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## GRADE 8 MATTERIALS OF PHYSICS 1 st semester

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**GRADE 8 MATTERIALS OF PHYSICS**1st semester • FORCE • WORK • ENERGY • PRESSURE**Standard Competence**• Students are able to understand the concept of motion, the role of effort, force, and energy in the daily life • Basic Competence • Students are able to analyst experiment’s data of contact and non contact force, resultant force, and the application in daily life SMP N 2 KENDAL**Key words:**• Force = gaya • Contact force = gaya sentuh • Non- Contact force = gaya tak sentuh • Frictional force = gaya gesek • Muscular force = gaya otot • Spring force = gaya pegas • Static frictional force= gaya gesek statis 8. Kinetic frictional force= gaya gesek kenetik 9. Newton’s Law = Hukum Newton 10. Inertia = kelembaman 11. Electric force = gaya listrik back next**FORCE**What is force? Push a door open. Pull a rubber band. Lift your books. In each case, you give some force. Force is a push or pull given by an object to another object. Sometimes the effects of force can be easily seen, like when a running car strikes a tree. Force effects on the tree can be observed. But not all force effects are observable.**How can you measure force? Force is measured**using a spring balance, as shown in Figure 5.1. The unit of Force is Newton (N).**force**Contact force Non –Contact force Muscular force Spring force Frictional force, etc Magnetism force Gravitational force Electric force**The Effect of Force on an Object**• The objects moves. • The objects at rest. • Force changes the speed of an object. • Force changes the shape • Forces can change the direction Figure 5.2 Forces given to the ball change the speed of the ball.**Balanced Forces and un Balanced Force**( RESULTANT FORCE ) • Forces on an object that are equal in size and have opposite are called balanced forces. • Unbalanced forces always change the speed and direction • of an object. a). + = + = 0 + = b). Figure 5.5 Two forces can be added up (a), can be balanced (b), or can be subtracted (c). c).**Newton’s First Laws of Motion**• Sir Isaac Newton (1642-1727) was finally able to formulate the laws explaining the effects of those forces on an object. • Newton's first laws of motion states that an object moving at a constant velocity will keep moving at the same velocity, unless there is a resultant force acting on the object. • If an object is at rest, it stays at rest, unless there is a resultant force causing it to move. • This law is sometimes called the inertia law.**Newton's Second Laws of Motion**• ( Accelerated motion ) • If you pull a toy-car, it will start to move. • The more strongly you pull the car, the faster it moves. • The bigger the force, the faster the acceleration. • If your toy-car is loaded, you must pull more strongly to move it. • The force acting on the toy-car is proportional to the mass and acceleration of the toy-car. • This statement can be written mathematically as follows: • Force = mass x acceleration • Or • F = m x a • This equation is known as the second of Newton's laws of motion.**With : F = Force ( N )**m = mass ( kg ) a = acceleration ( m/s2) 1 N = 1 kgm/s2 Figure 5.15 (a) The acceleration of the toy car depends on the force given by the child. (b) To move at the same acceleration, the child must pull more strongly.**The second of Newton's laws of motion states that a force**working on an object causes the object to be accelerated in the same direction as the direction of the force. The acceleration is dependent upon the force and the mass of an object.**Example**Someone is pushing a 250-kg table using the force of 75 N. Measure the acceleration of the table. Steps in solving the problem: 1. What is known? mass of the table, m = 250 kg force, F = 75 N 2. What is not known? acceleration, a 3. Choose the equation, F = m x a 4. Solution: F = m x a, so a = 0.3 m/s2**Problems**1. What is the force used to accelerate a motor- cycle having a mass of 200 kg while the rider has a mass of 70 kg and the acceleration is 4 m/s2 ? 2. A child is pulling a toy-car having a mass of 2.5 kg with a 4N force. How much is the acceleration of the toy-car?**Newton's Third Laws of Motion ( Action and Reaction)**• Action and Reaction • Action and reaction forces always work in pair. • 2. Action-reaction forces always have the same amount of force and are in the opposite directions. • 3. When the first object gives a force to the second object, the second object gives a force to the first. Action force and reaction**Newton’s third laws of motionstates a relationship between**action and reaction forces :When the first object gives a force to the second object, the second object gives a force to the first object of the same quantity but of an opposite direction.