Moments of force, also known as torques, are a fundamental concept in physics that refers to the ability of a force to produce rotation around a fixed point or axis.
That is, moments of force explain how a force can make an object turn instead of just move in a straight line.
Now let's delve a little deeper into the topic with some everyday examples that we have all experienced at some point.
1. Opening a door: the moment we apply when turning a handle
Think about the last time you pushed a door open. If you push near the hinges, where the door connects to the frame, you need to apply a lot more force to move it.
Instead, if you push or pull from the opposite end, where the handle normally is, it feels much easier.
Because?
This is where the concept of moment of force comes into play. Moment is the product of the force you apply and the distance from the pivot point (in this case, the door hinges). By applying force away from the hinges, we increase this distance and, as a result, the moment also increases.
This means that with the same amount of force, we get a greater effect when opening the door. That is why the handles are placed further away from the hinges, to make the action of opening and closing easier.
2. Tightening screws with a wrench: the effect of a lever
When using a wrench to tighten a screw, we apply a moment of force.
When we turn the wrench, we are generating a rotational force around the axis of the screw. The length of the wrench is essential here.
The longer it is, the less force we need to apply to turn the screw.
Imagine you are trying to tighten a screw very hard. If you have a small wrench, you will need to apply a lot of force. However, if you use a longer wrench, the moment increases because the distance between the force you apply and the axis of the screw also increases. As a result, tightening the screw becomes easier.
In other words, by making the lever (key) longer, we can make the task require less physical effort.
3. The steering wheel of a car: turning to change direction
When we drive a car and turn the steering wheel, we are applying a moment on the steering axis.
Have you noticed that the steering wheel has a considerable diameter? This is no coincidence. The further away our hand is from the steering axis, the easier it will be to apply a torque and turn the steering wheel.
Imagine trying to turn the steering wheel from the very center: it would take a lot of force. By having the steering wheel rim at a certain distance from the axis, we use the moment principle to achieve steering without excessive effort.
4. The swing: applying moments to swing
We've all enjoyed a swing at some point, and when we do, we're generating momentum around the hooks that hold the swing to the structure.
When you push back and forth, you are changing the angle and amplitude of the moment you apply, which allows you to control the speed and height of the roll.
In fact, even children intuitively realize that if they lean forward or back at the right moment, they can increase the amplitude of their swing.
This action generates a moment that maintains the pendulum movement and allows the swing to rise and fall with increasing momentum.
5. Pedaling a bicycle: generating movement through moments of force
When you pedal a bicycle, you are also applying moments of force. By pushing down on the pedals, you generate a moment about the axis of rotation of the bicycle, which is at the center of the chainring.
The force you apply to the pedal is multiplied by the distance from the center of the chainring, generating a moment that helps propel the bike forward.
Additionally, the chainring size, which is the distance from the center of the axle to where the chain teeth engage, influences the torque you transmit. The larger the chainring, the more force can be transmitted to the chain and wheels, making pedaling on certain terrains more efficient.
6. The lever: a simple and powerful machine
The lever is one of the clearest examples of the use of the moment of force.
Think of a situation where you need to lift a heavy rock off the ground. If you try to lift it directly, it might be impossible. But if you use a long bar as a lever and place a smaller rock as a fulcrum near the rock you want to lift, you could move it with relative ease.
This is because the lever allows us to apply a force at one end, which is converted into a moment about the fulcrum, making it possible to lift the object at the other end. The longer the bar you use, the less force you will need, as the moment will be greater.
This same principle is what makes a wheelbarrow, with its long handle and fulcrum close to the load, so effective for moving heavy objects.
7. The pulley: making lifting loads easier
Another simple machine that uses moments of force is the pulley.
A pulley is a wheel with a rope or cable running through it. By pulling on one end of the rope, you apply an input force, generating a moment about the axis of the pulley. This moment facilitates lifting loads at the other end.
Pulleys can be used in combination, such as in a compound pulley system, to further reduce the amount of force required. By increasing the number of pulleys, we increase the effective distance over which the force is applied, and therefore the moment.
This allows us to lift heavy objects with minimal force, as occurs in the pulley systems of cranes or elevators, as would be the case with the pulley in the image.
8. Steam turbines: generating electricity through moments
Steam turbines are an industrial example of the application of moments of force.
In a nuclear power plant, steam generated by thermal energy is directed towards the blades of a turbine, applying a tangential force to the wheel.
This force is perpendicular to the turbine axis, generating a moment that makes the blades and, consequently, the turbine axis turn.
This rotational motion is transmitted to a generator, which converts mechanical energy into electrical energy. Here, the torque generated by the steam is essential to produce the circular motion that gives rise to electricity.
This example can also be applied to any other turbine, such as wind turbines in a wind farm.