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Transcript
KS4 Physics Forces 1 of 31 © Boardworks Ltd 2005 Contents Forces Types of Forces Balanced and Unbalanced Forces Newton’s Laws Summary Activities 2 of 31 © Boardworks Ltd 2005 What is a force? A force is a push or a pull. A force cannot be seen but you can see how a force affects an object. 3 of 31 © Boardworks Ltd 2005 What type of force? 4 of 31 © Boardworks Ltd 2005 Contact and non-contact forces When two objects or materials need to be touching for a force to have an effect, it is a contact force. Examples: friction air resistance When two objects or materials do not need to be touching for a force to have an effect, it is a non-contact force. Examples: gravity electrostatic magnetic Non-contact forces act over a distance. Are these non-contact forces attractive, repulsive or both? 5 of 31 © Boardworks Ltd 2005 Effects of friction Friction is a type of force that always acts in the opposite direction to which an object is moving and slows it down. Whenever there is friction between two objects, heat is generated and their surfaces eventually wear away. The effect of friction can be reduced by using a lubricant. Oil is a common lubricant that is used in car engines and bike gears to reduce friction effects. 6 of 31 © Boardworks Ltd 2005 What is friction? If you rub your hands together they get warm. There is resistance to the rubbing motion. What is the name of this resistive force called? It is called friction. What causes this force? Your hands might look smooth, but on a microscopic level they have rough surfaces. So when you rub your hands together you feel the resistive force of friction. 7 of 31 © Boardworks Ltd 2005 In which direction does friction act? Friction always acts in the opposite direction to which an object is moving. An object will only start to move if the forces applied to it are greater than any frictional forces. What is the direction of friction acting on each moving ball? friction friction friction friction 8 of 31 © Boardworks Ltd 2005 Gravitational attraction Gravity is an attractive force that exists between all masses. The larger the mass, the greater the gravitational attraction. The greater the distance between masses, the smaller the gravitational attraction. The Earth has a large mass and so produces a strong gravitational force. The Moon is kept in orbit around the Earth by the pull of the Earth’s gravity. 9 of 31 © Boardworks Ltd 2005 Gravity and weight The pull of the Earth’s gravitational force on an object is called weight. The Moon also has a gravitational force. Why is the weight of an object on the Moon less than the weight of the same object on the Earth? The Moon is smaller than the Earth and so the pull of the Moon’s gravity is weaker than the pull of the Earth’s gravity. This means that the weight of the object is less on the Moon. 10 of 31 © Boardworks Ltd 2005 Mass and weight What is the difference between mass and weight? mass = 1 kg 1kg weight = 10 N 11 of 31 Mass is the amount of matter that makes up an object. Weight is a force due to the pull of gravity on an object. The mass of an object is always the same, wherever it is in the Universe. The weight of an object will vary depending on where it is in the Universe. The units of mass are kilograms (kg). The units of weight are newtons (N). © Boardworks Ltd 2005 Mass and weight – true or false? 12 of 31 © Boardworks Ltd 2005 Contents Forces Types of Forces Balanced and Unbalanced Forces Newton’s Laws Summary Activities 13 of 31 © Boardworks Ltd 2005 Force diagrams A force diagram uses arrows to show the forces acting on an object. The direction of each arrow shows you the direction of each force. The size of each arrow can be used to compare the sizes of the forces. air resistance What is the force diagram for this falling object when it first starts to fall? weight 14 of 31 © Boardworks Ltd 2005 Forces on still objects What forces are acting on Mel’s computer? The computer is pulled downwards by the force of gravity and causes it to have weight. The table exerts an equal and opposite force pushing upwards on the computer. This is called the normal force. weight reaction force These forces are balanced so the computer does not move. What forces are acting on Mel as she works at her computer? 15 of 31 © Boardworks Ltd 2005 Forces and motion If the forces on an object are balanced, the object will continue to do what it is already doing without change. If the object is stationary, it will remain stationary. If the object is moving, it will continue to move at the same speed and in the same direction. If the forces on an object are unbalanced, two things about the object can change: The speed – the object may speed up or slow down. The direction of motion. 16 of 31 © Boardworks Ltd 2005 Unbalanced forces and motion If an object is stationary and unbalanced forces act on it, what will happen to the object? The object will start to move – its speed and direction have changed. If an object is moving and unbalanced forces act on it, what can happen to the object? The speed of the object can change. It might speed up or slow down. The direction of the object can change. 17 of 31 © Boardworks Ltd 2005 Balanced or unbalanced forces? 18 of 31 © Boardworks Ltd 2005 Friction and movement 19 of 31 © Boardworks Ltd 2005 Contents Forces Types of Forces Balanced and Unbalanced Forces Newton’s Laws of Motion Summary Activities 20 of 31 © Boardworks Ltd 2005 Newton’s Laws of Motion 1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2nd Law – Force equals mass times acceleration. 3rd Law – For every action there is an equal and opposite reaction. 21 of 31 © Boardworks Ltd 2005 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 22 of 31 © Boardworks Ltd 2005 Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force. 23 of 31 © Boardworks Ltd 2005 Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. 24 of 31 Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop! © Boardworks Ltd 2005 Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – FRICTION! 25 of 31 © Boardworks Ltd 2005 There are Four main types of Friction: 26 of 31 Sliding Friction: Ice Skating Rolling Friction: Bowling Fluid Friction (air/liquid): Air or water resistance Static Friction: initial friction when moving an object © Boardworks Ltd 2005 Newtons’s1st Law and You Don’t let this be you. Wear seat belts! Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour. 27 of 31 © Boardworks Ltd 2005 Newtons’ 2nd Law of Motion The net force of an object is equal to the product of its mass and acceleration, or F=ma. 28 of 31 © Boardworks Ltd 2005 Newton’s nd 2 Law The net force of an object is equal to the product of its mass and acceleration, or F=ma. 29 of 31 © Boardworks Ltd 2005 Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. We know that objects with different masses accelerate to the ground at the same rate. However, because of the 2nd Law we know that they don’t hit the ground with the same force. 30 of 31 © Boardworks Ltd 2005 Newton’s 3rd Law For every action, there is an equal and opposite reaction. 31 of 31 © Boardworks Ltd 2005 According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. 32 of 31 © Boardworks Ltd 2005 There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces. 33 of 31 © Boardworks Ltd 2005 Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. 34 of 31 © Boardworks Ltd 2005 Contents Forces Types of forces Balanced and Unbalanced Forces Newton’s Laws Summary activities 35 of 31 © Boardworks Ltd 2005 Multiple-choice quiz 36 of 31 © Boardworks Ltd 2005