**Air Resistance**gravitation**The example OF Action force and reaction**1. Rocket Launching Reaction Force “Action force” of the gas pushes the rocket up. “Reaction force” of the rocket pushes the gas out.**2.**When the feet and the hands of the swimmer push the water, the water pushes the swimmer back. This Action force and reaction**Sir Isaac Newton**Born: 4 Jan 1643 in Woolsthorpe, Lincolnshire, EnglandDied: 31 March 1727 in London, England**FRICTION**Friction is a force to oppose motion between two touching surfaces. Kinds of friction: Static friction : on bodies at rest Kinetic friction: on bodies in motion Friction between two surfaces depend on: The nature of surfaces in contact. Friction is greater if the surfaces are rough. The force pressing on the surface together**THE HARMFUL AND BENEFICIAL FRICTION**• The following are the harmful frictions ( The Disadvantageous): • Friction between car engine and its clutch can produce exceed heat, which can break down engine. • Friction between air and car can resist the car movement. • Friction between the tire of a vehicle and the road causes the tire become thin • The following are the beneficial frictions (The advantageous): • Friction between our feet and the surface of a road makes us able to walk. • Friction in the braking system can slow down the vehicle. • Friction between a tire and the road surface so that vehicle is not slip.**Gravitational Force**• Gravitation is a pulling force of an object to another object • The gravitational force of objects is determined by two things: the mass of the objects and the distance between the objects. Figure 5.11 The coconut is falling down and accelerated to the ground because of the gravitation.**Weight**• Weight of an object arises because of gravitational force. • Weight of an object on or above the earth is the gravitational force that the earth exerts on the object. • The weight always acts downwards towards the center of • the earth. • On any other astronomical body , weight is the • gravitational force exerted on the object by that body. • Unit of weight is Newton (N), mathematically: W = m x g**EXAMPLE**• There are two bodies, which each has a mass of 2 kg and 5 kg • respectively. The two bodies are at the same place. • If acceleration due to gravity in that place is 9.8 m/s2, determine • the weight of each body. • 2. An Astronaut has a weight of 490 N on the earth. If on the moon • acceleration due to gravity is 1.6 m/s2, what is the astronaut’s • weight on the moon. g EARTH = 9.8 m/s2**Relation between Mass and Weight**• Mass and Weight are not the same quantity ! • Mass is a quantitative measure of inertia • Mass is an intrinsic property • Weight varies depending on the location (height from earths surface etc. • The Unit of weight is Newton (N), mathematically: W = m x g With : W = weight ( N) m = mass ( kg) g = acceleration of earth’s gravitation ( m/s2)**DRAW CONCLUSION**• Force acting on a body can cause the change of shape and size of the body, Change of body motion’s direction, and change of body condition (the body at rest becomes moving or moving body becomes at rest) 2. Based on its property, force grouped into touchable (contact) and untouchable force (non contact) force. 3. Based on its cause, force grouped into muscular force, spring force, magnetic force, electric force, machine force, etc. 4. A force can be drawn using a vector diagram which has shape of arrow.**5. The unit of force is Newton (N)**1 N = 1 kg m/s2 1 N = 105 dyne 6. Resultant of forces acting on a body have the same direction, opposite direction. 7. The formula of weight is w = m x g**EVALUATION**1. Explain the definition of Force and Acceleration . 2. Mention changes which can produced by a Force 3. Mention kinds of Forces based on their cause. Give each one example. 4. Draw vector diagram of the following forces, with length of each 1 N is equal to 1 cm. F1= 2N to the right, F2= 3N to the left, F3= 5N to the right. Then calculate the resultant of those three forces**5. Calculate weight of these two parachutist below, with**Earth gravitational acceleration is 9.8 m/s2! A=56 kg B= 60kg 6. Weight of a body around equator is 1960 N. While weight of the body in north pole is 2000N. If gravitational acceleration around equator is 9.8 m/s 2 , what is gravitational acceleration in the north pole?**7. A car of mass 200 kg is pushed by two men. The first man**push with force of 500 N, while the second man of 400N. Calculate the acceleration of the body. 8.Two children push a cupboard to the right. Resultant of those forces is 90N. If ratio of those two forces is 7:8, calculate value of each force**1 and 2**Each of the diagrams shows the forces on an objetc of mass 2.0 kg. Find the acceleration of the object. 1 2 24N 10 N 20 N 10 N 8N**11. Please explain the harmful and the beneficial of**friction!