Questions generally about force

[Sigmarus]

Hello.So recently i have 2 questions in mind,which are

1.How does a spring balance measure the force?

2.why is the frictional force opposite the direction of movement?

 

Thanks

[DrP]

1 - there is a linear scale that measures the displacement of a marker on the spring with the load added to the hook. Known weights can be added and displacement noted and the system can be calibrated... you might be able to predict the displacement using some maths and the Young's modulus of the spring or something, but basically you are measuring the displacement caused by the load on the spring.

 

2 - "For every action there is an equal and opposite reaction" - When you push a block across a surface the resistance to the pushing is experienced as a force in the other direction. If you look at the surface of the block and that of the table or floor it will be rough at the micro level..... like a mountain range even. Pushing these rough surfaces against each other causes a force in the opposite direction to motion as the rough peaks and troughs of the surface physically catch on each other and have to ride up and over or bust through to over come the resistance to the motion.

 

I am sure you might be able to come up with a better answer, but this is how I understand what you are asking about - I hope that helps. Any more questions, or if I haven't given you what you are looking for then please ask - If I cannot answer your question then I am sure someone else will.

[DrKrettin]

Without wishing to devalue the previous post, I give the same answers in a different form:

 

1) Hooke's Law for a spring says that the extension or compression of the spring is proportional to the force applied. So the linear movement of the spring is a direct measurement of the force, once it is calibrated.

 

2) If a body is not experiencing an acceleration, then the forces on the body are in equilibrium. That means that you can resolve all the forces in any direction and the sum must be zero. If they are non-zero, then there is an acceleration in that direction. It follows then that the frictional force must be equal to and in the opposite direction to the movement. If it were in any other direction, the sum total of the forces would not be zero.

[Sigmarus]

I understand the first part but what if the spring balance is used to measure friction?And is there a simpler explanation for frictional force being the opposite direction of movement(Since i'm only in lower-secondary)

 

But thank you so much for taking the time to write these invaluable answers!

[DrKrettin]

I understand the first part but what if the spring balance is used to measure friction?And is there a simpler explanation for frictional force being the opposite direction of movement(Since i'm only in lower-secondary)

 

But thank you so much for taking the time to write these invaluable answers!

 

I imagine that you are pulling an object along a horizontal surface with a spring balance which is showing some kind of measurement. That has to be the frictional force. As to the direction of that force, it has to be acting along the line of force of whatever is pulling the object, but acting in the opposite direction. If the frictional force were not acting along that line, then the object would start moving in a different direction to compensate. Perhaps we need a physics teacher here with the experience of explaining this in a clear manner.

[studiot]

Hello.So recently i have 2 questions in mind,which are

1.How does a spring balance measure the force?

2.why is the frictional force opposite the direction of movement?

 

Thanks

 

I understand the first part but what if the spring balance is used to measure friction?And is there a simpler explanation for frictional force being the opposite direction of movement(Since i'm only in lower-secondary)

 

But thank you so much for taking the time to write these invaluable answers!

 

Hello Sigmarus, I see your main interest is Chemistry and that you are in 'lower-secondary' so I will try to pace the answers accordingly.

 

To offer best help I will split my answer into two parts (posts) and also ask a couple of questions about your actual study level.

 

I am guessing that you have completed one year of what we call the sixth form in the UK which means you will have studied 'force statics' and understand what equilibrium means but not yet studied dynamics or kinematics.

As a chemist you will also have heard of the law of conservation of energy.

 

Please confirm or correct these guesses.

 

So first to answer your question as to why friction opposes motion.

 

This is entirely due to the law of conservation of energy.

 

Imagine a rough block sitting quietly on a rough surface

( We say a rough surface when friction is involved and a smooth surface etc when friction is not involved)

Now let us start to pull it along and then let go of the pull.

If the friction acted in the direction of motion and we removed the pull, the block would continue to move under the action of the friction alone.

 

We know that does not happen and I hope you can see that if it did it would contravene the law of conservation of energy.

 

What we observe is that the block slows to a standstill as the kinetic energy is dissipated as work against the frictional force.

 

If you like this we can study your other question (about the spring balance) in more detail.

 

These are good questions, by the way, so keep asking them. +1

Edited July 4, 2017 by studiot

[Sigmarus]

Well lower secondary school in my country means 13-14 years old

[studiot]

Thank you , that is younger than I guessed.

 

But I am more interested in what science you know and what you thought about my explanation.

So that I can connect it to what you already know.

[Sigmarus]

Basically the basics of forces.Such as effects of forces,different types of forces(frictional force etc),and pressure