 # Motion Speed What is Speed Speed distance time

• Slides: 28 Motion Speed What is Speed? Speed = distance time Speed is the distance traveled by a moving object per unit of time. You can calculate the speed of a moving object by dividing the distance the object travels by the time it takes to travel that distance Speed that does not change is called constant speed. But the speed of a moving object is not always constant. Dividing the total distance by the total time gives the average speed. Total Distance = Average Speed Total Time Velocity is speed in a given direction One day when Newton was drinking tea in the garden, he saw an apple fall to the ground. He started thinking about why it fell, and finally concluded that the same force that caused the apple to fall also kept the moon in orbit around the earth. This same force also kept the planets in orbit around the sun. What is the name of the force that this famous scientist described? GRAVITY • In other words, the force of gravity exists between any two objects. • The magnitude (strength) of the gravitational force depends on the MASS of the objects and the DISTANCE between the two objects. • The apple incident led Newton to put forth three laws which explain why objects move (or don't move) as they do and these three laws have become known as: _ _ _ 's three LAWS of MOTION Newton’s First Law An object at rest stays at rest And An object in motion stays in motion UNLESS A force acts on it. What forces do you see acting on these? Newton’s nd 2 Law An object accelerates, or changes direction, when a force is applied. Furthermore, the acceleration due to a force is always in the direction of the force. When there are unbalanced forces we know that F=ma. Acceleration is the change in velocity (that is a change in speed or direction) What is the change in this roller coaster? Newton’s rd 3 Law For every action there is an equal and opposite reaction. What is the action and reaction here? Consider friction – which always opposes motion. What is a Force? A push or a pull Friction effects motion Sliding Rolling Fluid Sliding friction When two solid surfaces slide over each other the sliding friction acts between the surfaces. When you push a chair across the floor, sliding friction opposes your motion. Sliding friction What kinds of things effect the amount of sliding friction present? The amount of sliding friction present depends on two factors: the weight of the object that is moving and the types of surfaces that the object slides across. There is more friction when a stack of cartons is pushed than when just one carton is pushed. But there is less friction opposing the motion if the cartons are pushed across a smooth floor rather than across a carpeted one. Sliding Friction Besides “Curling” the Olympic sport shown in the previous slide where else can you find sliding friction and is it helpful or a hindrance? Rolling friction Tends to oppose motion less than sliding friction. So wheels are often placed under objects to make it easier to use them. Rolling Friction Besides Bowling shown in the previous slide where else can you find rolling friction and is it helpful or a hindrance? Fluid friction All liquids and gases are fluids. Air, water, and oil are fluids. When an object moves through a fluid, fluid friction opposes the motion. Air resistance is an example of fluid friction. Substances called lubricants change sliding friction to fluid friction. Fluid Friction Again what examples of fluid friction can you think of be specific? How do we use this friction to help and how does it hinder us? Energy Kinetic Energy Potential Energy Kinetic Energy is energy of motion. When you walk, run, swim and jump, you have kinetic energy. Kinetic Energy The faster an object moves, the more kinetic energy it has. So kinetic energy is directly related to the velocity of an object. In baseball, a fast ball has more kinetic energy than a slow curve. You have more kinetic energy when you ______than when you ______. Kinetic Energy A battleship that is moving at 40 km/hr has much more kinetic energy than a mosquito moving at the same velocity. So kinetic energy must depend on something other than just velocity. The battleship has more kinetic energy because it has greater mass. Potential energy is energy stored in an object due to its position. Potential energy Stretch a rubber band between your thumb and index finger. Keep the rubber band stretch without any motion. How long can you hold it this way? After a short while you begin to sense the energy in the rubber band. Yet the rubber band is not moving! The stretched rubber band has energy stored in it. You cannot see this energy, but you know it is there because the stretched rubber band can do work as it returns to its normal shape. Energy Conversions Changes in the forms of energy are called energy conversions. One of the most common energy conversion involves the changing of potential energy to kinetic energy OR kinetic energy to potential energy. Energy Conversion Can you think of an example of an energy conversion? Either Kinetic to Potential OR Potential to Kinetic? How about in your Egg Bungee Jump?