Chapter 15 Work, Power & Simple Machines

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1 Chapter 15 Work, Power & Simple Machines Essential Questions: I. What is Work? (In Physics Terms!) II. What is Power? (In Physics Terms!) III. How do machines make work easier and how efficient are they? IV. What are the 5 types of simple machines? V. What are compound machines?

2 15-1 What is Work? Work Def. Work is done when a force acts on an object along the parallel direction the object moves In order for work to be done, a force must be exerted over a distance. Ex you can push on a wall for hours, you ll be real tired, but you haven t done any work in the scientific sense, anyway

3 15-1 Work Work The amount of work done in moving an object is equal to the force applied to the object along the direction the object moves times the distance through which the object moves Work = Force x Distance Units Force is measured in Newtons, Distance is measured in meters. So, the unit is Newton X meters. A Newton meter is known as a Joule (J)

4 15-1 Work A 700 N person climbs a 50 m cliff. How much work did she perform? GIVEN: W = F * d F = 700 N d = 50 m WORK: W = F * d W = (700 N) (50 m) W = 35,000 J

5 15-1 Work An object weighing 200 N is lifted 0.5 m. How much work was required? GIVEN: W = F * d F = 200 N d = 0.5 m WORK: W = F * d W = (200 N) (0.5 m) W = 100 J

6 15-1 Work A dog does 50 N-m (Joules) of work dragging a 0.05 N bone. How far did the bone move? GIVEN: W = F * d W = 50 J F = 0.05 N WORK: W = F * d d = W F d = (50 J) (0.05 N) d = 1,000 m

7 15-1 Work Mrs. O Gorman s superhuman strength allows her to lift a pickup truck 2.0 m above the ground. How much force was required if 25.0 Joules (J) of work was done? GIVEN: W = F * d W = 25.0 J d = 2.0 m WORK: W = F * d F = W d F = 25.0 J 2.0 m F = 12.5 N

8 15-2 Power Power Def: The rate at which work is done, or the amount of work per unit time. Power tells you how fast work is being done so it is a rate similar to the way speed, velocity and acceleration are rates. Power is work per unit time. Any measurement per unit time is a rate!! Formula: Power = Work Time

9 15-2 Power Power rate at which work is done measured in watts (W) P = W t P: power (W) W: work (J) t: time (s)

10 15-2 Power Formula: Since work s formula is force X Distance, the formula for Power may ALSO be written as: Power = Force x Distance Time

11 15-2 Power Units Work is measured in Joules (J), So, the unit for Power is a Joule per second (J/s). The short way to write a J/s is a Watt (W).

12 15-2 Power When do we use Watts in our Daily Lives? They are used to express electrical power. Electric appliances and lightbulbs are rated in Watts. Ex: A 100 Watt light bulb does twice the work in one second as a 50 Watt lightbulb.

13 15-2 Power A small motor does 4000 J of work in 20 sec. What is the power of the motor in Watts? GIVEN: W = 4000 J T = 20 sec P =? WORK: P = W t P = 4000 J 20 s P = 200 J s So P = 200 W

14 P = 2400 W W = 120,000 J T =? 15-2 Power An engine moves a remote control car by performing 120,000 J of work. The power rating of the car is 2400 W. How long does it take to move the car? GIVEN: WORK: t = W P t = 120,000 J 2400 W t = 50 sec

15 15-2 Power A figure skater lift his partner who weighs 450 N, 1.5 m in 3.0 sec. How much power is required? GIVEN: P =? F = 450 N d = 1.5 m t = 3.0 sec WORK: F x d P P = F x d t P = 450 N x 1.5 m 3.0 sec P = 675 J (N m) 3.0 sec P = 225 W t

16 GIVEN: F = 300 N d = 2.0 m P = 300 W t =? 15-2 Power A sumo wrestler lifts his competitor, who weighs 300 N, 2.0 m using 300 Watts of power. How long did it take him to accomplish this show of strength? WORK: P = W t W = F x d W = (300 N)(2.0 m) = 600 J t = 600 J 300 W t = 2.0 s P W t

17 15-3 Work Input & Work Output Machine def. Any device that changes the size of a force, or its direction, is called a machine. Machines can be anything from a pair of tweezers to a bus.

18 15-3 Work Input & Work Output There are always 2 types of work involved when using a machine Work Input - The work that goes into it. Work Output - The work that comes out of it. The work output can NEVER be greater than the work input!!!

