Force - Physics Notes PDF

# 3 Force ## Learn about - What is force? - Effects of force - Force of friction - Types of friction - Advantages and disadvantages of friction - How to increase and decrease friction ## What is Force? In our every day life, we often have to push or pull objects in order to move them. When you push...

# 3 Force ## Learn about - What is force? - Effects of force - Force of friction - Types of friction - Advantages and disadvantages of friction - How to increase and decrease friction ## What is Force? In our every day life, we often have to push or pull objects in order to move them. When you push an object, it moves away from you and when you pull an object, it moves towards you. For example, a box has to be pushed to shift it and a suitcase has to be pulled to roll it along a railway platform. A push or a pull acting on an object is called force. Whenever we bend, stretch, lift something, walk, or just stand still, a push or a pull is involved. ## Force as a Push - Pushing a stalled car to one side - Pressing a key on a keyboard. - Mowing the lawn with a lawnmower. ## Force as a Pull - A handcart used to move goods. - Opening a drawer. - A horse pulling a cart. Sometimes, both push and pull can be simultaneously applied. For example, when we try to move a heavy sofa, some people push it and others pull at it. We may also apply force that may not always end up in moving things, such as sitting on a chair or leaning on a wall. ## Forces Act in Pairs When an object exerts force on one object, th second object also exerts an opposite force on the first object. So, when we try to stretch a spring, the spring also tries to exert an equal and opposite force on our arms. ## Contact Forces These are forces in which contact or touch between objects is necessary for the forces to operate between them. We give different names for contact forces based on the type of objects in contact. ### Tension Force It is the force that acts on a string when an object is hung from a string. An example is the tension in the steel cables on a crane when a car being lifted by it. Tension force is always a pulling force. ### Spring force The force exerted by a compressed or stretched spring up any object that is attached to it is called spring force. ### Normal force The support force or reaction force exerted upon an object that is in contact with another stable object is called normal force. A book kept on a table exerts a downward force on it due to its weight. The table also exerts an upward force on the book. ## Non-contact Forces Some forces do not involve physical contact between two bodies. They act at distance. Gravitational force, electrical force, and magnetic force are examples of such forces. ### Gravitational force All bodies that have mass exert gravitational force on each other. It is always attractive in nature. If the objects involved have very small masses, then the gravitational force is very small. Since the Earth has a huge mass, it attracts everything on it or near the surface towards it. The gravitational force exerted by the Earth is called gravity. A fruit falling down from a tree, rain falling from the sky, and a ball thrown up coming down are all due to gravitational force. ### Electrical Force Matter has a property called 'charge', about which you will learn in later classes. The force exerted by charged objects on charged or uncharged objects is called electrical force. An example of this force is when a plastic comb rubbed on hair is brought close to bits of paper; the paper bits get attracted towards the comb and stick to it. ### Magnetic Force The force of attraction or repulsion exerted by magnets is called magnetic force. A magnet attracts objects made of iron, cobalt, or nickel. Magnets exert force of attraction or repulsion on other magnets. ## Effects of Force Force cannot be seen, but its effects are seen or felt. Some effects of force are given below: - Force can start to move an object at rest. For example, a football starts moving when it is kicked. - Force can stop a moving object if it is applied in the direction opposite to the motion. For example, a football being stopped by a goalkeeper. - Force can change the direction of the motion of an object. For example, in the game of cricket, the direction of a moving ball can be changed by applying force on it through a bat. - Force can change the speed of a moving object, i.e., force can make the object go either slower or faster. For example, if you are walking with a trolley, applying force in the direction of its movement can increase its speed, whereas decreasing force on it can slow it down. - Force can change the shape of an object but not the mass. For example, the shape of dough changes when we apply force on it while making chapattis but the mass does not change. ## The SI unit of force is newton (symbol N). In many situations, there is more than one force acting on a body. The total force (or net force) applied on a body is called resultant force. When two forces are acting in the same direction on a body, then the resultant force is obtained by adding the two forces together. If two forces are acting in the opposite direction and one force is greater than the other, then the resultant force will be the difference between the two forces and the body will move in the direction of the greater force. If two equal forces act in exactly opposite directions on a body, then the body does not move. We say that resultant force is zero. ## Questions 1. Write down whether the force is a push or a pull. 2. Divide the following forces into contact and non-contact forces. * Gravitational force * Normal force * Tension force * Spring force * Electric force * Magnetic force ## Force of Friction The force that opposes or resists the motion of one surface across another is called the force of friction. Some characteristics of friction are given below: - Friction always acts in the direction opposite to motion. So, unless the force that is moving the object is enough to overcome the frictional force, the object will come to a stop. That is why a rolling ball stops on its own after rolling for a while. - The frictional force depends on the type of surfaces in contact. If the surfaces are smooth, friction will be less. That is why a ball rolls for a longer time on a smooth floor. - The frictional force also depends on how heavy the object is. If the object