Force and movement

Newton‘s Three Laws of Motion

Based on a long-term study of movement, the basic principles of movement and changes in movement, which we also call motion laws, have been defined. They were formulated by Sir Isaac Newton. In addition to the basic three laws of motion, we also know others that are related to specific cases of motion, such as motion at the speed of light, and also motion in the microworld, at the atomic level. Consequently, Newton‘s laws of motion cease to apply in these specific situations.

The First Law of Motion tells us that a moving object is moving in the same direction and at the same speed until a force acts on it. This means that if we kick the ball, it will fly at a constant speed infinitely until another force acts on it. Although at first glance this law is not in line with reality, it is absolutely true. After kicking the ball, the forces on the moving ball act to slow it down and then fall. For example, the Earth‘s gravitational force pulls the ball down (this force acts on the ball while it lies), but also rubs on the air in which the ball moves.

The second motion law explains that the heavier object is, more power it will need to make to move the object (F = m.a). Among other things, it means that the more you kick the ball, the further it will fly. Thus, if we apply the same force to two objects of different masses, they will accelerate the object, which is more massive.

The Third Law of Motion explains that each action produces an equally great but opposite effect. This means that at each moment there are two forces interacting with each other. In the example with the ball, this means that when we put the power to kick the ball, the ball is equally forceful on our foot, just in the opposite direction. Especially when we kick the ball with our fingers. The force the ball exerts on our foot may be so great (depending on how hard we kick the ball) that it will cause a finger to break. The third movement law is more visible, for example, when shooting from a cannon (but also from other firearms). The explosion is the supplied cannon ball force, which is fired in a certain direction by the cannon. However, the cannonball on the cannon is equally powerful. The cannon moves in the opposite direction to the fired cannonball, but not as fast as the ball itself, because the cannon is much heavier and therefore more power is needed to accelerate it than to accelerate a much lighter cannonball.

Simple and compound motion

There are two explanations of final (resulting) movement of objects. The first view resolves the movement as a straightforward displacement, whereby the displacement speed can be constant, but due to forces acting in the direction of movement or in the opposite direction, the object can move more slowly or faster. More complicated is the movement in which the object changes its direction of movement. For example, rotating objects or a ball thrown by an slanting throw. Even with such movements, force must be applied to the object, but it does not act in the direction of movement of the object (or upstream) but at a certain angle.

Active and stored energy

To explain why things are happening around us, we use the term energy. We know different forms of energy, such as solar, atomic, electrical, chemical, and the like. When we apply force to an object, we change its energy. This energy is used either to work the object or to accelerate it. Energy, unlike motion (vector quantity), has no direction. These quantities are scalar quantities. In connection with forces and movement we talk mainly about kinetic (motion) energy, potential energy and energy of strings (energy of elastic materials). Energy cannot be felt by touch or otherwise perceived, it is just something through which we explain how things are happening around us. Energy is measured in joules.

The movement and its explanation is related to the conversion of potential energy to kinetic. When the object moves, we say it has kinetic energy. Potential energy is „stored“ in objects and its size depends on the object itself, but also where and how it is located. A classic example of potential energy is the effort to lift a brick. When the brick is on the ground, it has a certain amount of potential energy. When we lift a brick, we exert a certain force on the brick, we work. This work has added extra energy to her potential energy. The higher we plot it, the more energy we add. When we release the brick from above, the brick can, by its energy, affect other objects, for example, it can break something. This means that the brick we brought up there and holds it has more potential energy than the brick that lies on the ground.

Work

Working in the physical sense of the word happens when the force we are exerting on the objects causes them to move. The amount of work done is obtained by multiplying the force exerted by the distance by which the object moves (W = F.d). Thus, work is measured in newton-meters, which is the same as joules. This means that the unit of labor and energy is the same. Force and work are not the same. For example, if we hold a brick in the outstretched hand, the brick will gradually become heavier and heavier, even only by our feelings. We exert specific amount of force to keep the brick, but as the brick does not move, we do no work. When our hands get tired and we put the brick and lift it again, we do the work. We do less work when we consider a brick and when it is raised. Despite the fact that the path that the brick passes through when laying and lifting is the same, when lifting we must use more force (to counteract the effect of gravitational force) and therefore the resulting work is greater when lifting than when laying bricks. Since the term work is also used in normal speech, it is important to distinguish the physical quantity from the commonly used term work.

