NOTES FOR WORK AND ENERGY

Work is equal to a Force X a distance.

Force is measured in Newtons and distance is measured in meters. Thus the unit of work is the N - m. We give these combined units a new name called the Joule. In the English system work is measured in foot-pounds.

Remember for work to be done there must be movement. Also for work to be done the distance the object moves MUST be in the same direction as the force.

If you have to lift twice the load you do twice the work.  If you have to lift the load twice as high you do twice the work.  If you lift it slow or fast you do the same amount of work.

Work video

Power

Power is the rate of doing work. To find power take the Force (N) X the Distance (m) and divide it by the time (s) it takes to do the work. The units for power are N-m/s. We give this a new name called the watt. 1000 watts is equal to 1 kilowatt. In the English system power is measured in horsepower. A horsepower is equal to 550 foot-pounds/second.  

Metric English conversion:  1 horsepower = 746 watts

Power video

In the power video who does more work person A or B? Why They do the same amount of work because the moved the same force through the same distance.

In the power video who has more power Person A or B? Why Person B has more power. Even though the do the same amount of work, person B does it much faster. Remember, Power is the rate of doing work

Horsepower lab

Your instructor with a mass of 180 pounds runs up the stairs, (see video) in 9.2 seconds. The stairs have a VERTICAL distance of 10 meters. 1 pound = 4.45 N and 1 horsepower = 746 watts. Can you fill in the following chart? See your instructor for the answers.

Example work and power problems. (See me for the answers to these practice problems)

1.) A person lifts a 25 Newton box and places it on a shelf 2 meters high. How much work did they do?

2.) A person stands and holds a 50 Kg barbell over their head. They then walk 5 meters. How much work did they do?

3.) A person pushes with a force of 60 Newton's against a crate and move it a distance of 10 meters. How much work was done.

4.) A person stands and pushes against a wall. How much work is being done

5.) Bob lifts a 40 Kg barbell a distance of 2 meters in 4 seconds. How much work did he do? How much power was developed?

6.) A machine lifts 4500 pounds of dirt 20 feet in 10 seconds. How many foot-lbs of work was done? What was the horsepower put out by the machine.

Energy is the ability to do work. Energy can appear in many different forms they are:

1.) Heat
2.) chemical
3.) electromagnetic
4.) Nuclear
5.) Mechanical

Mechanical can be subdivided further into

A.) Kinetic energy: energy of motion. KE = (mass)(velocity squared)/2 Look at this formula carefully. What happens to the KE when you double the mass? What happens to the KE when you double the velocity? They are not the same answers see your instructor to check your answers.

B.) Potential energy: energy of postion. PE = For potential energy if it is a spring it is Force x distance and if it gravitational potential energy it is (mass)(g)(height)

For example: Lets say the you pull back on the bowstring of a bow and arrow with a force of 50 newtons. YOu pull it back a distance of .60 meters. Then the PE stored is 50 N x .60 m = 30 Joules. If you take a 5 kg box and place it 4 meters above the floor you now have gravitational PE. It is equal to the work done which is (5 Kg)(10 m/s/s)(4 meters) = 200 joules. Remember the work done to compress a spring is stored in the form of PE for the spring. If work is done to lift and object then this work is stored in the form of gravitational potential energy. If 100 joules of work is done then neglecting friction 100 joules of energy can be stored.

Energy conversions: Energy cannot be created or destroyed but it can be be transferred between forms. However, in any energy conversion not all of the energy is converted into the form we want. For example when you drive your car you convert the chemical energy of the gasoline into the mechanical energy of the car moving. Not all of the chemical energy can be converted to mechanical energy. Why? Some of the chemical energy is lost to heat. This does not violate the law of conservation of mass and energy. Why? Because the sum of the heat energy and mechanical energy will equal the original chemical energy of the gas.

Look at this picture of two ramps.  
1.)Which ramp do you think that the small cart will get to the ground first?  Why?
2.) Which ramp do you think the car will have the greatest speed at the bottom? Why?
3.)Which ramp do you think the car will have the greatest average speed? Why?

For example. Rub your hands together. What happens to the temperature of your hands? You are converting the KE of moving your hands together to heat energy. However, if you stop moving your hands then you no longer have any heat being produced.. Look at the video of the bowling ball. Where is the KE the highest? Where is the PE the highest? Where are there equal amounts of KE and PE. How does this pertain to these students on the swings?

Conversely, to increase or decrease KE you must do work on the object and the change in KE is equal to the work done. This is known as the work-energy theorem. If your car is moving down the road and you slam on the brakes the road must do work on the car to make it stop. The force of friction X the skid distance would be equal to the work done. This then would also be equal to the change in KE.

Sample energy problems set 1 Formula's = Gravitational PE= mgh, Spring PE = F x D, and KE = (mass)(velocity squared)/2

1.) A 4 kg box is placed on a shelf 2 meters high. a.) How much work was done. b.) How much PE is stored in the box?

2,) A ball with a mass of 5 kg moves down the road with velocity of 3 m/s. What is the KE of the ball? b.) If the ball moves with a velocity of 6 m/s what is the KE of the ball? c.).) If a 10 kg ball moves with the same speed what is the KE of the 10Kg ball?

