Lesson Explainer: Triangle of Forces Mathematics

In this explainer, we will learn how to solve problems about the equilibrium of a particle under the action of three forces using the triangle of forces method.

Forces are vectors, and so a force can be represented graphically as an arrow pointing in the direction of the force, with a length proportional to the magnitude of the force.

The addition of forces represented as arrows is represented by placing the arrows head-to-tail, as shown in the following figure.

The resultant force, ⃑𝐹R, of βƒ‘πΉοŠ§ and βƒ‘πΉοŠ¨ is given by ⃑𝐹=⃑𝐹+⃑𝐹R and is represented graphically as an arrow that has its tail at the same point as the tail of βƒ‘πΉοŠ§ and its head at the same point as the head of βƒ‘πΉοŠ¨, as shown in the following figure.

Let us define a force ⃑𝐹 of equal magnitude to ⃑𝐹R, acting in the opposite direction to it. If ⃑𝐹 is added to ⃑𝐹R, we obtain ⃑𝐹+⃑𝐹=βƒ‘πΉβˆ’βƒ‘πΉ=0.RRR

We can see from this that ⃑𝐹+⃑𝐹+⃑𝐹=0.

This is represented graphically in the following figure.

The three arrows representing the three forces are all joined head-to-tail, forming a triangle.

Let us suppose that these forces all act on a particle P, as shown in the following figure.

It has been established that the resultant of these forces is zero. If these are the only forces acting on P, the net force on it is zero, and P is in equilibrium.

Forces βƒ‘πΉοŠ§ and βƒ‘πΉοŠ¨ need not be perpendicular to be two of the forces in a triangle of zero resultant force. The following figure shows an example of a force triangle where none of the forces are perpendicular to each other; the diagram also shows these forces acting on a particle P that is in equilibrium.

We can define a triangle of forces in equilibrium as follows.

Definition: Triangle of Forces in Equilibrium

Three force vectors that form a triangle for which the directions of the forces are all either clockwise around the triangle or counterclockwise around the triangle have a zero resultant, and hence the forces are in equilibrium.

Let us look at an example where forces are graphically represented by a triangle of arrows.

Example 1: Using a Triangle of Forces to Solve a Word Problem

Bassem is attempting a mechanics problem in which three coplanar forces βƒ‘πΉοŠ§, βƒ‘πΉοŠ¨, and βƒ‘πΉοŠ© are acting on a body. He needs to determine whether the body is in equilibrium or not. He remembers his teacher saying something about checking whether he could arrange the forces into a triangle. So, he draws the shown figure.

Bassem concludes that the three forces are in equilibrium. Is he correct?

Which of the following best describes what he has done?

  • He has not paid attention to the direction of the forces. All the forces should meet head-to-tail. However, in his diagram, βƒ‘πΉοŠ¨ and βƒ‘πΉοŠ© meet head-to-head. Therefore, the forces do not actually form a triangle.
  • He has done nothing wrong.
  • He has used the wrong method; a force triangle is not a valid way of checking for equilibrium.
  • He has put the forces in the wrong order. He should have started with the force represented by the longest arrow and worked his way to the shortest.

Answer

To determine whether the forces are in equilibrium, we can refer to the figure in the question.

From the figure, we see that ⃑𝐹+⃑𝐹=⃑𝐹.

The sum of the three forces is therefore given by ⃑𝐹+⃑𝐹+⃑𝐹=⃑𝐹+⃑𝐹=2⃑𝐹.

As βƒ‘πΉοŠ© is nonzero, the forces acting do not have zero resultant and so are not in equilibrium.

We can eliminate two of the options for the second part of the question immediately.

Something clearly has been done wrong as the forces are not in equilibrium despite forming a triangle, so it is incorrect to say that nothing has been done wrong.

The mistake made is not that a force triangle cannot be used to show that three forces are in equilibrium, as this is a valid method.

One of the remaining options states that

β€œHe has put the forces in the wrong order. He should have started with the force represented by the longest arrow and worked his way to the shortest.”

This can be understood to mean that we should join the arrows head-to-tail in order of length, as shown in the following figure.

Clearly ⃑𝐹R is nonzero if this is done, and so the forces are not in equilibrium, so this could not show that the forces are in equilibrium.

The final remaining option is that

β€œHe has not paid attention to the direction of the forces. All the forces should meet head-to-tail. However, in his diagram, βƒ‘πΉοŠ¨ and βƒ‘πΉοŠ© meet head-to-head. Therefore, the forces do not actually form a triangle.”

It is true that βƒ‘πΉοŠ¨ and βƒ‘πΉοŠ© meet head-to-head, and it is true that, in a triangle of forces, all the arrows must meet head-to-tail. If the arrow representing βƒ‘πΉοŠ© is reversed, a triangle of forces with zero resultant is formed, as shown in the following figure.

This option identifies what was done incorrectly.

It is important to note that, by changing the direction of βƒ‘πΉοŠ© so that its head is at the tail of βƒ‘πΉοŠ§ rather than βƒ‘πΉοŠ¨, βƒ‘πΉοŠ© has been removed and replaced by another force that has the same magnitude as βƒ‘πΉοŠ© but in the opposite direction. The three forces shown in the question cannot be in equilibrium.

Now let us look at an example where a triangle of forces is used to determine the magnitude of an unknown force.

Example 2: Using a Triangle of Forces to Solve an Equilibrium Problem

Three coplanar forces βƒ‘πΉοŠ§, βƒ‘πΉοŠ¨, and βƒ‘πΉοŠ© are acting on a body in equilibrium. Their triangle of forces forms a right triangle as shown.

Given that ⃑𝐹=5newtons and ⃑𝐹=13newtons, find the magnitude of βƒ‘πΉοŠ©.

