in high school , i solve friction questions by making FBD but recently i got a doubt while imagining a situation
suppose there is a block of mass 4kg kept on ground and we apply a force of 20N and lets assume the limiting friction is 14N and kinetic friction is 12N , so the block will start sliding and when we make equation of motion for this block we will get
20N – 12N = 4a
a = 2m/s²
so the block will have acceleration 2m/s²
but if we think of what will happen after our force, the block will stop after sometime
so what does actually happen after this 2m/s² acceleration which causes the block to stop , it will be friction obviously ( if we neglect air resistance ) but does friction keep resisting the motion of block and if yes then can anyone explain thoroughly what exactly happens cause this was never taught in our class ,
thanks
In: Physics
Let’s say that you push for one second, then let go. The moment you let go, the block has a velocity of 2 m/s. Now, instead of a net force of 8N acting in the direction of its travel, you have only 12N of friction acting in the opposite direction.
That means the block is now accelerating in the opposite direction at 3 m/s² and reaching a velocity of 0 after 2/3 s.
The interaction between block and surface “doesn’t care” whether the block is pushed such that it accelerates (F > 12N in your example) , stays at a constant speed (F=12N) or slows down (F< 12N). As long as the Block moves, it will exert a set force opposed to the direction of movement.
Having just 2 types of friction is a simplification of the truth. Actually there is a nonlinear friction profile depening on the velocity.
ABS breaking systems abuse this behavior to keep the relative velocity of the tire versus the road close to the maximum friction that is pretty close to zero slip.
Friction is only a reactionary force. It helped me to think that it only exists to oppose motion, so when there is nothing trying to cause motion, there will be no force of friction. So when the block comes to rest, the static friction is zero until additional force is applied, and then it is a force up to the maximum (14N in your example). But the force of friction is 12N until the block does come fully to rest, at which point it becomes zero until additional forces are applied. Once the block is stationary, the static force is whatever it needs to be to make the block stay, up to 14N. So if you apply 10N, it will push back 10N, 11N -> 11N, etc.
I think that’s what you’re asking at least? If that wasn’t what you were asking or if you need more clarity let me know and I can try to elaborate
If you keep applying a force, the block will keep moving. If you stop applying force, the block will slow down and then stop. The reason it slows down is because there’s still kinetic friction acting opposite to the direction of the block’s motion. And when you stop applying the force, there’s nothing to work against friction. So the friction causes negative acceleration until the block becomes stationary.
Friction is essentially interactions between objects and the surface they’re sitting on. Imperfections in the bottom of the block and the surface of the floor tend to oppose the block’s motion. The rougher a surface is, the more friction you’ll get.
Whilst you are pushing you have a net force of 8N accelerating the block, from the 20 you push with and the 12 friction is resisting.
As soon as you stop pushing the only force now acting on the horizontal movement is the friction with the ground which is still 12N. So your block will slow down at a rate of 3m/s2 until it stops.
You got it right, the kinetic friction is going to apply as long as that block is in motion. That’s why if you throw a book across the floor it eventually stops.
The issue is that pushing a block across the floor is actually two different problems. The first problem is more complicated and happens when the block has a force applied to it. The second problem is often ignored because its much simpler. You’ve identified the transition from problem 1 to problem 2 and that problem 2 is different.
Friction is proportional to the amount of force applied. If you push on something harder or slide something heavier on the same surface, there will be more friction. Also, assuming that only friction is at play to slow it down. Then friction is decelerating the object just like the pushing was accelerating it. Its just an invisible push in the opposite directioj.
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