Equilibrium

Practice

practice problem 1

A 100 N sign is to be suspended by two cables. Determine the tension in each cable for the given angles …
  1. 90°, 90°
  1. 180°, 45°
  1. 120°, 30°

solution

For all solutions, let T1 be the cable on the left and T2 be the cable on the right. The sign always has weight (W), which points down. The sign isn't going anywhere (it's not accelerating), therefore the three forces are in equilibrium. Describe this state using the language of physics — equations; in particular, component analysis equations. As always, make a nice drawing to show what's going on. Use a ruler and a protractor if you wish.

  1. [slide]
    The two upward components should equal one another. Together they should equal the weight, which means each one is carrying half the load.

    T1 = T2 = ½W = ½(100 N) = 50 N

  1. [slide]
    Weight points down (270°) and T1 points to the left (180°). These are both good vectors — good in the sense that they are easy to deal with. T1 is the troublemaker. Break it up into components and state the conditions for equilibrium in the vertical and horizontal directions. I like to put negative vectors on the left side of the equals sign and positive vectors on the right side. I also suggest working through the vertical equation first.
     
    horizontal
    ∑ Fx =  ∑ F+x
    T1 =  T2 cos θ2
    T1 =  141 N cos 45°
    T1 =  100 N
     
    vertical    
    ∑ Fy =  ∑ F+y  
     
    W =  T2 sin θ2  
    T2 =  W  =  100 N
    sin θ2 sin 45°
    T2 =  141 N  
     
  1. [slide]
    Weight is the only force with a convenient direction. Resolve the tensions into their components. State the equilibrium condition along both axes. I suggest working with the horizontal equation first.
     
    horizontal
    ∑ Fx =  ∑ F+x
    T1 cos θ1 =  T2 cos θ2
     
    vertical
    ∑ Fy =  ∑ F+y  
    W =  T1 sin θ1  + T2 sin θ2

    That's the end of the physics. The rest of the work is math. Solve the horizontal equation for T1. Substitute the result into the vertical equation.

    T1 = T2  cos θ2  
    cos θ1  
    W = T2  cos θ2  sin θ1 + T2 sin θ2
    cos θ1

    Solve that for T2, substitute values, and compute T2.

    T2 =  W  
    sin θ1 cos θ2/cos θ1 + sin θ2  
    T2 =  100 N
    sin 60° cos 30° / cos 60° + sin 30°
    T2 =  50.0 N  
     

    Substitute back into the horizontal equation and compute T1.

    T1 = T2  cos θ2  = 50.0 N  cos 30°  = 86.6 N
    cos θ1 cos 60°

My, that last one wasn't very much fun. Let's see if there isn't a simpler solution. We used component analysis since it's the default approach. Whenever you're given a pile of vectors and you need to combine them, components is the way to go — especially if you have no expectation of any special relationships among the vectors. We use this brainless, brute force approach to problems all the time. Understand the rules, describe them using commands a computer understands, put numbers in, get answers out.

Sometimes, however, there are clever solutions available. They don't work all the time, but when they do we should use them. In this practice problem, the vectors are rigged so that the alternate solution is easier than the default solution. The graphical method for addition of vectors requires placing them head to tail. The sum would be the resultant vector connecting the tail of the first vector to the head of the last. When forces are in equilibrium, their sum is zero and their will be no resultant. This means, it should be possible to arrange the three vectors in this practice problem into a closed figure — a triangle. Let's try it.

  1. This is what we call a degenerate triangle. Sure it has three sides, but it covers no area. The two short sides lie on top of the long side. Symmetry tells us the two short sides should have equal length. Thus each tension equals half the weight. We already said this, so there is no advantage to this method over the previous one.

    T1 = T2 = ½W = ½(100 N) = 50 N

  1. The horizontal tension and the vertical weight are the legs of a 45–45–90 triangle whose hypotenuse is the diagonal tension. These forces should form the ratio 1 : 1 : √2.
     
    T1  =  W  =  T2
    1 1 √2
    T1  =  100 N  =  T2
    1 1 √2
    T1 = 1(100 N) = 100 N  
     
    T2 = √2(100 N) = 141 N  
     
  1. The two tensions are the legs of a 30–60–90 triangle and weight is the hypotenuse. This means the sides should form the ratio 1 : √3 : 2. Just be sure to get the tensions to correspond to the correct parts of this ratio.
     
    T2  =  T1  =  W
    1 √3 2
    T2  =  T1  =  100 N
    1 √3 2
    T1 =  √3  100 N = 86.6 N
    2
    T2 =  1  100 N = 50 N
    2

And there are other ways to solve this problem.

practice problem 2

Three forces act on a point: 3 N at 0°, 4 N at 90°, and 5 N at 217°.
  1. What is the net force?
  2. What fourth force will put the point in equilibrium?

solution

  1. Answer it.
  2. Answwer the rest.

practice problem 3

Write something different.

solution

practice problem 4

Write something completely different.

solution

Answer it.