is heavy, friction will be more. - Friction is a relatively complex phenomenon to explain. One theory is that any smooth surface is actually quite rough, consisting of depressions and elevations, when examined closely. As one surface slides over the other, these depressions and elevations lock into each other causing resistance to motion. - Another theory is that when two surfaces come in contact, their atoms and molecules pull at each other due to electrostatic forces So, when we try to slide one surface over the other. these offer a resistance to motion. ## Friction produces heat (e.g. your palms feel warm when you rub them together). Friction is also produced when objects move through air (this is called air resistance) or water, especially at high speeds. So, any vehicle moving through air like cars and aeroplanes, ships, and boats all experience friction. ## Space shuttles do not experience friction in outer space as there is no air in outer space. But when the shuttle re-enters the earth's atmosphere, it slows down and also gets heated up due to friction. ## Activity **Aim:** To find out which of the given surfaces offers the most friction **Materials required:** A stack of books, a wooden plank, a towel, a sheet of aluminium foil, a plastic sheet, a small lunch box, and a stopwatch **Procedure:** 1. Stack the books and place the wooden plank as shown in Figure A. 2. Let the lunch box slide down the surface and note the time it takes to come to rest. 3. Repeat the activity by wrapping the lunch box with the towel, then the sheet of aluminium, and finally the plastic sheet. **Conclusion:** The surface on which the lunch box takes the maximum time to come to rest offers the most friction. ## Types of Friction Some types of friction are given below. **Static Friction** Static friction is the maximum value of frictional force between two surfaces which can prevent one surface from sliding over the other. It keeps the object stationary when it is placed on a surface. For example, it prevents a car from sliding down a hill when parked. Static friction varies based on the force applied on the object, but has a maximum value. When the applied force overcomes static friction, the object starts moving. At this point, static friction ceases and kinetic friction or sliding friction takes over. For example, if the maximum static friction between a washing machine and a tile floor is 50 N, and you try and move the machine with a force of 25 N, the static friction will be 25 N. If you increase the force you exert to 35 N, the static friction also will increase to 35 N. This continues until the force you apply is greater than the maximum static frictional force which is 50 N. At this point the washing machine budges and starts sliding. Once the washing machine starts sliding, there is no longer static frictional force but only sliding friction. **Sliding Friction** Sliding or kinetic friction is the friction that results from two surfaces being pressed against each other while sliding. **Rolling Friction** Rolling friction occurs when one object rolls across the other. For example, when a ball or a tyre or a wheel rolls on a surface. Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. This is because rolling friction is less than sliding friction. ## Disadvantages of Friction Some disadvantages of friction are given below: - Friction is disadvantageous in situations where we want an object to move at a high speed. This is because friction causes moving objects to slow down. - The heat produced due to friction is the wastage of energy and can damage machines if it is not controlled. - Friction causes wear and tear. A shoe sole wears out after repeated use. In the same way, tyres of vehicles wear out due to friction. ## Advantages of Friction Friction is advantageous in many situations. Some of these are given below: - Friction between the wheels and the road helps a vehicle to move steadily on the road, otherwise it would skid. - Friction between the brake and the wheels makes a vehicle stop when the brakes are applied. - Friction between our feet and the ground helps us to walk steadily. ## How to decrease friction Friction can be a disadvantage; some ways in which friction can be decreased are given below: 1. Friction can be reduced by polishing the surfaces or by applying oil or grease between the surfaces in contact. 2. Wheels can be fixed on an object to reduce friction. Rolling friction is lesser than sliding friction. For example, it is much easier to put a box on a trolley than pushing it. 3. When aeroplanes or ships move they encounter friction from air and water. To reduce this friction aeroplanes and ships are shaped in a way to reduce the amount of friction. This is called streamlining. Bodies of birds and fish are also streamlined. Racing cars are streamlined due to the same reason. ## How to increase friction Some ways to increase friction are given below: 1. Grooves (long narrow cuts) are made into soles of shoes to prevent us from slipping. The grooves make the surface of the sole uneven, increasing friction. Similarly, 2. Handles of cricket bats and badminton racquets have a rough finish so that we can get a good grip. 3. Mobile phone users usually cover their phone with a rough case so that it does not slip out of their hand. ## Wrap Up - A push or a pull acting on an object is called force. - Force can: * move an object * change its direction, speed, or shape * stop a moving object - Friction exists between two surfaces in contact, and always opposes the motion of one surface over the other. It depends on the types of surfaces in contact. - Some types of friction are: static, sliding and rolling friction - Static friction is the maximum value of frictional force between two surfaces which can prevent one surface from sliding over other. - Sliding or kinetic friction is the friction that results from two surfaces being pressed against each other while sliding. - Rolling friction occurs when one object rolls across the other - Disadvantages of friction: * Slows down a moving object * Causes wastage of energy due to production of heat * Causes wear and tear - Advantages of friction: * Provides us with grip and help us to stop vehicles when brakes are applied - Friction can be reduced by polishing, oiling or greasing, also by using wheels or streamlining. Friction can be increased by making grooves and giving a rough finish.