Kinetic Friction on a Sliding Block

[collegelife101]

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Hello everyone!

After a block began to slide, how did its speed vary with time? (Assume the tension and kinetic friction forces on the block were constant in magnitude).

Since they mentioned that the forces were equal in magnitude, I assumed that fnet was zero and that the velocity was constant. However, the problem mentions that the coefficient of friction is always lower than that of static friction. Therefore, there is a net accelerating force on the block once it starts to slide, producing a constant acceleration, making the velocity increase with time.

Can someone please explain this? Why mention that the two forces are equal? It just really confused me.

Thanks!

 

[tdod]

it seems to me that the force of tension and kinetic friction are equal in magnitude.. The force of gravity is greater than the combined net force of tension and kinetic friction. Of course, this is hard to answer given that we don't know the direction of the tension....

 

[collegelife101]

Sorry there was a diagram included in the problem, with a sliding block on a horizontal surface attached a string. This string was hooked to a smaller mas which hung over a table along with a pulley (I think that's what they're called). So even if the Fnet on the block is zero, do we assume that the block still slides because of the force of gravity pulling it down? It just seemed to conflict with Fnet= 0, implying constant velocity.

Thanks!

 

[DrknoSDN]

The classic example is if you imagine trying to slide a heavy box along a floor.

At first when you are pushing it doesn't move... Once you push hard enough it breaks free and begins to slide easily.
This is because at first you are pushing against the force of static friction. Once the box starts sliding you only have to overpower the force of kinetic friction (which is always smaller) so it slides more easily once it gets going.
*Example taken from ExamKrackers audio osmosis.

You can actually test this with any object lying around to get a very clear demo of how a stationary object can resist more force than it can after it starts sliding. =)

 

[NextStepTutor_1]

So, what might have gotten you confused is reading that force of kinetic friction was equal to force tension. If that is the case then, yes, there would be no net acceleration. However, if they mentioned force of static friction is equal to force tension, then like those above me said, the force of static friction is always greater than that of kinetic friction. Therefore, after the block overcomes static friction it will accelerate.

 

[collegelife101]

Thank you for your response. I'm still a little confused because the problem did state that the kinetic friction force and tension force were equal in magnitude, but that there was still a net acceleration. Why would this be the case? Wouldn't there be no net acceleration in this case?

Thank you.

 

[DrknoSDN]

Gravity can accelerate at 9.8 m/s... If the friction force is less than the force of gravity then there will be a net acceleration in the direction of the pulley. The object will be accelerating (slower than gravity).

The force of friction is still equal to the tension force but less than the force of gravity.

 

[mehc012]

Sorry there was a diagram included in the problem, with a sliding block on a horizontal surface attached a string. This string was hooked to a smaller mas which hung over a table along with a pulley (I think that's what they're called). So even if the Fnet on the block is zero, do we assume that the block still slides because of the force of gravity pulling it down? It just seemed to conflict with Fnet= 0, implying constant velocity.

Thanks!

It NEVER states that they are equal, only that they are CONSTANT.

If the block is moving, than gravity was a stronger force than the tension and the static friction combined, otherwise it would not have started sliding.
Since kinetic friction forces are smaller than maximum static friction, we KNOW that gravity will continue to be larger than tension + friction.

Thus, there is a downramp acceleration and speed increases.

 

[Czarcasm]

The typical example given is you trying to push say an oven into your home. You keep pushing, harder and harder with increasing force, but each time it does not budge. The (static) friction exactly balances the force you apply each time. Eventually, with just enough force to overcome the maximum of force of the opposing (static) friction, the oven budges and begins moving. At this point the two forces of consideration is the force you are applying to the object and the opposing (kinetic) friction. This might be more intuitive if you recall ever pushing a heavy object. It's more difficult to push a heavy object initially at rest, but once it starts sliding, it's a little easier.

In this problem though, in order for the object to begin moving at all, we'd need to overcome the static friction. If you somehow applied enough force to equal the maximum static friction - the object would not move. It would have a net force of zero but remain at constant velocity (it this case zero). When you apply just enough force to overcome it, then there is a net force and the object accelerates. So whether or not you really knew kinetic friction was less than static friction, you should still have been able to figure out it's accelerating and therefore, increasing in velocity over time.