Friction

Friction is a force that slows the movement of a sliding object. Friction occurs wherever two objects come into contact. The friction force always acts in the opposite direction to the sliding direction of the object. To stop the car on the red traffic light, the car stops due to the friction of the brakes (pads, discs) by the moving wheels. If we run down the hill on the asphalt road and want to stop, we can do it by rubbing the shoes on the asphalt. However, if the asphalt is wet, or if there is a thin layer of mud on it, we will not stop so well. Similarly, with the car moving on the road. The friction is high on the dry road, the friction is lower on the wet and therefore the car has a longer stopping distance. If the wheels are in a thin layer of water, they may lose contact with the asphalt (the friction is lost because the wheel is not in contact with the road and so it cannot be called friction) and braking is ineffective – then we are talking about aquaplaning.

Air resistance

Frictional force is generated only by the friction of solids. However, by moving in gases and liquids (in liquids), the movement of the object itself slows due to the resistance of these substances to the moving object. If the fluid in which the object moves is called air resistance. Air resistance, for example, causes thatmovement of an asteroid entering the earth‘s atmosphere causes compression of the air molecules, as it moves at high speed. This compression causes heating of the air as well as the meteorite itself. If the meteorite is small, it can also burn in the atmosphere due to this heat.

Air resistance is used, for example, in the construction of parachutes. On the contrary, we need to reduce air resistance if we want objects to move in the air as quickly as possible, with the least possible force to accelerate – for example, in the construction of aircraft, but also in cars and ships. In the construction of these devices are used so-called. principles of aerodynamics – motion in fluids (liquids and gases). For example, new car body models are tested in a wind tunnel to measure the air resistance that arises when the car moves against the air stream. That is why the cars have the same shape as they do. The figure shows examples of air resistance values for various bodies moving in the air. It is also clear why the water drop has the shape it has. Since the water is liquid and can change its shape, it changes shape when it falls from a height as the air resistance acts on the drop and the resulting shape is the one that penetrates the light most easily (the air resistance is the smallest).

Pendulum

The pendulum consists of a bob and a string, a wire, a chain, or a bar that is fixed at a fixed point and allows the bob to oscillate from one side to the other. When the weight is just below the fixed point, we say it is in a steady position, it does not move. If by force we cause the pendulum bob to move to a certain side, the pendulum will oscillate regularly back and forth. The real pendulum gradually slows down, especially because there is no fixed point without friction and the pendulum also moves in the air (fluid) environment causing resistance (pendulum movement) and gradual slowing of the pendulum. When swinging the pendulum, the bob reaches a certain maximum position. The angle between the string in the equilibrium and the maximum position is called the amplitude of the oscillation. The time needed by the pendulum to perform one whole oscillation is called the oscillation time. The vibration time depends mainly on the magnitude of the gravitational force acting on the weight. For this reason, the same pendulum has a smaller period of oscillation (oscillating faster) on the earth‘s poles because the gravitational force is slightly greater than on the equator.

Foucault‘s pendulum

The first pendulums were used to measure time periods and also to detect the impending earthquake long before defining the pendulum theory by Galileo Galilei, a known scientist. It is he who is officially attributed to the discovery of the pendulum. The first attempted to explain how and why the pendulums oscillate. The way the pendulum moves is considered to be one of the first evidence that Earth rotates around its own axis. If we want the Earth´s revolution to be manifested, we need to construct a very large pendulum that is suspended at a fixed point with minimum friction. This kind of pendulum is called Foucault‘s pendulum, according to the French physicist Léon Foucault, who designed the pendulum for the first time (in Paris in 1851).

If we swing the pendulum along the line on the ground, each one of the pendulums swings is out of the original direction even by very small pieces. Respectively, it‘s not a pendulum that is bent, but a Earth that rotates causes the observed deviation. To emphasize this pendulum movement, it originally drew the pendulum of the line into the sand poured under the pendulum. The sand was later replaced by plates, which are placed under the pendulum in the circle. If Earth did not rotate, the pendulum would only throw oppositely standing plates. But as the Earth rotates around its own axis, the pendulum gradually sheds further and more plates in the circle are thrown down (see Picture). The time that the pendulum throws away all the plates we call a pendulum day. This period of time varies across different parts of the Earth.