Answer

For the body that forces βƒ‘πΉοŠ§, βƒ‘πΉοŠ¨, and βƒ‘πΉοŠ© act on to be in equilibrium, it must be true that ⃑𝐹+⃑𝐹+⃑𝐹=0.

The triangle formed by the forces is right-angled, so using Pythagoras’s theorem, we see that 𝐹=𝐹+𝐹.

Rearranging to make 𝐹 the subject, we obtain 𝐹=πΉβˆ’πΉ.

Substituting the magnitudes of the forces, we find that 𝐹=13βˆ’5=144𝐹=√144=12.newtons

The negative square root of 144 is not considered, as magnitudes of vectors are necessarily positive valued.

Let us now look at an example where three forces act at a point.

Example 3: Finding the Ratio between Three Forces Acting Parallel to the Sides of a Right Triangle Given That the System Is in Equilibrium

In the figure, three forces of magnitudes 𝐹, 𝐹, and 𝐹newtons meet at a point. The lines of action of the forces are parallel to the sides of the right triangle. Given that the system is in equilibrium, find 𝐹∢𝐹∢𝐹.

Answer

The length of the hypotenuse of the right triangle, β„Ž, is not known, but using Pythagoras’s theorem, we see that β„Ž=87+208.8=5116.44β„Ž=√5116.44=226.2.cm

To form a triangle of forces with zero resultant, the magnitudes of the forces must be in the same ratio as that of the lengths of the sides of the triangle.

Comparing the side lengths of the triangle, we see that 226.287=2262870=135 and 226.2208.8=22622088=377348=1312.

Hence, we see that 𝐹∢𝐹∢𝐹=5∢12∢13.

Let us now look at an example involving determining a force in an isosceles triangle of forces rather than a right triangle of forces.

Example 4: Equilibrium of a System of Three Forces Acting through a Triangle

A body is under the effect of three forces of magnitudes 𝐹, 𝐹, and 36 newtons, acting in the directions 𝐴𝐡, οƒŸπ΅πΆ, and 𝐢𝐴, respectively, where △𝐴𝐡𝐢 is a triangle such that 𝐴𝐡=4cm, 𝐡𝐢=6cm, and 𝐴𝐢=6cm. Given that the system is in equilibrium, find 𝐹 and 𝐹.

Answer

To form a triangle of forces with zero resultant, the magnitudes of the forces must be in the same ratio as that of the lengths of the sides of triangle 𝐴𝐡𝐢, which is shown in the following figure.

The triangle of forces that corresponds to this triangle is shown in the following figure.

The lengths of 𝐡𝐢 and 𝐢𝐴 are equal, so the magnitude of 𝐹 is 36 N.

The lengths of 𝐢𝐴 and 𝐴𝐡 are related as follows: 𝐴𝐡𝐢𝐴=46=23.

Hence, the magnitude of 𝐹 is given by 𝐹=36ο€Ό23=24.N

Let us look at an example where a problem involving the equilibrium of a suspended object is solved using a triangle of forces.

Example 5: Finding the Tension in the Strings That Keep a Uniform Rod in Equilibrium

A uniform rod of length 50 cm and weight 143 N is freely suspended at its ends from the ceiling by means of two perpendicular strings attached to the same point on the ceiling. Given that the length of one of the strings is 30 cm, determine the tension in each string.

Answer

The rod and the strings are represented qualitatively in the following figure.

The rod and strings form a right triangle. Using Pythagoras's theorem, we can determine 𝑙, the length of the unknown side: 50=30+𝑙𝑙=50βˆ’30=2500βˆ’900=1600𝑙=√1600=40.cm

The forces acting are the weight of the rod and the tensions in the strings. These forces are in equilibrium, so they can be drawn acting at the same point. This is represented qualitatively in the following figure, where βƒ‘π‘‡οŠ§ corresponds to the tension in the 40 cm string and βƒ‘π‘‡οŠ¨ corresponds to the tension in the 30 cm string.

These forces can form a triangle of forces, as shown in the following figure.

As the forces are in equilibrium, we see that π‘Š=𝑇+𝑇.

It is stated that π‘Š=143;N hence, 143=𝑇+𝑇.

From the triangle formed by the rod and strings, we can know that the sides of the triangle that correspond to the strings have lengths of 40 cm and 30 cm. The ratio of the lengths of the sides of this triangle equals the ratio of the forces corresponding to the tensions in the strings, and so we see that 𝑇=4030𝑇=43𝑇.

Substituting the expression for π‘‡οŠ§ into 143=𝑇+𝑇, we obtain 143=𝑇+ο€Ό43π‘‡οˆ143=𝑇+ο€Ό169οˆπ‘‡143=ο€Ό259οˆπ‘‡π‘‡=143ο€Ό925οˆπ‘‡=143ο€Ό35=4295=85.8.N

As𝑇=43𝑇,𝑇=43ο€Ό4295=171615=114.4.N

Now that we have seen a variety of examples, let us recap some key points of the explainer.

Key Points

  • For two forces βƒ‘πΉοŠ§ and βƒ‘πΉοŠ¨ that have a resultant ⃑𝐹R, it is the case that ⃑𝐹+⃑𝐹+ο€Ίβˆ’βƒ‘πΉο†=0,R where βƒ‘πΉοŠ§, βƒ‘πΉοŠ¨, and βˆ’βƒ‘πΉR form a triangle of forces that has a zero resultant.
  • A body that is acted on only by forces that form a triangle of forces is in equilibrium. Arrows representing the forces in the triangle must all meet head-to-tail.
  • The ratios of the lengths of the sides of a triangle are equal to the ratios of the magnitudes of the forces in a triangle of forces.

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