constant velocity means 0 force in this case?

[blackmi4]

yes, I understand that constant velocity means 0 acceleration and 0 acceleration means F=0 based on
F=ma. please don't repeat that equation to me...

the question is: a 50 kg box is moved across the floor at a constant velocity of 5 m/s. the
coefficient of friction between the box and the floor is 0.1. what is the net force on the block?

the answer given is 0 N "because there is no acceleration there is no force"

this doesn't sit well with me.

this is my thought: the answer is teling me if I use my hand to push a block at a constant 5 m/s along my carpet that I am not exerting a force on the block?

I think I am exerting a force.

can someone help me with this?

thank you!

---

Members don't see this ad.

 

[Lambda Phage]

yes, I understand that constant velocity means 0 acceleration and 0 acceleration means F=0 based on
F=ma. please don't repeat that equation to me...

the question is: a 50 kg box is moved across the floor at a constant velocity of 5 m/s. the
coefficient of friction between the box and the floor is 0.1. what is the net force on the block?

the answer given is 0 N "because there is no acceleration there is no force"

this doesn't sit well with me.

this is my thought: the answer is teling me if I use my hand to push a block at a constant 5 m/s along my carpet that I am not exerting a force on the block?

I think I am exerting a force.

can someone help me with this?

thank you!

Pushing =/= exerting a force

 

[TommyTman]

I could be wrong on this, but the BLOCK is the system so you exerting a force doesn't matter. What matters are the forces acting on the block. If you drew a free body diagram, the sum of forces in the X-direction would be: T-ukN where T = tension (pretend a rope is providing the force to the left) and ukN is the frictional force acting to the right. If T=ukN, then the sum of forces acting on the block would be 0.

 

[brood910]

In short,

Ftotal =/= Fyours

 

[inasensegone]

In short,

Ftotal =/= Fyours

F you too bro! 🙂

 

[brood910]

F you too bro! 🙂

 

[blackmi4]

Thank you for all the responses.
I really appreciate the time you put in.

I still don't understand though..

If I am sitting in my living room and using my hand to push a block on the floor, I still don't see how I am not exerting a force.

The forces on the block:
Fmy hand
Ffriction
Am I missing a force here?

The block is moving because Fhand>Ffriction.

Can someone explain to me, using this example, what other forces are acting on the block that result in F=0?

Thank you again.

 

[Ramzy]

Your force exerted equals the force exerted back by friction; hence, the net force is zero, box does not accelerate, and velocity remains constant.

 

[farnell]

Just wanted to add to this since you are having trouble. Think about it this way: what would happen if you applied a constant on the block on a completely frictionless surface? The answer is that the block would accelrate, because F = ma. Similiarly, in the question, the block is moving but it is NOT accelerating. In other words, it has constant velocity. This is because the force of friction (F=mg*uk) is exactly equal to the force applied by your hand. That is why you can apply a force, have the block be moving, but still have a net zero force.

 

[SOOyC52H1Sof]

You have to realize that when you are dealing with cars accelerating, or pushing blocks, dont think about the real world when you are doing these problems. In the real world, you have friction.

Suppose in a physics problem you are told a car accelerates to 60 miles per hour, and then maintains constant velocity. In a problem with that scenario, a person would not touch the gas pedal after they accelerated to 60 mph.


In the real world, air resistance and friction from the tires would slow the car down, so we normally keep the pedal down a certain amount to account for the friction.

Similarly, whenever in a physics problem you see constant velocity, remember that the derivative of the velocity (which is acceleration), if velocity is constant, is ZERO.

 

[blackmi4]

Thank you again for the replies.

I think I understand it now. I will give my explanation.

Does this sound OK? (I apologize for the caps -- it is how I hammer stuff in my brain.)


The block started at rest, but, later on, did move.

Therefore, it must have overcame static friction.

Therefore, the block must have accelerated.

Time 0: v=0m/s. Timef: v=5m/s. dv=acceleration=force.

Therefore, my hand was exerting a force. A net force.

However, the blocks velocity STOPPED CHANGING.

Why would velocity change and then stop changing? That doesn't make sense.

Something must have happened.

A force made that block stop moving faster. kinetic friction.

This doesn't mean that my hand isn't exerting a force.

When a mass hangs from a ceiling with a rope we don't stop calling tension a force or gravity a force -- so why would we stop here?

It just means that the Fhand is cancelled out because another force opposed it equally and oppositely.

The opposing force must be equal.

If it wasn't, the velocity would keep increasing -- or the block would stop moving.

The only thing that changed is that there is NO NET FORCE.

We can still say forces are being exerted. However, we can no longer say that there is a net force.

We have to recognize that net force is not so much concerned with what is happening as much as what is CHANGING with what is happening.

Don't tell me I'm not exerting a force! Something is happening! But what is happening isn't changing! No net force!

/my explanation

 

[SpaceBuff12]

The block started at rest, but, later on, did move.
Therefore, it must have overcame static friction.
Therefore, the block must have accelerated.
Time 0: v=0m/s. Timef: v=5m/s. dv=acceleration=NET force on block.

Sounds good. I've added bolded part for clarity.

Therefore, my hand was exerting a force. A net force.

You exerted a force but the force that you exerted (F_exert) is not equal to the net force since F_exert is not the only force on the block, there is also a frictional force opposing the motion of the block.

However, the blocks velocity STOPPED CHANGING.
Why would velocity change and then stop changing?

Simple. The force propelling the block changed to equal the magnitude of the opposing frictional force.

But none of that matters. Let's look at the actual question.

a 50 kg box is moved across the floor at a constant velocity of 5 m/s. the
coefficient of friction between the box and the floor is 0.1. what is the net force on the block?

Notice that the question doesn't mention anything about how the block started to move. All you know is that the block is moving at a constant velocity despite a frictional force opposing that motion.

The opposing force must be equal and opposite to the applied force.
If it wasn't, the velocity would keep increasing -- or the block would stop moving.

The only thing that changed is that there is NO NET FORCE.

I know that you are referring to the speculated conditions of moving the block from rest, but according to the question there is no change taking place, the initial conditions are irrelevant; that is the point. As you said, "there is NO NET FORCE".

We can still say forces are being exerted. However, we can no longer say that there is a net force.

Correct.

 

[Blueprint MCAT Tutor]

Just to bake your noodle:

How hard you pushed also had to change. To get the thing started, you had to overcome static friction, which has a max value higher than kinetic friction. So you had to push hard to get started, then back off a bit and just keep pushing enough to keep the thing moving.

 

[neurodoc]

Think about it this way: Suppose you have a block of mass m lying on an inclined plane (at any angle, say X degrees to the horizontal) and with some
frictional force between the plane and the block). The free body diagram will show the forces acting on the block (gravity, "g") and the frictional force.

The gravitational force (mg) will have a component of mgSinX directed down the inclined plane. The frictional force will not allow movement of the
block unless the frictional force is overcome by the component of mg in the direction of the inclined plane.

Let's assume that the block begins sliding down the plane because the plane is inclined just enough that mgSinX exceeds frictional resistance. In that case,
we need to look at the velocity of the block on the inclined plane. If the velocity is constant, there is no application of force to the block.

 

[Blueprint MCAT Tutor]

If the velocity is constant, there is no NET application of force to the block.

 

[blackmi4]

Thank you for the replies. If velocity is constant, there is no NET application of force to the block. That is what I really needed to hear.

Thank you again.

Michael