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Transcript
Forces and Motion What is a Force? • A force is a push or pull. • SI unit for force is Newton Force • A force is described by its strength(magnitude) and by the direction in which it acts. Combining Forces • When forces act in the same direction they add together. • When forces act in opposite directions, you subtract the forces and it moves in the direction of the stronger force. • Once forces are combined known as net force Unbalanced forces • Unbalanced forces acting on an object result in a net force , and causes a change in the object’s motion. Balanced Forces • Equal and opposite forces that result in no change of direction or position. Friction • Friction is a force that 2 surfaces exert on each other when they rub against one another. • Friction depends on 2 factors: – How hard objects push together – Type of surfaces 4 types of friction • Static Friction: friction on an object not moving (ex: trying to move a desk) • Sliding friction: friction from sliding (Ex: Skis) • Rolling Friction: friction from rolling (Ex: Bike) • Fluid Friction: Friction with liquid or gases. (ex: Air resistance) Gravity • Force that pulls objects toward each other • Universal Gravitation: all objects exert a gravitational force. 2 factors Affect Gravity • Mass of objects • Distance between objects Weight and Mass • Weight- how much force is exerted on an object due to gravity • Mass- how much matter in an object. Free Fall • In free fall the only force acting on an object is gravity, so object accelerates toward ground. Air Resistance • Objects falling through air experience an opposite force known as air resistance. Terminal Velocity • Greatest velocity an object reaches is called its terminal velocity. Newton’s 3 Laws of Motion The first Law of motion • Newton’s first law states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force. • VIDEO: CAR CRASH • https://www.youtube.com/watch?v=y118jLg2 0i0 Inertia • Inertia is the tendancy of an object to resist a change in motion. Newton’s first law is called the law of inertia. Inertia depends on mass. The more mass the more inertia. The Second law of Motion • According to Newton’s second law of motion, acceleration depends on the object’s mass and on the net force acting on the object Acceleration= Net Force Mass Newton’s third Law • Newton’s third law states that if one object exerts a force on another object , then the second object exerts a force of equal strength in the opposite direction on the first. • https://www.youtube.com/watch?v=BiLq5Gn po8Q Action-Reaction Pairs • When you jump you push off the ground with your feet, this is an action force. The ground pushes back on your feet with an equal and opposite force, this is the reaction force. Detecting Motion • You can’t always detect motion. • For example when you drop a pencil, you see the pencil fall towards the ground, but you don’t see the pull of the ground to the pencil because the ground is much larger and has a much greater inertia (resistance to move) Action and reaction forces don’t cancel out • Because the 2 forces act on different objects • Example: Girl hitting a volleyball a force on her wrists and on the ball. Momentum • “Quantity of motion” • Momementum = mass x velocity • The more momentum an object has the harder it is to stop. Conservation of Momentum • In the absence of outside forces, the total momentum of objects that interact does not change. • The total momentum of any group is conserved or remains the same unless outside forces act on the objects. • See page 358 Key term to know • Centripetal force: Any force that acts on an object to move in a circular path