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In the absence of the forces P and Q, the force R would produce the same turning effect on the bar as do the forces P and Q jointly.

If the magnitude of R were not equal to the magnitude of W, the vector sum of the forces would not be zero, there would be a torque about any point on the barother than at the point C, and the bar would not be in equilibrium.

Exercise involving a bar supported at only one point

Tactile graphics

The file named Phy1110c1.svg contains a vector diagram that represents this scenario. All of the action occurs near the right end of the bar, so only theright end is shown in the image.

Figure 8 shows the mirror image that is contained in that file for the benefit of your assistant who will create the tactile graphicfor this exercise.

Figure 8 . Mirror image from the file named Phy1110c1.svg.
Missing image

Figure 9 shows a non-mirror-image version of the same image.

Figure 9 . Non-mirror-image version of the image from the file named Phy1110c1.svg.
Missing image

Figure 10 shows the key-value pairs that go with the image in the file named Phy1110c1.svg.

Figure 10 . Key-value pairs for the image in the file named Phy1110c1.svg.
m: P = 15 N n: File: Phy1110c1.svgo: B = 0 p: a = 1q: b = 2 r: C = 3s: W = -10 N t: A = 1u: Bar v: Q = -5 Nw: Exercise involving a bar supported at only one point

Unlike or antiparallel

It's time to modify your drawing to reflect a different scenario.

Make your bar 20 units in length. Draw a downward force labeled Q at the right end of the bar. Label this as point C on the bar.

Draw an upward force labeled P somewhere on the bar. Label the location of this point on the bar as A

In this case, we have two parallel forces, P and Q pulling in opposite directions but not in the same line of action. These forces are said to be unlike or antiparallel .

Add another downward force

Draw a downward force labeled W somewhere to the left of A. Label this point as B and consider it to be the horizontal origin. Directions to the right of Bare positive directions.

Label the horizontal distance from B to A as a. Label the horizontal distance from A to C as b.

What is required for equilibrium?

We can look at this scenario and tell that the forces labeled W and P, both of which point down, must be on opposite sides of the force labeled P.Otherwise, the bar will rotate in a clockwise direction.

The following two conditions must be true for the bar to be in equilibrium.

P + W + Q = 0, or

P = - (W + Q) (vector sum must be zero)

a*P + (a+b) * Q = 0 (moments around point B)

The plan

We will set values for all the variables in the twoconditions other than a and P and then solve for values of a and P that will result in the bar being in equilibrium .

  • Let W = -10
  • Let Q = -5
  • Let b = 2

The computations

Substituting these values into the two conditions and rearranging terms gives us:

P = -(W + Q), or

P = 15 , this is one of the answers

a*P +(a+2)*(-5) = 0, or

15*a -5*a -10 = 0, or

10*a = 10, or

a = 1 , this is the other answer

The answers

Therefore, for the values of W, Q, and b given above , P must be equal to 15 and a must be equal to 1 for the bar to be in equilibrium.

Questions & Answers

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Source:  OpenStax, Accessible physics concepts for blind students. OpenStax CNX. Oct 02, 2015 Download for free at https://legacy.cnx.org/content/col11294/1.36
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