A mass m₁ rest on top of another mass , m₂, which is connected to m₃ by a light rope

(passes over a frictionless pulley).

This java applet show the force diagram and the motion of the system when the frictional forces are present.

The mass of each block is shown at the lower-right corner.

Assume the coefficients of friction μ between any two surfaces are the same.

Force diagram for mass m₂ is shown to the left .

（不同來源的作用力以各色箭頭代表）:

Color of arrow	black	blue	green	cyan	red	yellow
force	gravity	normal	tension	friction	friction	Net force on m2
source	earth	table	rope	m1	table

Click within the white area of each block to change its mass.

Click right mouse button to increase one mass unit,

click left mouse button to decrease one mass unit.

Press Start will start the animation,

click mouse button to suspend the animation, click it again to resume.

Press Reset for default parameters.

1. m₁ and m₂ would not be accelerated? (Net force =0.)
2. m₁ and m₂ are accelerated together.
3. m₁ and m₂ are accelerated differently. (a₁ ≠ a₂ which one is larger?)

The mass of the blocks are m₁, m₂, m₃ , accelerations are a₁ , a₂ , a₃ .

The normal force between m₁ and m₂ : N₁₂= m₁ g,

so the frictional force f₁₂ ≦ m₁ gμ .

The normal force between m₂ and the table : N₂= (m₁+m₂) g

so the frictional force f₂ ≦ (m₁+m₂) gμ

The tension of the rope is T。

For m₁ ： f₁₂ = m₁ a₁

For m₂：If T > ( f₁₂ + f₂ ) then T - ( f₁₂ + f₂ ) = m₂ a₂

otherwise f₂= T and a₂ = 0.

For m₂： T - m₃ g = m₃ a₃

From the above equations, we can find out the accelerations.

I did that calculation already! It is your turn to do it now! O.K.

Your suggestions are highly appreciated! Please click hwang@phy03.phy.ntnu.edu.tw

Author：Fu-Kwun Hwang, Dept. of physics, National Taiwan Normal University

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