3 CHANGING THE MOVEMENT OF AN OBJECT REQUIRES A NET FORCE TO BE ACTING ON IT  # Force and movement

Force and movement are dealt by area of mechanics, which is considered a key area of physics. It is possible to claim that everything in the universe is moving. It can only be a very small movement and a very slow motion, but it is happening. Physics of Movement deals mainly with forces. If we want to move the object or change its movement, we need to affect the object by force. No motion is done by itself, it is always a change in the effect of forces on a given object. Changing the effects of forces on an object can cause a change in its direction of movement and / or a change in the speed of its movement.

## Speed, acceleration, mass and momentum of a moving object

When changing movement, the speed, acceleration and weight of the object must be taken into account. The speed of an object reflects the way the object travels over time. Acceleration expresses how the speed of movement of an object has increased over time. The movement of objects also depends on their weight. Weight is the amount of material that an object is made up of (expressed in kilograms, grams). So, for example, a moving car has a certain speed, acceleration and weight. These qualities must be taken into account when assessing its movement, especially when trying to change its movement, for example, when trying to stop the car.

Both speed and acceleration are of vector character. TThey are characterized by size amount and direction as well. The easiest way to explain this is by using arrows. The arrow determines the direction of the speed, and its length determines how fast the object moves in a particular direction. Acceleration is also a vector quantity. The object accelerates when both acceleration and speed are in the same direction. If the direction is opposite, the object slows down. Specific is the so-called constant acceleration.Acceleration is constant when a constant force is applied to an object. An example of constant acceleration is the so-called the gravitational acceleration that results from the Earth‘s constant gravitational force. Gravity acts on the object with constant force in the direction of the planet Earth‘s center. The gravitational force of the Earth decreases as the distance from the planet‘s surface increases. It is also interesting to note that the gravity of other planets differs from Earth‘s Gravity, as the planets may have more or less weight than the planet Earth. Therefore, objects in the fall accelerate differently to Earth and comparing to other planets.

If we multiply the speed of the object by its mass, we get another vector quantity – momentum. Momentum always has the same direction as speed. In principle, this quantity expresses how difficult it is to stop a moving object. For example, if a person runs 10 kilometers per hour and weighs 50 kg, his momentum is 500 kg.m/s. If flying ball weights 1 kilogram at a speed of 10 kilometers per hour and it hits, it hits us less than the ball that weighs more and/or moves faster. The momentum of such a ball is 10 kg.m/s.

Even very tiny objects (light ones) can hurt us more than large objects (heavy ones) if their speed is very large. A typical example is a bullet fired from a gun. The bullet itself is very light and if anyone just throws it, it won‘t hurt us at all. It is the momentum that causes injuries, while the large charge momentum is due to the disproportionately larger portion of the bullet, not its mass. The opposite is true as well. Even very slow moving, but heavy objects can hurt us, for example, if we pinch our fingers under moving wardrobe.

The momentum of the object represents the kinetic energy of the object. This is also maintained if such an object encounters an obstacle and bounces off it. However, this is only true if it is an ideally flexible object. The momentum of an object with ideal elasticity changes the momentum of the object, but only in the direction of its action. The momentum remains the same, only the direction in which the object moves is changed. In principle, the ideal flexibility does not exist, we know only objects with different flexibility. The less flexible the material is, the more kinetic energy itlooses when bounced.

The ideally flexible and ideally inflexible impact of the object and the impact on the momentum are shown in the figure. Since there are no ideally flexible and inflexible materials, it is not possible to demonstrate this phenomenon, but differences between very high and very low elasticity objects, such as a ping-pong ball and plasticine, can be observed.

If we throw a plasticine ball on the ground, it will remain deformed lying on the floor. Part of the kinetic energy of plasticine has turned into heat (plasticine is slightly heated) and sound (vibration on the floor and sound waves in the air). Since the speed is zero after impact, the momentum is zero. Kinetic energy has not disappeared, it has just turned into another form of energy. The more flexible the material, the more energy remains in the form of kinetic energy, for example, a ping-pong ball, unlike plasticine, bounces off the obstacle. Pic 22: Impact and its impact on the impulse

## 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.

The amount of friction between the two surfaces depends on how much the two surfaces are pressed together and to what extent the surfaces are rough. Friction acts against any movement. It is the resistance that occurs when two surfaces rub against each other. No surface is perfectly smooth. Tiny indentations or protrusions, and of course larger ones, on the rough surfaces, interlock with each other, creating friction. In addition, physico-chemical interactions of molecules of two mutually rubbed surfaces often act on the contact of two surfaces.

## Measurement of friction force

Friction can be measured. Its size depends on the materials that slide against each other. For example, concrete on concrete has a high coefficient of friction. The friction coefficient expresses how easily the object moves relative to another object over a certain surface. If the coefficient is high it means that there is high friction between the materials. While concrete has a high coefficient of friction, for example teflon has a low coefficient of friction for all materials. Even lower friction coefficient have our joints.

