Physics - Force and Machines PDF

3 Force Learn about WHAT IS FORCE? TP " What is force? " Effects of f...

3 Force Learn about WHAT IS FORCE? TP " What is force? " Effects of force Inour everyday life,we often have to " Force of friction push or pull objects in order to move Types of friction them. When you push anobject, it moves " Advantages and disadvantages qWay from you and when you pull an of friction object, itmoves towards you. " How to increase and decrease friction For example, a boOx hasto be pushed to shift it and asuitcase has to be pulled to roll it along a railway platfornm. A push or Force oas a Pull apul acting onan object is caled force. Ahandcart used to move goods, openinga drawer, and ahorse puling a Whenever we bend, stretch, lift something. walk, or just stand still, a push or a pull is Cart (Fig. 3.2) are examples of force as a involved. pull. Force as a Push Pushinga stalled car to one side, pressing akey on akeyboard,and mowing fhe lawn with a lawnmower (Fig. 3.1) are examples of force as a push. (a) Handcart used to move goods (b) Opening a drawer (a) Car being pushed (c) Horse pulling a cart Fig. 3.2 Force as a pull Sometimes, both push and pull can be simultaneously applied. For example, (b) Pressing a key on (c) Lawnmower mowing a lawn when we try to move a heavy sofa, some a keyboard people push it and others pull at it. Fig. 3.1 Force as apush We may also apply force that may not 39 always endup in moving things, sucn ds Spring force Siffingon a chair or leaning on a wall. The force exerted by a stretched spring upon any Forces Act in Pairs is attached to it is called spring object When an obiect exerts force on one the Figure (3.4(0)J we cun see thot 1orce object,the second object also exerts spring can both push the bol an opposite force on the first object. So, when we try to stretch a spring, the spring pull it inward. torwoId also tries to exert an equal and opposie Force of Friction force on our arms (Fig. 3.3). The force that opposes motionis friction. It is applied by one col eg another. The striker on a carom surtaboard ce over comes to stop after we strike it Fig, 3.3 Forces act in pairs of the force of friction. because Another example is that when a book rest on atable, the book exerts a force on Normal force the table and the table exerts a force on The support force or reaction force the book. That is why the book is able to exerted upon an object that is in contor rest on the table. with another stable object is called normal force. A book kept on a table Forces can be divided into contact and exerts a downward force on it due to its non-contact forces. Let us learn more weight. The table also exerts an upward about these forces. force on the book [Fig.3.4(c)]. 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. Let us learn more about some types of contact forces (a) Tension force N Normal Tension Force force It is the force that acts on a string when anobject is hung from a string. An example is the tension in the steel cables (0006000066000 w on acrane when a car is being lifted by Weight it[Fig. 3.4()].Tension force is always a pulling force. (b) Spring force (C) Normalforce Fig. 3.4 Contact Forces 40 Non-contact Forces EFFECTS OF FORCE TP Someforces do not involve physical effects can Force cannot be seen. but its contact between two bodies. They act at of force are beseen or felt. Some effects adistance. Gravitational force, electrical given below. force, and magnetic force are examples object of such forces. Force can start to move an football starts atrest. For example, a Gravitationalforce moving when it is kicked. if All bodies that have mass exert Force can stop a moving object if opposite aravitational force on each other. It is is applied in the direction football to the motion. For example, a always attractive in nature. being stopped by a goalkeeper. the objects involved have very small Force can change the direction of example, masses, then the gravitational force is the motion of an object. For direction very small. Since the Earth has a huge in the game of cricket, the mass, it attracts everything on it or near of amoving ballcan be changed a bat the surface towards it. The gravitational byapplying force on it through force exerted by the Earth is called [Fig. 3.5(a)]. gravity. Force can change the speed of a Afruit falling down from a tree, rain falling moving object, i.e., force can make from the sky, and a ball thrown up coming the object go either slower or faster. down are alldue to gravitational force. For example, if you are walking with a trolley, applying force in the direction Elecirical Force of its movement can increase its speed, whereas decreasingforce on it Matter has aproperty called 'charge', Can slow it down. about which you will learn in later Force can change the shape of an classes. The force exerted by charged object but not the mass. For example, objects oncharged or uncharged the shape of dough changes when objects is called electrical force. we apply force on itwhile making An example of this force is when a plastic chapatis [Fig. 3.5(b)]but the mass comb rubbed on hair is brought close to does not change. bits of paper; the paper bits get attracted towards the comb andstick to it. Magnetic Force The force of attraction or repulsion exerted by magnets is called magnefic force.A magnet attracts objects made of iron, cobalt, or nickel. Magnets exert force of (a) Changing the direction of (b) Changing the shape an object of an object aftraction or repulsion on other magnets. Fig. 3.5 Effects of force 41  20N Ihe Sl unit of force is newton 10N (synbol N) In many situations, there is more than one force actingon abodyThe total 70 N force (or net force) applied on a body Is 50N called resultant force When wo forces are acting in the same 25 N 25N directionon abody, then the resultant force is obtained by adding the two forces together. force depend Fig. 3.6 Resultant both on magnitude and direction If two forces are acting in the opposite directicon and one force is greater than Both the amount (magnitude) and the other, then the resultant force will be direction of a force are important the difference between the two forces and the body will move in the direction of We can also see from the above examples that only unbalanced the greater force. Can make an object move froma forces If two equal forces act in exactly stationary position. If the forces are opposite directions on a body, thern the body does balanced, the object willnot move fom not move. We say that resultant force is a stationary position. zero (Fig. 3.6). Questions 1. Write down whether the force is a push ora pull. h 2. Divide the following forces into contact and Gravitational force, normal force, tension force,non-contact spring force, forces. force electric force, magnetic 42 FORCE OF FRICTION to electrostatic forces So whern we over the othr The force thai opPOsES OI resists the Try fo slhde on Surfoco motion of one surface acOSS another Is these offer a ros1stance to motion alled the force of friction Depressons and Some characternstics of friction are given elevations lock into below eachother Frction always acts in the direction opposite to motion. So, unless the Friction acts in the force that is moving the object is direction opposite enough to overcome the frictional to motion. force. the object will come to a stop. That is why arolling ball stops on its own after rolling for awhile. Direction of The frictional force depends on the motion. type of surfaces in contact. If the surfaces are smooth, friction will be less.That is why a ball rols for a longer of how Fig. 3.7 Diagrammatic representation fime ona smooth floor. friction works The frictional force also depends on Friction produces heat (e.g.. your how heavythe object is. If the object palms feel warm when yOu rub them is heavy, friction will be more. together). Friction is a relatively complex Friction is also produced when objects phenomenon to explain.One theory move through air (this is called air is that any smooth surface is actually resistance) or water, especially at high quite rough, consisting of depressions' speeds. So,any vehicle movingthrough andelevations, when examined air like cars and aeroplanes, ships, and closely. As one surface slides over boats al experience friction. the other, these depressions and elevations lock into each other Fact File Causing resistances to motion Space shuttles do not experience friction (Fig. 3.7). in outer space as there is no air in outer Another theory is that when two space.But when the shuttle re-enters the Surfaces come in contact, their atoms earth'satmosphere, it slows down and due and molecules pull at each other also gets heated up due to friction. other parts around it "depression: apart of a surface that is lower than the other parts around it