3.) To compress a spring a person pushes with a force of 90 newtons. The person's hand moves .10 meters. How much PE is stored in the spring?

4.) A bowling ball and tennis ball are held from the top of the school building. Which one has more PE? Why?

Einstein's contribution: Under the right conditions, mass can be converted to energy or energy can be converted to mass. This pertains to very small particles traveling at very fast speeds or very small particles involved in nuclear reactions.

Simple Machines

Machines make work easier by reducing the force required to move an object. In order to do this and not violate any laws of physics you need to sacrifice distance. That is to reduce the force you must work through a greater distance.

Work input is the effort force X the distance the effort moves

Work output is the resistance force X the distance the resistance moves.

Machines make work easier by reducing the force required to move an object or by changing the direction of the force. Remember to reduce the force the work must take place through a greater distance.

The efficiency of a machine is the ratio of the work output to the work input. That is work output/work input. The efficiency should never be greater than 100%. If it was then we would be creating energy. At most the efficiency can be 100%. This can hardly ever occur because of friction.

The Actual mechanical advantage (AMA) of a machine is the resistance force/effort force. It takes into account friction.

The Ideal mechanical advantage (IMA) of a machine is unique to each type of simple machine. It does not include frictional forces.

Example IMA for a pulley equals the number of supporting strands. IN a single fixed pulley the IMA is 1 because only one strand is supporting the weight. Therefore neglecting friction you would need an effort force of 50 N to lift a 50 N weight. In a single movable pulley the IMA is 2 because both strings support the weight. Therefore neglecting friction you would only need 25 N to lift a 50 N weight. However, in order to move the weight 10 cm up you would need to pull out 20 centimeters of string. Remember in order to reduce the force you have to sacrifice distance. Video of a pulley in action (notice how far the scale needs to move in order to move the resistance weight)

Pulley lab done in class. Can you fill out the chart given the following information?

Types of simple machines.

1.) Inclined plane and Wedge: A wedge is an inclined plane that moves through the object

An inclined plane is a flat slanted surface.

For more information on a wedge.

For more information on an inclined plane.

Examples of Inclined Plane
1. Hill
2. Oil changing ramps
3. Wheel Chair Ramp

Examples of Wedge
1. Ax/ Hatchet
2. Metal Wedge

IMA of Wedge = Length of wedge/ width of wedge
IMA= Length of the ramp/height of the ramp.

Inclined plane Pictures: Wheel chair ramp, hallway ramp

Wedge Pictures: zipper, Wedge in wood, Wedge seperating wood, screw wedge

3.) Screw

A screw is another form of an inclined plane. It is an inclined plane wrapped around a central bar or cylinder that forms a spiral. It multiplies the effort force but it increases the distance.

For more information on the screw see this site.

IMA= The IMA of a screw is the length of the thread divided by the length of the screw.

Pictures : Top of a jar, end of light bulb, C-clamp, Screw clamp, bottom of a chair. In each case what does the screw allow the object or user to do?

Examples= wood screws, cork screws, clamps, some chair bottoms, nuts and bolts, jar lids, and light bulbs.

4.) Levers: A lever is a bar that pivots on a fixed area. The fixed point is called the fulcrum. The part that you lift is the resistance and the part you push or lift on is the effort force. In a first class lever, the order goes effort force, fulcrum, and resistance force. In a second-class lever, the order is fulcrum, resistance force, and effort force. In a third class lever, the order is resistance force, effort force, and fulcrum.

Examples of a first class lever are scissors, crowbar, and a screwdriver.
Examples of a second-class lever are clamps, paper cutters, and wheelbarrow.
Examples of a third class levers are fishing pole, tweezers, and teeter-totters.

Levers. Levers can be used to change the distance and power of movement.
In the above example the distance from the weight to the ...
Levers. Objectives The closer a load is to the fulcrum, the less effort
is needed. The less effort means more distance to move the effort.

IMA= effort length/resistance length

pictures: tweezers ,scissors ,clamp ,and paper cutter

5.) Pulley's

Pulley's are lines rapped around groved wheels. They either reduce the resistence force or change the direction of the applied force.

Examples: a garage door opener, lifting bales into the hay loft, and lifting heavy objects up (like a engine)

IMA = The number of supporting lines that does not include the haul rope.

Pictures: Chute pulley, Barn pulley, door pulley, grinder (pictures courtesy of Eric) What types of pulleys are these, single fixed, single movable, or combination pulleys? Look close at the grinder picture and count how many support strands are on this pulley. Also notice that is is attached to a crank (which is a wheel and axle) so this is really a combination machine. Here are some more pictures: single fixed, engine hoist, combination pully, and engine pulley.

Here are two good sites for pulleys. site 1 and site 2.

6..) Wheel and axle: A wheel and axle is a simple machine made up of two circular objects of different sizes. The wheel is the larger object. It turns about a smaller object called the axle. Because the wheel is larger than the axle, it always moves through a greater disatnce than the axle.

pictures: ,pencil sharpener, tech gears, door handle, car tire, steering wheel, and volleyball cable winch

IMA= The radius of the wheel divided by the radius of the axle.

Here are two good sites about the wheel and axle. Site one, Site two