ENERGY, WORK & SIMPLE MACHINES

the nature of Energy

Energy — the ability to cause change Forms of energy include: Electrical Chemical Thermal Radiant

Kinetic Energy — energy in the form of motion KE depends on mass and velocity of moving object. Example of KE: Going down a slide

Potential Energy — energy that is stored Example of PE: Waiting at top of slide Elastic Potential Energy – energy stored by something that can stretch or compress, such as rubber band or spring

Chemical Potential Energy – energy stored in chemical bonds A glass of milk has CPE until you drink it then calories are used as energy for your body Gravitational Potential Energy – anything that can fall has stored GPE A ball on a ledge has GPE

Conservation Of Energy

Mechanical Energy — total amount of kinetic energy and potential energy in a system  Mechanical Energy = PE + KE POINT A: PE MAXIMUM, KE = O POINT B: KE , PEx POINT C: KE , PE POINT D: PE MAXIMUM, KE = O : TOTAL MECHANICAL ENERGY SWING

The Law of Conservation of Energy – energy may change form but it cannot be created or destroyed under ordinary conditions. No energy is lost, just friction converts mechanical energy into thermal energy The breakfast you eat converts chemical energy into mechanical energy, heat energy so your muscles can help you pump your legs. Air resistance converts mechanical energy into thermal energy

Work

Work — transfer of energy that occurs when a force makes an object move For work to be done, something has to move and the motion must be same direction as force. When work is done, a transfer of energy always occurs.

Calculating Work Ex. You push a wheelbarrow with a force of 100N Calculating Work Ex. You push a wheelbarrow with a force of 100N. You moved the wheelbarrow 5m. How much work did you do? Work (joules) = Force (Newtons) x Distance (meters) W = F x d W = F x d W = (100N) x (5m) W = 500 J

Power (watts) = work(J) P = W Power – the rate at which work is done. Ex. You do 900 J of work pushing a fridge. I t took 5 seconds. What was your power? Calculating Power Power (watts) = work(J) P = W time (s) t P = W = 900J = 180J/s = 180 watts (w) t 5s

Using Machines

Machine — device that makes doing work easier Machines make work easier by increasing the force that can be applied to an object.

Two forces are involved when machine does work (FIN) Input Force The Force applied to machine Output Force (FOUT) The force applied by machine Output Force Input Force

Machines can be made more efficient by reducing friction. Mechanical advantage – the ratio of the output force to the input force Efficiency – measure of how much work put into a machine is changed into useful output work by the machine. Machines can be made more efficient by reducing friction. Mechanical advantage = output force (newtons) Input force (newtons) MA = FouT FIN

Simple Machines

Simple Machine — machine that does work with only one movement of the machine LEVER PULLEY Bar that is free to pivot around fixed point (fulcrum) 1st Class Lever – fulcrum is between the input force and output force 2nd Class Lever – output force is between fulcrum and input force 3rd Class Lever – input force is between fulcrum and output force Grooved wheel with a rope, chain or cable running along groove Fixed pulley – attached to something that doesn’t move Moveable pulleys – one end of the rope is fixed and the wheel is free to move Block and Tackle – system of pulleys consisting of fixed and moveable pulley

WHEEL AND AXLE Consists of an axle attached to center of a larger wheel so that the wheel and axle rotate together INCLINED PLANE Sloping surface, such as a ramp that reduces the amount of force required to do work THE SCREW Inclined plane wrapped in a spiral around a cylindrical post THE WEDGE An inclined plane with one or two sloping sides