the objects involved. By itself, it will not spontaneously speed up the block.
 
  • #5
PeroK said:
No, because friction is a resisting force and there is friction in both directions!

hutchphd said:
Be aware that the massive block, when in motion with some velocity, will not stop instantly when the pushing force is removed. The will be a finite interval where the friction forces (we then call it kinetic friction) will decelerate the block until its velocity is zero.
(Kinetic friction)≤(static friction) and so once in motion the block will usually continue for a little distance. The nature of kinetic friction, because it involves complicated motions of many particles, is usually to take away energy from the objects involved. By itself, it will not spontaneously speed up the block.
According to you, If I hold and push the block with force F>2f (f = max static friction)then the object start accelerating? If F<=2f it move with constant velocity? and friction have maximum = 2f ?
 
  • #6
Clockclocle said:
According to you, If I hold and push the block with force F>2f (f = max static friction)then the object start accelerating? If F<=2f it move with constant velocity? and friction have maximum = 2f ?
That makes no sense and is not what was said.

Read these : static friction, and kinetic friction.
 
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  • #7
hmmm27 said:
That makes no sense and is not what was said.

Read these : static friction, and kinetic friction.
so why the object accelerate ? whenver I exert harder force then also friction cause third law of Newton. The net force always zero
 
  • #8
Clockclocle said:
so why the object accelerate ? whenver I exert harder force then also friction cause third law of Newton. The net force always zero
Well, I guess that's the end of physics then!
 
  • #9
Clockclocle said:
so why the object accelerate ? whenver I exert harder force then also friction cause third law of Newton. The net force always zero
To give you an answer. Newton's third law does not say that if you apply an external force ##F## on an object, then something else applies an equal and opposite external force on the same object and so the net external force on any object is always zero.

That's not what it's saying. That's absurd.

Newton's law says that if you apply an external force ##F## on an object, then it applies an external force ##F## on you (in the opposite direction).

If there is a friction force, then there is no law that says the fiction force is equal and opposite to the external force applied. Instead, the friction force has a maximum possible value. If an external force greater than that maximum is applied, then the object will move under the net force:
$$F_{net} = F_{applied} - F_{max friction}$$
If the friction is between the object and a table, say, then Newton's thid law says that the object exerts a force on the table of ##F_{max friction}## in the opposite direction.
 
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  • #10
PeroK said:
To give you an answer. Newton's third law does not say that if you apply an external force ##F## on an object, then something else applies an equal and opposite external force on the same object and so the net external force on any object is always zero.

That's not what it's saying. That's absurd.

Newton's law says that if you apply an external force ##F## on an object, then it applies an external force ##F## on you (in the opposite direction).

If there is a friction force, then there is no law that says the fiction force is equal and opposite to the external force applied. Instead, the friction force has a maximum possible value. If an external force greater than that maximum is applied, then the object will move under the net force:
$$F_{net} = F_{applied} - F_{max friction}$$
If the friction is between the object and a table, say, then Newton's thid law says that the object exerts a force on the table of ##F_{max friction}## in the opposite direction.
Since the friction is applied from surface to object while object moving,it has its maximum depend on what kind of surface?
 
  • #11
Clockclocle said:
Since the friction is applied from surface to object while object moving,it has its maximum depend on what kind of surface?
The maximum friction force depends on the roughness of the surfaces.
 

1. What is friction?

Friction is a force that resists the relative motion or tendency of motion between two objects in contact. It is caused by the microscopic irregularities on the surface of the objects that interact with each other.

2. How does friction affect motion?

Friction can either slow down or stop the motion of objects. It also causes objects to wear down over time due to the constant rubbing against each other.

3. What factors affect the amount of friction?

The amount of friction is affected by the types of surfaces in contact, the force pressing the surfaces together, and the speed at which the surfaces move against each other. Rougher surfaces, higher force, and faster speeds result in higher friction.

4. How can friction be reduced?

Friction can be reduced by using lubricants, such as oil or grease, to create a smooth layer between the surfaces in contact. Another way is by using rolling instead of sliding motion, as rolling causes less friction than sliding.

5. What are some real-life applications of friction?

Friction plays a crucial role in many everyday activities. It allows us to walk, write, and grip objects. It is also essential in transportation, as it helps vehicles to stop and turn. Friction is also used in machines, such as brakes and gears, to control and adjust motion.

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