Work, Power and Simple Machines. Chapter 4 Physical Science

Transcription

1 Work, Power and Simple Machines Chapter 4 Physical Science

2 Work, Power and Simple Machines Machines make jobs easier by increasing the applied force on an object. The trade-off is that this also requires an increase in the distance over which the force must be applied.

3 Types of Simple Machines: Types of Simple machines: 1. inclined plane 2. wedge 3. screw 4. lever 5. pulley 6. wheel and axel Modern machines use a combination of these simple machines.

4 Work: Work is the amount of force applied to an object times the distance the object moves in the direction of that force. i.e. work = force distance or W = Fd 1 joule (J) = 1 newton (N) 1 meter (m) or 1 joule (J) = Newton meter (N m)

5 Common misconceptions about work: misconception Truth Work involves time It does NOT involve time. Even if an object does not move, work is still done No, work is NOT being done. Work is always positive it CAN be negative.

6 Calculating Work: Q. What amount of work is done when a force of 300 N moves an object a distance of 2 m? A. W = Fd W = (300 N)(2 m) = 600 N m = 600 J Q. A force of 550 N was used to move a stone 23 m. How much work was done? A. W = Fd = (550 N)(23 m) = N m = J

7 Calculating Work: Q. If 34,560 J of work was done with a force of 960 N, how far was the object moved? A. W = Fd d = W F = 34,560 J 960 N = 36 m

8 Power: Power is the rate at which work is done. This requires a unit of time. Power = work = W time t Units of power are joules/second (J/s) or watt (W). The watt rating on a lamp bulb or electric motor is a measure of its power.

9 Horsepower: James Watt, Scottish inventor, 1782, as a marketing tool for the sale of his improved steam engine, developed a comparative measure of power.

10 Horsepower: Horsepower (hp) is equal to the amount of sustained power of 1 draft horse. One draft horse can move a 330-pound object 100 feet in one minute or 33,400 footpounds per minute (550 ft-lbs/s). The metric equivalent is a 750-newton object pulled 1 meter in 1 second. i.e. 1hp = 750 watts (more precisely W)

11 Horsepower: Relating horsepower Car engine = ~ 170 hp Diesel train = ~ 10,000 hp Nuclear power plant = ~ 300,000 hp Although the European Union as of 1 January 2010 has banned the use of horsepower rating, it will likely continue in use for years to come in the U.S.

12 Calculating Power: Q. What is the power of a 900 N force applied over a distance of 40 m for 45 seconds? (2 steps) 1) W = Fd 900 N x 40 m = 36,000 N m 2) P = W/t = 36,000 J 36,000 J / 45 s = 800 J/s = 800 W

13 Calculating Power: Q. What is the power of a 300 N force applied over 20 m for 20 seconds? (2 steps) W = Fd 300 N x 20 M = 6000N m P = W/t = 6000 J 6000 J / 20 s = 300 J/s = 300 W

14 Machines: People by nature seek to make tasks easier (more efficient use of the time.) Machines make work easier by changing the size or direction of the applied force.

15 Forces of Machine Use: Effort Force (F E ) is the force applied to the machine. Resistance Force (F R ) is the force applied by the machine. Resistance force always opposes effort force and is usually equal to the weight of the object being moved.

16 Effort: When effort force is applied, work done is measured as work input (W I ). Effort distance (d E ) is the distance that a machine moves due to effort force (F E ) W I = F E x d E i.e. Work input using a lever is equal to the force applied to the handle times the distance the handle moves.

17 Resistance: Resistance force (F R ) is the force applied by the machine. Resistance distance (d R ) is the distance the machine moves the resistance. Work output (W O ) is the work done by the machine.

