Work in Mechanical Systems

I. Work is done when a force is exerted on an object causing it to move or change.

Example: Work is done when this man applies forces to the boxes causing them to move. Would it require work to lift them?

Linear Work When a force is applied in the direction of motion. Then, W = F • d

What are the units of work? Work is measured in Newton-meters (N·m). A Newton-meter is called a “JOULE” (sounds like ‘jewel’) and is abbreviated with a capital “J.”

Ex: A weight lifter lifts a 30-N barbell 2-m into the air Ex: A weight lifter lifts a 30-N barbell 2-m into the air. How much work is done? Ex: A robot cart in a factory pulls a load of supplies 200-m. If the force needed to pull the load is 900-N how much work is done?

Example: No work! Although this woman may be doing a lot of thinking, from a physics point of view she is not doing any work. Why? Is ‘home work’ really work at all?

There are six types of Simple Machines... levers pulleys wheel and axles screws wedges inclined planes

How Simple Machines Work... The basic principle is: f • D = F • d a small force applied over a large distance yields a large force over a small distance.

Pulleys A pulley system has two main functions to multiply force to change direction A series of pulleys hooked together is called a block and tackle

Levers...

How machines are rated... The mechanical advantage of a machine is the ratio of the force out to force put in. MA = Fout/Fin The efficiency of a machine is a ratio of the work out to the work put in Eff = Wout/Win

Ex: What is the efficiency of a block and tackle that allows you to lift a 150-N engine 1-m if the operator pulls with a force of 20-N over 2-m?

Can you name the five forms of energy? Mechanical - moving things, sound Electromagnetic - light, magnetic, electric, microwaves Thermal - total molecular energy Chemical - bonds, reactions Nuclear - the nucleus

Recall, energy can be manifested in two ways. Kinetic Energy - energy of motion Potential Energy - energy that is stored; energy due to position or condition

Gravitational Potential Energy G.P.E. depends on weight and height, if you are above the ground you have G.P.E. GPE=weight x height What are the SI units?

Ex: What is the G.P.E. of a 30-N diver at the top of a 10 meter high dive? Ex: What is the GPE of the 35-N diver when she is 8 meter under the water?

Kinetic Energy depends on mass and speed K.E. = 1/2•m•v2 What are the SI units?

Ex: At the same speed what has more kinetic energy, the car or the boat? Why?

Ex: What is the K.E. of a 60 kg skier moving at 8.5 m/s?

Work and Energy Formulas W = F • d MA = Fout/Fin Eff = Wout/Win GPE=weight x height K.E. = 1/2•m•v2