Friction force vs external force

Fu-Kwun Hwang

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1 Friction force vs external force

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on: June 25, 2013, 02:54:21 pm » posted from:Taipei,T'ai-pei,Taiwan

A block with mass $m$ is put on top of a surface. The friction coefficient between block and the surface is $\mu$ .
The normal force is $m g$ , so the maximum static friction $F_\mu\le m g \mu$ .
It means when the external force is less than $m g$ , the friction force is $- m g$ .
However, if the external is larger than the maximum static friction force then the friction force stay constant $- mg \mu$ .
And the block starts to move.
The following simulation let you play with the above case.
You can drag the spring horizontally to change the external force.

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Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License

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Fu-Kwun Hwang

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2 Re: Friction force vs external force

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Reply #1 on: June 26, 2013, 08:04:59 am » posted from:,,Satellite Provider

Here is another more complicated version.

Bock A with mass m is put on top of a surface. There is another block (B) with mass m2 on top of block A.
Friction coefficient between block A and surface is $\mu$ , between block A and block B is $\mu_2$ .

The normal force between block A and surface is $(m+m_2) g$ , between block A and block B is $m_2 g$
The external force is F:

1. $F\le (m+m_2)g \mu$ : both block would not move
Friction force between block A and surface is $-F$ , between block A and block B is 0.
2. $(m+m_2)g\mu<F\le (m+m_2) g (\mu+ \mu_2)$ : block A and block B move together
( $\frac{F-(m+m_2)g\mu}{m+m2}m_2< m_2 g\mu_2$ Force on block B less than maximum static friction force)
Friction force between block A and surface is $(m+m_2)g \mu$ , acceleration of both blocks is $\frac{F-(m+m_2) g\mu}{m+m2}=\frac{F}{m+m_2}-g\mu$
friction force between block A and block B is $\frac{m_2}{m+m_2} (F-(m+m_2) g\mu)=\frac{m_2}{m+m_2}F-m_2 g \mu$
3. $F>(m+m_2) g (\mu+\mu_2)$ : block A and block B move separately.
Friction force between block A and surface is $(m+m_2)g \mu$ , acceleration of blocks A is $\frac{F-(m+m_2) g\mu-m_2 g\mu_2}{m}$
friction force between block A and block B is $m_2 g \mu_2$ , acceleration of block B is $g \mu_2$

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Please feel free to post your ideas about how to use the simulation for better teaching and learning.
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RUPESH BABU

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3 Re: statics and dynamics

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Reply #2 on: July 24, 2013, 08:39:23 am » posted from:Pondicherry,Puducherry,India

please, help me..!
differentiate between
1. Body at rest which has no net force
2. Body moving at constant velocity which has no net force Sad


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Fu-Kwun Hwang

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4 Re: statics and dynamics

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Reply #3 on: July 24, 2013, 08:51:52 am » posted from:Taipei,T'ai-pei,Taiwan

A person sit on a car moving with constant velocity.

For observer in the same car: that person is at rest which has no net force.
For observer at rest relative to the ground: that person is moving at constant velocity(the same as the car) which has no force.


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RUPESH BABU

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5 Re: statics and dynamics

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Reply #4 on: July 25, 2013, 09:22:40 am » posted from:Pondicherry,Puducherry,India

parameters :
velocity = 0 , final velocity = v ;
acceleration = 0 , acc_mid = a [from 0 to v] , final acceleration = 0 ;

concept:
1. Initially ,velocity is zero.
2. Force is applied to reach constant velocity (i.e) to overcome friction and to accelerate , producing net force.
3. After reaching velocity 'v' , net force is zero. (i.e) force is applied only to overcome friction.

My understandings says that, at CONSTANT VELOCITY,
FORCES are there , only NET FORCE is zero.
If my concept is right then acknowledge me else help me with CORRECT concept...

SIR, many questions are with me. I want to get clarified one by one. Here after ,you can see my posts frequently. Huh

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Fu-Kwun Hwang

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6 Re: Friction force vs external force

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Reply #5 on: July 25, 2013, 06:24:21 pm » posted from:,,Satellite Provider

Yes. Net force is zero when an object move with constant velocity.
The external is required if there are friction force or other kind of force resist the motion of the object.


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RUPESH BABU

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7 Re : mechanics

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Reply #6 on: July 26, 2013, 09:44:55 pm » posted from:Pondicherry,Puducherry,India

What is moment of inertia ?
- polar moment of inertia ?
Please explain the term briefly with its importance in dynamics with real time examples.
please brief brief.....


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Fu-Kwun Hwang

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8 Re: Friction force vs external force

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Reply #7 on: July 27, 2013, 05:48:46 pm » posted from:,,Satellite Provider

Please check out wikipedia first.

You are welcomed to post in more detail what you do not understand and we will try to help.
But you should learn to check out information by yourself.


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RUPESH BABU

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9 Re: MEchanics

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Reply #8 on: September 12, 2013, 09:08:36 am » posted from:Pondicherry,Puducherry,India

"Moment of Inertia or (Polar I) is the tendency to resist the rotational motion".
this is the fact I grabbed...
But I need some insight meaning related statics as well as dynamics...(ie. about its role in these areas).
PLease be brief.! that will help me a lot


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