Click on a number to view the explanation of the corresponding item.

A number of assumptions underlie the applet. These are described in "Assumptions".

Basic Control Buttons

There are four basic control buttons. They are, from left to right:

Rewind. Resets the simulation to the initial position with the previously chosen initial values. If you have the particle path displayed, Rewind will allow you to compare paths traversed under different conditions, e.g., different initial velocities.

After clicking Rewind, click Play to restart the motion.

Play/Pause. Starts the motion.

After Play has been clicked, the button will change into the Pause button .

Clicking Pause will freeze the motion. To resume the motion, click Play once more.

Step. Lets you step through the motion in equal time steps.

Reset. Resets the applet to its default setting.

Adjustments in the initial conditions can only be made after first clicking REWIND or RESET.



Data

Selecting the Data button will open the Data box shown below. The box can be closed by clicking the "X" in the upper right-hand corner or by clicking the Data button a second time. The box can be dragged across the window.



The Data box displays the following values.

• g: magnitude of the acceleration due to gravity,
• m₁: mass of Block 1; this mass is fixed at 600 g
• R: radius of the pulley wheel; R has a fixed value equal to 0.500 m; the pulley wheel is assumed to be a solid disk of uniform mass density
• I: moment of inertia of the pulley wheel; the moment of inertia is taken to be equal to (M/2)R² where M is the mass of the pulley


• t: time elapsed from the start of the motion
• v: instantaneous speed of the blocks
• a: magnitude of the acceleration of the blocks


• T₁: tension in the string on the side of the pulley leading to Block 1
• T₂: tension in the string on the side of the pulley leading to Block 2


• KE₁: the kinetic energy of block 1,
• KE₂: the kinetic energy of block 2,
• KE_pulley: the kinetic energy of the pulley,
• PE₂: the potential energy of block 2 (more correctly: the potential energy of the pair consisting of block 2 and the earth),
• E: the mechanical energy of the system, which is the sum of the four precedinig energies.

The two tension values are displayed in the Data box only when the free-body diagrams are displayed. See the following Point 3.



Free-Body Diagrams
.

The FBD toggle button will display/hide the free-body diagrams for the two blocks. The diagrams can be dragged by clicking on their center points and dragging.

The free-body diagrams for Blocks 1 and 2 are shown in the following snapshot.



The free-body diagrams in the snapshot above apply at a moment when the blocks are in motion. The diagrams look different when the blocks are being dragged. In the latter case, force vectors whose magnitudes are undetermined are shown dimmed and their labels are shown with question marks. The free-body diagrams also look different when the blocks are held before the start of the motion or when Block 2 is resting on the platform. For details, please go to "ShowMe".

Values for the tensions T₁ and T₂ are displayed in the Data box. See Point 2 above. These values are replaced by question marks when unknown forces are present in the physical situation that is being simulated.



Magnifier

The Magnifier button allows you to change the scale of the energy bar. Clicking on the button repeatedly will let you cycle through the magnification factors 1X, 2X, 3X, 4X, 0.5X, and back to 1X.



Sliders

There are two sliders, shown in the following image. Clicking on a slider to the left or right of the slider tab allows fine adjustment of the slider setting.



Clicking on the Input Dialog button of a slider will open a dialog for entering an exact value for the slider setting. The dialog for the slider that sets the mass of Block 2 is illustrated below. The range in which values can be entered, from 200 g to 2000 g in this case, is indicated above the data entry field.



The two sliders allow you to adjust the values of the following quantities.

• Mass of Block 2. The mass m₂ of Block 2, the downward moving block, can be varied between 200 g and 2000 g, in steps of 1 g. The mass of Block 1, the horizontally moving block, is fixed at 600 g.
• Mass of Pulley. The mass M of the pulley can be varied between 0 and 2.00 kg, in 0.01 kg steps. Since the pulley radius R is fixed, the pulley mass will determine the moment of inertia I of the pulley. I can be calculated from M via I = 0.5MR². The values of R and I are listed in the Data box.



Speedometer



The speedometer indicates the speed v of the blocks by a red needle moving horizontally. The range of the speedometer is from 0 to 10 m/s. The value of the speed is displayed in the Data box (Point 2).



Platform

The yellow platform can be dragged up or down to positions of either positive or negative elevation h.



Energy Bar

The values of the energies are displayed graphically by means of an energy bar:



The individual energies are color-coded. The total height of the energy bar, measured from the zero-level indicated by the short horizontal line across the energy bar, represents the energy E. In the situation represented above, the moment of inertia of the pulley and therefore the kinetic energy of the pulley are zero. Therefore the energy bar contains no segment for the pulley's kinetic energy.

The energy bar can be scaled by clicking the magnifier button repeatedly.

Note. The potential energy is 0 when h = 0. For details, please go to "Assumptions".



Moving the Blocks

Initially, or after clicking REWIND or RESET, you can move Block 1 by clicking on it and dragging it. Block 2 will automatically move also, so that the connecting string stays taut. A marker next to the h-scale displays the elevation of the lower edge of Block 2.