TOPIC 10: FORCES TERM 3 STARTING OFF: PAGE 129 WHAT IS A “FORCE”? • A force is an action that can be described as a push or pull. • We are unable to see forces, but the effect of forces can be seen everywhere. • We exert a force when we pull, push twist to change the shape, direction and speed of a substance. • Force is measured in Newton ( named after Sir Isaac Newton. DESCRIPTION OF FORCES? REFER TO FIGURE 3 ON PAGE 130 DO ACTIVITY 2 ON PAGE 131 WHAT ARE FORCE PAIRS? • when you apply a force to an object, the object resists the force. • the resisting force is equal in size as the initial force, but it is exerted in the opposite direction. • figure 9(a) on page 132 shows us how a boy is pushing against a wall (f1) and the wall resists in the same force size but in the opposite direction (f2) • we call these two opposing forces force pairs. BALANCED AND UNBALANCED FORCES • Balanced forces are those that display no visible effect ( no movement) because these forces are equal in size; just acting in different directions. • Figure 9(b) on page 132 shows us the two forces acting on the books. what are these two forces? • These two forces acting on the books are balanced. Why? • But if one force acting on an object is greater than the other force acting on the same object, then the forces acting on that object will be unbalanced. • Consider figure 11(b) the two forces acting on the box are the force of friction and the pushing force of the boy on the box. • If the pushing force of the boy is greater than the frictional force, then the two forces are imbalanced, and the box will start moving. • If there is an imbalance of forces, then a visible effect will be seen in the form of movement or change in the object’s shape. CONTACT FORCES AND FIELD FORCES ( NON-CONTACT FORCES) • Forces acting on an object can be classified into two broad groups : contact forces and non-contact forces • A contact force is exerted when two objects are in contact (when they touch each other) • A non-contact force ( field force) acts over a distance. the two objects exert field forces on each other without touching each other. TYPES OF CONTACT FORCES • FRICTION • TENSION • COMPRESSION • FRICTION Contact force that a surface exerts on an object. • A ball rolling on the ground will slow down and eventually stop, this happens because of friction. the surface of the ground rubs against the surface of the ball. • Frictional force always acts parallel to the surface and it also acts on the opposite direction. • It can prevent an object from moving. • If you are pushing against a heavy box that is on the grass like on page 134, the grass will exert a frictional force on the box against your pushing force. • If your pushing force is smaller than the frictional force the box will not move. • But if it’s greater than the frictional force the force will move. • Friction sometimes produces heat. READ CASE STUDY ON PAGE 135 TENSION • Contact force in a rope or cable when it is used to carry a load. • If you hang an object from the ceiling using a rope tension is created in the rope. If the object is too heavy, the tension in the rope will be too much and the rope will break. • Some materials stretch if too much tension increases in them. • Figure 15 on page 135 shows you another example of tension. COMPRESSION • A contact force that pushes or squeezes something into a smaller space. • Compression can push something into another space. when you push toothpaste from a tube, you are compressing the tube of the toothpaste to force the toothpaste onto your toothbrush. DO ACTIVITY 4 ON PAGE 134 AND ACTIVITY 5 ON PAGE 135 FIELD FORCES • Gravitational force • Magnetic force • Electrical force • Weight GRAVITATIONAL FORCE • Is a pulling force that bodies exert on each other over a distance due to their masses. • Any object in the universe will exert a force of attraction on any other object because all matter have mass. • Gravitational forces act in a gravitational field. You exert a tiny gravitational force on all people and object in the classroom and each of these exert a tiny equal gravitational force on you. GRAVITATIONAL FORCE WORKS IN PAIRS • You exert a gravitational force on the person sitting next to you. that person exerts exactly the same size gravitational force on you, but in