Friction generates heat. The amount of heat generated depends on the magnitude of the friction force. This means that more heat is generated when rubbing materials with high friction coefficient. Sometimes we can use this phenomenon (for example, we can warm ourselves by rubbing our palms together), but mostly friction causes unwanted heat. For example, various complex machines, such as car engines, contain many components that, when they work, rub against each other. This friction (in addition to components such as brakes), we try to reduce as much as possible in the engines. In addition, by directly rubbing the metal parts together, the components themselves can wear, a large amount of heat is generated by friction, which can cause local expansions of the components that can then jam. Therefore, we use different procedures to reduce friction. If the friction is very high in addition to heat, sparks can be observed (for example, when using a circular saw, see picture, or when the train is suddenly braked on rails). Pic 23: Circular saw

## Reducing friction

Sometimes we need friction and sometimes we try to prevent it. A good example is the operation of a wheel or ball. In this case, it is a change of friction by sliding to friction by rolling. This friction always has less value. Their shape causes them to roll (not slide) and the result is that friction is reduced. This principle works well, for example, by simply laying marbles under heavy loads, but also by attaching different wheels on the axle fixed freely on the load. A more refined idea is the ball bearing. Between the two surfaces on which we want to reduce friction, we put a set of beads of sufficiently strong material. In principle, the surfaces do not rotate as the balls rotate one by one and the friction of the two surfaces is significantly reduced. Some toys, such as a spinner, also work on a ball bearing principle (see picture).

Lubricants are often used to reduce friction. They are liquid to semi-liquid substances that fill small irregularities in the surfaces and thus the two surfaces move more easily against each other, friction is reduced. Lubricants are liquid but must have a higher viscosity so that they do not run off the two surfaces. The problem is that many substances have suitable friction-reducing properties when they are at room temperature, but lose their viscosity by heating, are more fluid, and escape very quickly (even by object movement) from the interface of the frictional surfaces (e.g. pork ointment). Therefore, it is not easy to find a substitute for lubricants that are used in a professional manner (e.g. petrolatum) Pic 24: Ball-bearing, Fidget-Spinner

Another way to reduce friction is to change the materials that are in contact while sliding. For example , steel moves with very low friction on ice. If we reduce the contact area (we create a kind of blade, for example, in the case of skating), the friction of these two surfaces is greatly reduced. On the contrary, if we don‘t want to slip on the pavement, we use rubber soles. When rubbing rubber and asphalt, there is a great friction and therefore this shoe is very suitable for the movement in which we need friction. As mentioned, friction reduces the efficiency of the machines by losing force (and thus causes loss of energy). In addition, friction generates heat that must be discharged from the machine. However, not all friction effects are undesirable. Friction, for example, allows you to reduce the speed of a bicycle, cars, allows us to walk, run, but also write on paper.

## 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. Pic 25: Air resistance

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). Pic 26: Air resistance values for different bodies

## 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.

It is interesting to explore what influences (what variables, pendulum properties) pendulum oscillation. In view of the pendulum design, the following characteristics (variables) can be taken into account: the length of the hinge (string), the weight of the bob and the angle from which the pendulum is triggered when swinging. By experimentally examining the influence of these variables, the data presented in the table can be obtained. In experiments 1–5, the weight of the pendulum bob was gradually increased. Other variables were constant, followed by a correlation between increasing weight and time of oscillation. It is clear from the data obtained that increasing the weight of the bob does not have an effect on the pendulum oscillation. This means that the pendulum with a lighter bob oscillatesjust as quickly as pendulum with a heavier bob. In experiments 6–9, the pendulum bob weight is maintained the same (0.2 kg), and the pendulum launch angle is the same (15°). The experimental variable is the length of the pendulum string. In this way it is possible to determine whether the pendulum oscillation rate depends on the length of the string on which the bob is hanged on. In contrast to the weight of the bob, the results show that the length of the string has an effect on the pendulum velocity. The longer the string is, the slower the pendulum oscillates. Thus, there is a direct correlation between the pendulum oscillation rate and the length of the string. The last four results illustrate the relationship between the pendulum vibration velocity and the pendulum angle change. The weight of the pendulum bob and the length of the string remain constant, changed is only the angle from which the pendulum is triggered. From the results it is clear that the trigger angle does not affect the pendulum oscillation rate. The result of all measurements is that only the pendulum string length affects the pendulum oscillation rate.

Table 1: Pendulum speed when changing variables
Experiment No.Weight of the cart Length of the splitsOscillation angleSwing time
1. 0,02 0,4 15 1,25
2. 0,05 0,4 15 1,29
3. 0,1 0,4 15 1,28
4. 0,2 0,4 15 1,24
5. 0,5 0,4 15 1,26
6. 0,2 0,6 15 1,56
7. 0,2 0,8 15 1,79
8. 0,2 1,0 15 2,01
9. 0,2 1,2 15 2,19
10. 0,2 0,4 10 1,27
11. 0,2 0,4 20 1,29
12. 0,2 0,4 25 1,25
13. 0,2 0,4 30 1,26

## 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.

Foucault‘s original pendulum had a 28 kg heavy bob that was hung on a 67 meter long string. Such a pendulum has been deflected from the original direction by about 11.3° in one hour , so that it has reached its original position in about 31.8 hours. At the Earth‘s poles, any pendulum returns to its original position in exactly 24 hours, rotating the pendulum clockwise and counterclockwise at the north pole.

To manifest the Earth‘s rotation on the Foucault pendulum, the key is how the pendulum will be triggered. The amplitude should be a maximum of 20 degrees, and the trigger should be free of any lateral forces. For example, burning a string that holds the pendulum bob is usually used. Foucault‘s pendulums are found in many science museums. Since the pendulum is affected by air resistance, it will gradually slow down, so different mechanisms are installed in the pendulum structures to ensure steady oscillation of the pendulum. For example, the movement of the pendulum is accelerated by electromagnetic action on the bob so as to accurately compensate for the deceleration due to air resistance. A simpler way, which is also quite often used, is to restart the pendulum. Pic 27: Foucault's pendulum