Test 6: Chapter 5 Work & Machines Honors Physical Science

Assignment: Vocabulary Self-Awareness Chart Directed Reading Packet p.22-24

Section 1: Work Work – transfer of energy that occurs when a force makes an object move For work to occur, an object must move. ◦(No movement – No work!) The motion of the object must be in the same direction as the applied force on the object.

Work and energy are related, since energy is always transferred from the object doing the work to the object on which the work is done. Work is done on an object only when a force is being applied to the object and the object moves. Calculating work – work equals force (in Newtons) times distance ◦W = F d

Math Skills Activity Complete the Math Skills Activity on p.128 in your notes.

POWER Power – amount of work done in a certain amount of time; rate at which work is done Calculating power – power equals work divided by time ◦P = W/t Power is measured in watts (W). Since work and energy are related, power can also be calculated by dividing energy by time. ◦P = E/t.

Math Skills Activity Complete the Math Skills Activity on p.130 in your notes.

Practice Questions: 1. What must you ask to determine if work is being done? 2. How are work and energy related? 3. A person pushed a bowling ball 20 meters. The amount of work done was 1,470 joules. How much force did the person exert? 4. It took five minutes to move a refrigerator. You did 3,150 joules of work in the process. How much power was required to move the refrigerator? 5. How much power is required to push a car for 10 seconds if the amount of work done during that time is 5,500 joules?

Assignment: Packet p.29

Section 2: Using Machines Device that makes doing work easier is a machine. Machines increase applied force and/or change direction of applied force to make work easier.  Same amount of work can be done by applying a small force over a long distance as can be done applying a large force over a short distance, since work equals force times distance.  Increasing distance reduces the amount of force needed to do the work.  Some machines change the direction of the applied force to do the work.

Machines help move things that resist being moved. Force applied to machine is the effort force. (F E ) Resistance force – (F R ) force applied by machine to overcome resistance Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine. ◦a. Some energy transferred is changed to heat due to friction. ◦b. An ideal machine with no friction would have the same input work and output work.

Mechanical Advantage Mechanical advantage (MA) is the number of times a machine multiplies the effort force. It is calculated by MA equals resistance force divided by effort force. ◦MA = F R /F E

Efficiency Efficiency – measure of how much of the work put into a machine is changed into useful output work by the machine ◦Calculating efficiency – efficiency equals (output work divided by input work) times 100%. Efficiency = (W out /W in ) x 100 ◦Efficiency of a machine is always less than 100%. ◦Lubricants can make machines more efficient by reducing friction.

Practice Questions: 1. Why machine’s efficiency less than 100%? 2. A claw hammer is used to pull a nail from a board. If the claw exerts a resistance force of 2,500 N to the applied effort force of 125 N, what is the MA of the hammer? 3. Give an example of a machine you have used recently. How did you apply effort force? How did the machine apply resistance force? 4. Suppose you want to use a machine to lift a 6,000 N log. What effort force will you need if your machine has a mechanical advantage of 25? …15? …1? Show your calculations.

Section 3: Simple Machines A machine that does work with only one movement is a simple machine. Six types: lever, pulley, wheel & axle, inclined plane, screw, and wedge

Lever Lever – bar that is free to move about a fixed point called a fulcrum. ◦Effort arm is the part of the lever on which effort force is applied. ◦Resistance arm is the part of the lever that exerts the resistance force. ◦Three Classes of Levers ◦Calculating the ideal mechanical advantage (IMA) of a lever –  IMA lever = length effort arm /length resistance arm

First class lever The fulcrum is located between the effort and resistance arms; multiplies force; changes direction of force; Ex. Screw driver, pry bar, neck and head

Second class lever The resistance force is located between the effort force and the fulcrum; always multiplies force; Ex. Wheel barrow; foot when standing on toes

Third class lever The effort force is between the resistance force and fulcrum; doesn’t multiply force but does increase the distance over which the force is applied; Ex. Baseball bat, forearm when doing curls

Pulley Grooved wheel with a rope, simple chain, or cable running along the groove is a pulley, which is a modified first class lever. Three types – fixed, movable, block and tackle

Fixed Pulley A fixed pulley is attached to something that doesn’t move; force is not multiplied but direction is changed; IMA = 1.

Movable Pulley A movable pulley has one end of the rope fixed and the wheel free to move; multiplies force; IMA = 2.

Block and Tackle Block and tackle – system of pulleys consisting of fixed and movable pulleys; IMA = number of ropes supporting the resistance weight.

Wheel and Axle Wheel and axle – machine with two wheels of different sizes rotating together; modified lever form IMA = radius of wheel divided by the radius of the axle Gears are a modified form of the wheel and axle. Ex. Pencil sharpener, door knobs, bicycle tire

Inclined Plane Inclined plane – sloping surface that reduces the amount of force required to do work IMA = length of slope/divided by height of slope Less force is required if ramp is longer and less steep.

Screw Screw – inclined plane wrapped around a cylindrical post Examples – jar tops, light bulb base

Wedge Inclined plane with one or two sloping sides is a wedge. Examples – knife blade, axe

Compound Machine Compound machine – uses a combination of two or more simple machines Example – ◦Can opener (lever- handles; wedge –blade; wheel and axle – when you turn the handle); ◦Scissors – wedge and lever

Practice Questions: 1. What are the two basic machines, and what other machines can these be modified to become? 2. A 6-m ramp runs from the porch to the ground level sidewalk. The porch is 2m off of the ground. What is the ideal mechanical advantage of the ramp?

Lab Activity Copy and complete the lab on p.147 in your textbook. Turn in the completed lab.

Assignment: Complete packet and get stamped. In your standardized test practice workbook, make sure all pages are completed for chapters 2-5.