# Need help in understanding friction

Clockclocle
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?

Homework Helper
Gold Member
2022 Award
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!

• Lnewqban
Homework Helper
Gold Member
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.

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).

Homework Helper
2022 Award
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.

Clockclocle
No, because friction is a resisting force and there is friction in both directions!

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 ?

Gold Member
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.

• PeroK
Clockclocle
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

Homework Helper
Gold Member
2022 Award
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!

Homework Helper
Gold Member
2022 Award
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.

• hutchphd and Lnewqban
Clockclocle
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?