Introduction to Machines Force Work Mechanical Advantage

Objectives Explain how machines make work easier. Calculate mechanical advantage and efficiency. CLE 3202.4.3 Demonstrate the relationship among work, power, and machines.

A. Using Machines A machine is a device that makes work easier. Machines can be simple. Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier.

A. Using Machines 4. Machines make work easier by: a. increasing the force that can be applied to an object. b. increasing the distance over which a force can be applied. c. changing the direction of an applied force.

A car jack is an example of a machine that increases an applied force. Using Machines A car jack is an example of a machine that increases an applied force. The upward force exerted by the jack is greater than the downward force you exert on the handle. The jack increases the applied force, but doesn’t increase the work done.

Using Machines 5. Increasing distance reduces the amount of force needed to do the work.

Using Machines 6. 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 = Force X distance

Some machines change the direction of the force you apply. Using Machines B. Changing Direction Some machines change the direction of the force you apply. 2. The wedge-shaped blade of an ax is one example.

Using Machines The blade changes the downward force into a horizontal force that splits the wood apart.

C. Forces Two forces are involved when a machine is used to do work.

C. Forces Effort Force (Fe) force applied to the machine “what you do” Resistance Force (Fr) force applied by the machine to overcome the resistance “what the machine does”

D. Work Two kinds of work: The work done by you on a machine is called the input work ( Win .) The work done by the machine is called the output work ( Wout.)

D. Work Win Wout Work Input (Win) work done on a machine Work Output (Wout) work done by a machine Wout

E. Conserving Energy 1. A machine cannot create energy, so Wout is never greater than Win. When a machine is used, some of the energy transferred changes to heat due to friction. 3. The energy that changes to heat cannot be used to do work, so Wout is always smaller than Win.

F. Ideal Machine Win = Wout Win > Wout In an ideal machine... One in which there was no friction. Win = Wout But in the real world…An ideal machine DOES NOT EXIST! some energy is lost as friction Win > Wout

G. Mechanical Advantage Mechanical Advantage (MA) number of times a machine increases the effort force

H. Ideal Mechanical Advantage The mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA.

Mechanical Advantage Fe Fr MA Fe = 20 N MA = Fr ÷ Fe Fr = 500 N A worker applies an effort force of 20 N to open a window with a resistance force of 500 N. What is the crowbar’s MA? GIVEN: Fe = 20 N Fr = 500 N MA = ? WORK: MA = Fr ÷ Fe MA = (500 N) ÷ (20 N) MA = 25 MA Fr Fe

Mechanical Advantage Fe Fr MA Fe = ? Fe = Fr ÷ MA Fr = 2000 N Find the effort force needed to lift a 2000 N rock using a jack with a mechanical advantage of 10. GIVEN: Fe = ? Fr = 2000 N MA = 10 WORK: Fe = Fr ÷ MA Fe = (2000 N) ÷ (10) Fe = 200 N MA Fr Fe

I. Efficiency Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine. A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output work.

Efficiency 3. Lubricants, such as oil or grease, can make machines more efficient by reducing friction 4. A lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily. ***Some energy is lost due to friction.

Efficiency Efficiency measure of how completely work input is converted to work output Efficiency of a machine is always less than 100% due to friction

Efficiency Fe = 500 N Win = (500N)(4.0m) = 2000 J de = 4.0 m A worker exerts a force of 500 N to push a 1500 N sofa 4.0 m along a ramp that is 1.0 m high. What is the ramp’s efficiency? GIVEN: Fe = 500 N de = 4.0 m Fr = 1500 N dr = 1.0 m WORK: Win = (500N)(4.0m) = 2000 J Wout = (1500N)(1.0m) = 1500 J E = 1500 J × 100% 2000 J E = 75% 4.0m 500N 1.0m 1500N

Why is a machine’s efficiency less than 100%? Some energy is converted to heat due to friction.

Section Check 2 Question 1 What do a knife, a doorknob, and a car jack have in common? Answer These are all machines, because they are devices that make doing work easier.

Section Check 2 Question 2 When a machine is used to do work, the force that is applied to the machine is the __________. fulcrum effort force mechanical advantage resistance force

Answer The answer is B. The input force is applied to the machine. Section Check 2 Answer The answer is B. The input force is applied to the machine.

Section Check 2 Question 3 What is the effect of increasing a machine’s efficiency? Answer Increasing efficiency increases the amount of input energy converted to useful output.

Objectives Explain how machines make work easier. Calculate mechanical advantage and efficiency. CLE 3202.4.3 Demonstrate the relationship among work, power, and machines.