the opposite direction. THE SIZE OF GRAVITATIONAL FORCE • Sir Isaac Newton discovered that the gravitational force that two objects exert on each other become smaller if the two objects are moved further apart. • For example: if you move away from the classroom wall, the gravitational force that the wall exerts on you becomes smaller. the gravitational force that you exert on the wall will also become smaller. • He also noticed that the more mass the objects have, the larger the gravitational force they exert on each other. refer to figure 21 on page 137. WEIGHT • The gravitational force that a celestial body ( from outer space) such as a moon or a planet exerts on an object close to its surface. • weight is measured in newtons because weight is a measure of the force of gravity on a physical object. • your mass on earth will be different from what it will be on the moon or mars. the amount of gravitational force exerted depends on the masses of the bodies. • mass is a measure of the amount of matter that it is made up of, measured in grams (g) or kilogram (kg). if your mass is 65kg on earth, it will be 65kg on the moon and on any other planet. your mass will remain the same, unlike your weight. • Your weight on earth = mass in kilograms × 9,8 • Your weight on the moon = mass in kilograms × 1,63 • A spring balance or force meters are used to measure the weight of the object on earth. MAGNETIC FORCE • Is a force that two magnetic materials exert on each other over a distance. • Magnetic objects have magnetic fields around them ( see figure 26(b) ) • Iron, cobalt and nickel, and man-made steel are all magnetic. Types of Magnetic force • Force of attraction- a pulling force that two objects exert on each other. • Force of repulsion- a pushing force that two objects exert on each other. • A magnet has a north and south pole therefore when two magnets face each other with the same charge, they will push each other away. • Unlike charges will attract each other. MAGNETIC FIELD OF EARTH • EARTH IS ONE LARGE MAGNET; IT HAS A NORTH POLE AND A SOUTH POLE. • IT HAS A MAGNETIC FIELD AROUND IT WHICH IS CAUSED BY THE ELECTRIC CURRENT THAT IS CONSTANTLY CIRCULATING WITHIN IT. • FIGURE 29 ON PAGE 141 SHOWS HOW THE FORCES OF ATTRACTION AND REPULSION WORK. ELECTROSTATIC FORCE • IS A FORCE THAT TWO ELECTRICALLY CHARGED OBJECTS EXERT ON EACH OTHER OVER A DISTANCE. THESE FORCES ACT IN AN ELECTROSTATIC FIELD, WHICH EXISTS AROUND ANY CHARGED OBJECT. HOW DO OBJECTS BECOME CHARGED? • ALL SUBSTANCES ARE MADE UP OF ATOMS. • DESCRIBE THE STRUCTURE OF AN ATOM • IT TAKES A LOT OF ENERGY TO SPLIT THE NUCLEUS OF AN ATOM APART. • THE ATOM IS ELECTRICALLY NEUTRAL. WHY IS THIS? • ELECTRONS ARE TRANSFERRED FROM ONE SUBSTANCE TO ANOTHER EASILY, ONE COMMON WAY IS BY RUBBING THE SUBSTANCES TOGETHER. • THE SUBSTANCE THAT GAINS ELECTRONS BECOMES ELECTRICALLY NEGATIVE WHILE THE SUBSTANCE THAT LOSES ELECTRONS BECOMES ELECTRICALLY POSITIVE. ELECTROSTATIC POTENTIAL ENERGY • POTENTIAL ENERGY= STORED ENERGY IN A SYSTEM. • WHEN YOU LIFT A BOX ABOVE THE GROUND, YOU NEED TO DO EXTRA WORK TO OVERCOME THE GRAVITATIONAL FORCE OF THE EARTH ON THE BOX. • IF THE BOX IS HELD ABOVE THE GROUND, THE BOX AND EARTH FORM A SYSTEM THAT STORES GRAVITATIONAL POTENTIAL ENERGY. • IF YOU LET GO OF THE BOX AND IT FALLS, THE STORED POTENTIAL ENERGY BECOMES KINETIC ENERGY. • AN EXAMPLE OF ELECTROSTATIC POTENTIAL ENERGY USUALLY OCCURS ON A COLD DAY WHEN YOU WEAR A JERSEY ON TOP OF YOUR UNIFORM. • THE UNIFORM AND THE JERSEY ARE IN DIRECT CONTACT AND AS YOU MOVE ELECTRONS ARE RUBBED OFF FROM ONE CLOTHING ITEM TO ANOTHER. BOTH ITEMS BECOME ELECTRICALLY CHARGED. • THIS MEANS THAT BOTH ITEMS HAVE ELECTROSTATIC POTENTIAL ENERGY. LIGHTNING • LIGHTNING IS A GOOD EXAMPLE OF THE EFFECTS OF ELECTRIC CHARGE. WHAT HAPPENS DURING LIGHTNING? • CLOUDS ARE MADE UP OF WATER MOLECULES. AIR MOVES INSIDE A THUNDER CLOUD AND IT CREATES FRICTION BETWEEN THE WATER PARTICLES AND THE AIR PARTICLES. • THIS CAUSES THE CLOUD TO BECOME POSITIVELY CHARGED AT THE TOP AND NEGATIVELY CHARGED AT THE BOTTOM. • IF THERE IS A GREAT BUILD UP OF CHARGE, THE NEGATIVE CHARGES JUMP FROM THE CLOUDS TO THE GROUND. • THIS IS CALLED DISCHARGE WHICH CAUSES THE CLOUD TO BE LESS NEGATIVE AND THE GROUND TO BE LESS POSITIVE. • WE OBSERVE IT AS A LIGHTNING STRIKE.
