Transcript Slide 1
Force, motion, and machines Force of gravity • Gravity is a force that pulls objects toward each other. • All matter has gravity Force of gravity • 2 main things affect how gravity pulls on an object: – The objects mass (more mass = more pull from gravity) • Example: Why do the planets orbit the sun. – The distance between the two objects (The further the distance = less pull from gravity.) • Example: Why does the moon orbit the earth? Weight and mass • Mass is a measurement of how much matter is in something. • Weight is a measurement of how gravity is pulling down on something. • Weight is affected by gravity, mass is not! Laws of motion • Isaac Newton came up with 3 laws of motion in the late 1600s. Laws of motion • The first law of motion says that an object at rest will stay at rest, and an object in motion will stay in motion unless it is acted upon by a force. • Inertia is when an object resists change in motion. • The more mass something has, the more inertia it has. Friction • Friction is the force created when two surfaces rub against each other. Friction • There are 4 main types of friction – – – – Static friction Sliding friction Rolling friction Fluid friction Friction • Some devices use friction to control the motion of an object. Simple Machines • A machine makes work easier by changing one of 3 things: – The amount of force you have to put out. – The distance you use the force over. – The direction of the force. Simple Machines • A machine’s mechanical advantage is the number of times that a machine increases a force. • (Example: if it makes your job 3 times easier, it would have a mechanical advantage of 3) Simple Machines • There are 6 types of simple machines: – – – – – – Inclined plane Wedge Screw Lever Wheel and axel Pulley Simple Machines – inclined plane • An inclined plane is a flat sloped surface. (Like a ramp). • An inclined plane lets you exert your force over a longer distance. • Where in the school would you find inclined planes? Simple Machines – inclined plane • You can find the mechanical advantage of an inclined plane by using the formula: – Length of incline/height of incline. Simple Machines – wedge and screw • A wedge and a screw are other simple machines that are made from an inclined plane. • Where are some places in the school that you might find wedges and screws? Simple machines - Levers • A lever is a rigid bar that is free to rotate or pivot on a fixed point. • The point that the lever rotates on or around is called the fulcrum. • Levers change the amount of force you have to put out, and they can change the direction of the force. Simple machines - Levers • First class lever: – A first class lever has the fulcrum in the middle. – Examples of first class levers include: • • • • Prying open a paint can Scissors Rowing a boat Pliers Simple machines - Levers • A second class lever has the resistance (load) in the middle. • Examples of second class levers include: – Wheelbarrows – Nutcrackers Simple machines - Levers • A third class lever has the effort in the middle • Examples of third class levers include: – – – – – Brooms Tweezers Tongs Fishing poles Hockey sticks What type of lever is this? Simple machines - Levers • You can find the mechanical advantage of a lever using the formula: – Distance from the fulcrum to the input force / Distance from the fulcrum to the output force. – Input force is sometimes called “Effort” – Output force is sometimes called “load” or “resistance” Calculate the mechanical advantage for the levers ? Simple machines – Wheel and axle • A wheel and axle is made of 2 cylinders of different sizes that rotate together. • A wheel and axle changes the distance over which a force is exerted. • Examples include: – Screwdriver – Doorknob – A faucet Simple machines – pulleys • A pulley is made of a grooved wheel with a rope wrapped around it. • Pulleys change the amount of force needed and the direction of the force. • Where might you find pulleys at the school? Simple machines – pulleys • The mechanical advantage for a pulley is found by counting the number of rope segments (not counting the one that is attached to the effort.)