18 Resistance: W O = F R x d R Work output is NEVER greater than work input. Machines multiply force, not work.

19 Example of Work Output and Input: A force of 50 N is required to move a 200-N block of granite a distance of 1 m using a lever. Assuming that W I and W O are equal, what is the effort distance of the lever? F R =200 N F E = 50 N d e =? d R = 1 m Since W O = W I then W I = 200 J W O = F R x d R = 200 N x 1 m = 200 J W I = F E x d E d E = W I = F E 200 J = 50 N 4 m

20 Mechanical Advantage: Mechanical advantage (MA) is number of times a machine multiplies the effort force. MA = F R / F E Back to the example: Mechanical advantage of using the lever to lift the granite block. MA = 200 N / 50 N = 4

21 Mechanical Advantage: A machine used to change the direction of an object may have a mechanical advantage of one. e.g. If I need to lift a 1000-N object 1m upward, it would be easier to use a lever or fixed pulley and apply my 1000-N body to raise the object rather than using my back. A machine that has a mechanical advantage less than one is used to move objects greater distances or faster.

22 Efficiency: Efficiency (Eff) is the ratio of W O to W I expressed as percent. Eff = W O / W I x 100 W O can never be greater than WI because of friction. i.e. Efficiency can never be greater than 100%. No machine is 100% efficient. Modern machines use lubricants and bearings to reduce friction and increase efficiency. Still, the average car is only about 20% efficient.

23 Simple Machines: 1. Inclined plane An inclined plane is a slanted surface or ramp used to raise an object. Inclined planes reduce the force needed to lift an object, but require a greater distance. MA of inclined plane = length of plane height of plane

24 Simple Machines: Resistance Effort MA will always be greater than one, since the length of the plane must be greater than it s height. i.e. When an inclined plane is used, effort is always less than resistance, but some work is always lost by friction.

25 2. Wedges: A wedge is a moveable inclined plane. The thinner (sharper) the wedge, the less effort is needed to overcome a resistance force; therefore, the greater the MA.

26 3. Screws: A screw is similar to an inclined plane by multiplying effort force through a longer effort distance. The closer the threads are, the greater the MA.

27 4. Levers: A lever is any bar that pivots on a fulcrum (fixed point).

28 4. Levers Cont d: Classes of Levers: 1st class - multiplies and changes the effort force. e.g. pliers, crowbar, ice tongs 1st class levers can have a MA of one, more than one, or less than one, depending on the placement of the fulcrum.

29 4. Levers Cont d 2nd class - multiplies the effort with no change in direction. e.g. wheelbarrow, door Resistance force is between the fulcrum and the effort force. 2nd class levers always multiply the effort force (see MA of levers below).

30 4. Levers Cont d: 3rd class - multiplies the distance, not the effort force since the effort arm is always less than the resistance arm. e.g. hammer, baseball bat Effort is between the resistance and the fulcrum

31 Mechanical Advantage of Levers: MA = effort arm length resistance arm length Where: effort arm length is the distance from the fulcrum to the effort force and resistance arm length is distance from the fulcrum to the resistance force If a lever has a mechanical advantage of one, then the effort equals resistance Resistance arm Effort arm

32 6. Wheel and Axle A wheel and axle is a rotating lever made up of two different size wheels. The effort force applied to a wheel is multiplied at the axle (inner wheel). MA = wheel radius (r w ) axle radius (r a )

33 6. Wheel and Axle The fulcrum is the center of the wheel and axel. The wheel radius is the effort arm. The axle radius is the resistance arm. i.e. The bigger the wheel, the greater the MA.

34 5. Pulley: A pulley is a chain, belt, or rope wrapped around a wheel. Pulleys can either change the direction or the amount of effort force. Fixed pulleys (attached to a ceiling or frame) can only change the direction of an effort force, NOT increase an effort force. i.e. MA = 1 using a fixed pulley

35 5. Pulley, Cont d: A moveable pulley can multiply an effort force, but the effort distance increases. Additional pulleys added increase the MA. MA = number of supporting ropes.

36 Compound Machines Note: Compound Machines use a combination of 2 or more simple machines. e.g. cars, washing machines, tape players, watches, fishing reels, etc.