A Tightrope Mechanic Problem

A 675 Newton acrobat walks on a tightrope. The man's weight makes the tight-rope sag so that it looks like this:

Each rope's Θ \Theta is 3. 7 3.7^\circ

What is the tension of the rope on each side of the acrobat?

(The red line is just to show the angles of the rope's slants.)


The answer is 5229.938.

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1 solution

Connor Switala
Dec 6, 2016

Let T 1 T_1 = the tension of the first side of the rope, T 2 T_2 = the tension of the second side of the rope, and w w = the weight.

T 1 x = T 1 c o s ( 3. 7 ) = 1.0 T 1 T_{1x} = T_1 \cdot cos(3.7^\circ) = 1.0 \cdot T_1

T 1 y = T 1 s i n ( 3. 7 ) = 0.065 T 1 T_{1y} = T_1 \cdot sin(3.7^\circ) = 0.065 \cdot T_1

T 2 x = T 2 c o s ( 176. 3 ) = . 9979 T 2 T_{2x} = T_2 \cdot cos(176.3^\circ) = -.9979 \cdot T_2

T 2 y = T 2 s i n ( 176. 3 ) = . 06453 T 2 T_{2y} = T2 \cdot sin(176.3^\circ) = .06453 \cdot T2

w x = ( 675 N e w t o n s ) c o s ( 27 0 ) = 0 w_x = (675 Newtons) \cdot cos(270^\circ) = 0

w y = ( 675 N e w t o n s ) s i n ( 27 0 ) = 675 w_y = (675 Newtons) \cdot sin(270^\circ) = -675 Newtons

Now that we have everything split up into x- and y- components, we can some the forces in each dimension and make sure that they equal zero. (Since the acrobat is staying at the same height the whole time and the rope isn't going up or down, that means we have a Translational/Static Velocity. We are using Newton's second law here ( Σ F = m a \Sigma F = ma ) and so know we need to make sure that the sum of the forces in each dimension = 0

In the x- dimension we have:

1.0 T + 1 + . 9979 T 2 + 0 = 0 1.0 \cdot T+1 + -.9979 \cdot T_2 + 0 = 0

Although we can't solve anything here, we can rearrange the equation:

T 1 = 1.0 T 2 T_1 = 1.0 \cdot T_2

Now we can go to the y- dimension:

. 065 T 1 + . 06453 T 2 + 675 .065 \cdot T_1 + .06453 \cdot T_2 + -675 Newtons = 0 = 0

Since this equation had two unknowns in it, we can use the equation from the y- dimension to replace T 2 T_2 with 1.0 T 1 1.0 \cdot T_1 :

. 065 T 1 + . 06453 ( 1.0 T 1 ) + 675 .065 \cdot T_1 + .06453 \cdot (1.0 \cdot T_1) + -675 Newtons = 0 = 0

T 1 = 675 N e w t o n s . 130 = 5.19 × 1 0 3 T_1 = {{675 Newtons} \over {.130}} = 5.19 \times 10^3 or 5190

I believe the answer to the problem should be around 5232.558 Newtons. I think there are several apparently small mistakes at the beginning of the solution which propagate through to the end and give you a result off by around 42.558 Newtons.

First of all, cos(3.7) is exactly equal to -cos(176.3) as the two angles are exactly complements of each other. Same goes with sin(3.7) being exactly equal to sin(176.3). If you therefore use the values you obtain from a calculator, I think you should get 5232.558 Newtons at the end.

Bogdan Profir - 4 years, 6 months ago

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