19 15-3 Work Input & Work Output So, if machines do not increase the work we put into them, how do they help us? Machines make work easier because they change either the size or the direction of the force put into the machine.

20 15-3 Work Input & Work Output Let s analyze this Machines can not increase the amount of work, so work either stays the same or decreases. The formula for work is: Work = force x distance

21 15-3 Work Input & Work Output Again, the formula for work is: Work = force x distance So, mathematically speaking, to end up with the same or less work: If the machine increases the force then the distance must decrease. If the machine increases the distance, then the force must decrease.

22 15-3 Efficiency Why is it that machines can t have more work output than input? Where does all the work disappear to? A machine loses some of the input work to the force of friction that is created when the machine is used. Part of the input work is used to overcome the force of friction. There is no machine that people have made that is 100% efficient

23 15-3 Efficiency If machines make our work easier, how much easier do they make it? The ratio of how much work output there is to the amount of work input is called a machine s efficiency. Efficiency is usually expressed as a percentage (%).

24 15-3 Efficiency Efficiency measure of how completely work input is converted to work output Efficiency = W W out in 100% It is always less than 100% due to the opposing force of friction.

25 W W out E = in GIVEN: 100% F i = 500 N d i = 4.0 m F o = 1500 N d o = 1.0 m 15-3 Efficiency 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? WORK: W in = (500N)(4.0m) = 2000 J W out = (1500N)(1.0m) = 1500 J E = 1500 J _ J 1.0m E = 75% 1500N 500N 4.0m

26 15-3 Mechanical Advantage Mechanical Advantage is another way of expressing how efficient a machine is. Mechanical advantage is the ratio of resistance force to the effort force OR the ratio of the effort distance to the resistance distance.

27 Equations for MA Mechanical Advantage = force of resistance force of effort Mechanical Advantage = distance of effort distance of resistance

28 F MA = F GIVEN: res effort F e = 500 N F r = 1500 N Mechanical Advantage 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 mechanical advantage of the ramp? WORK: MA = F resistance F effort MA = 1500N 500 N 500N 4.0m 1.0m MA = N

29 MA = D D effort resistance Mechanical Advantage A person is pedaling a bike with an axle radius of 3 inches. They use a pedal with a radius of 8 inches. What is the mechanical advantage of the pedal? GIVEN: D e = 8 in D r = 3 in WORK: MA = D effort D resistance MA = 8 in 3 in MA = 2.7

30 15-4 Simple & Compound Machines Simple Machines There are six types of simple machines. They are the: 1 - Inclined plane 2 - Wedge 3 - Screw 4 - Lever 5 - Pulley 6 - Wheel and axle

31 15-4 Simple & Compound Machines 1 - Inclined Plane Def - A slanted surface used to raise an object. The force needed to lift the object decreases because the distance through which the object moves increases.

32 15-4 Simple & Compound Machines 2 - Wedge - Inclined Plane Type #1 Def an inclined plane that moves in order to push things apart. Examples are forks, axes, knifes.

33 15-4 Simple & Compound Machines 3 - Screw - Inclined Plane Type #2 - Def - An inclined plane wrapped around a central bar or cylinder, to form a spiral. Ex screw duh!!!

34 15-4 Simple & Compound Machines 4 - Lever Def - A rigid bar that is free to pivot, or move around a fixed point called a fulcrum. Ex see saw There are three main types (classes) of levers.

35 15-4 Simple & Compound Machines 3 classes of levers: First-class levers have the fulcrum placed between the load and the effort, as in the seesaw, crowbar, and balance scale. Ex - a see-saw or scissors

36 15-4 Simple & Compound Machines 3 classes of levers: Second-class levers have the load between the effort and the fulcrum. Ex - a wheel barrow

37 15-4 Simple & Compound Machines 3 classes of levers: Third-class levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load. Ex - a hammer or tweezer

38 15-4 Simple & Compound Machines 5 - Pulley Def - A rope, chain or belt wrapped around a grooved wheel. It can change the direction of force or the amount of force needed to move an object.

39 15-4 Simple & Compound Machines To calculate how much mechanical advantage a pulley system creates Count the number of ropes that are attached to the MOVEABLE pulley that # is your mechanical advantage!!!

40 15-4 Simple & Compound Machines 6 - Wheel & Axle Def - Made of 2 circular objects of different sizes attached together to rotate around the same axis.

41 15-4 Simple & Compound Machines Compound Machine Def - A combination of 2 or more simple machines