Static Mechanics: Levers Physics

by Prof. André Astro 04 Nov

The Physics of Levers

One of the mechanics of content with greater application in our daily life is the study of levers. Coupled to this, the concept of torque is present from the operation of complex machines like cars, to the functioning of simple machines, as our jaw when chewing a food. Therefore, it is imperative that we teachers to idealize our classes such content, do the theory into practice, always associating experimentation with existing tools and machines in our daily lives.

On the levers

The levers are simple machines that aims to facilitate and optimize the execution of a job. All levers have in their structure at least one resistance point, a support point (or fulcrum) and a point to make the driving force. And the relationship between these points and the distance between them is what makes the levers work for different purposes.

“Give me a lever big enough and strong support point, and I will move the world” Archimedes

In high school physics, generally, we study the situations of static equilibrium where the sum of the torques have to be zero, but it is enough to demonstrate several important points on the levers. The concept of torque is related to the rotational motion of a body after one applied force on it.

We calculate the torque module (also known as time), by: (force times the distance from the fulcrum force) in case of force being at right angles to the lever arm. Interpreting the physical formula, it can be said that it is possible to increase the ability to make a rotating object, increasing the driving force or increasing the distance that force the fulcrum, or by increasing the two quantities.

Another possible way of interpreting the equation, is thinking of the work done by the motor and resistant forces. On a machine, there is no gain in work, but you can gain in strength or distance, determining what we call mechanical advantage. Our mechanical kit brings several possibilities to explore in the classroom or in the laboratory these relationships.

A proposal for presentation of content

With scissors in hand, ask students, as with that scissors, they would cut a satin ribbon, and would like to cut with the same scissors a barbecue skewer? Intuitively, students already use the physics concepts of the levers without knowing.

When we want to cut something little resistance, we used a greater lever arm in the resistant part, therefore, the cutting force is less than the force to do with the fingers. But to cut anything tougher as the barbecue stick, we need to increase the cutting force, and we do this by lowering the lever arm, that is, bringing the stick closer to the fulcrum.

Although the case of the bamboo skewer cut, bring mechanical advantage, since the cutting force will be greater than the force made by your fingers, a disadvantage appears, you lose on the move. For the cutting of open a little, your fingers will have to open much more, as is the figure below.

The work done by your fingers and the cutting blade is equal. The fingers produce less force on a larger displacement, while the blade produces greater power at a lower displacement.

In view of the above concept, one might ask: If we want to cut a small branch of a tree, what would be the most appropriate tool? And what would be a disadvantage of this tool?

(1)                                   (2)                          (3)

Often in a lever, we do not want to gain in strength, as in the case of the branch, but on the move. An interesting example is the paddle.

The arm of the motive force is small compared to the resistance, ie the rower will have enough strength to make row, but the displacement of the blade will be much greater than the arm, making the rower gain in speed.

About our Mechanics Kits

The experimental part related to the levers in the Mechanic Set is very complete and is a great addition to the extensive body of static classes. With a relatively simple configuration, the teacher, along with student will be able to mount interfixed levers, interpotentes and inter-resistant, analyze possibilities, and perhaps the most important part, quantify.

 It is important that students see in practice and know quantify what is the basis for so many tools and machines, and even recurring content in various selection processes, such as ENEM for example. It is possible to demonstrate both the equilibrium conditions, the relationship between the force and displacement, as quoted in example rower. You can calculate the mechanical advantage in different configurations and relate this advantage with the various existing tools in daily life.

Another way to explore the kits is using the gamification. Basically the gamification is borrow game elements and apply in contexts that are not necessarily games. The versatility of the kit brings the possibility of launching challenges that encourage students to learn not only the theoretical, but the practical part. This part will the creativity of each teacher, but a suggestion would be to launch a challenge to students, using the ruler, place a mass at a certain distance from the point of support and challenge them with only a try, using the materials previously made available to, to leave the rule in balance. Or, given different masses scattered around the rule, with a movement only, balances it.

This kind of play / challenge in the classroom or in the laboratory, generates, as well as relaxation, encouraging logical thinking and reasoning of the learned concepts. It is worthwhile to perform!

Learn about the kits with which you can study levers:

Mechanic Set with Electromagnetic Trigger
Forces Panel
Static Mechanics Set (Basic)
Static Equilibrium of a Rigid Body

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