CHAPTER 14 Work, Power and Machines

14.1 Work and Power Work requires motion. Work is the product of force and distance. Figure 1 work is only being done when the weight lifter is lifting the barbell. Therefore work requires motion For a force to do work on an object some of the force must act in the same direction as the object moves. No movement, No work is done.

14.1 Work and Power Work depends on direction. Amount of work done on an object depends on the direction of the force and the direction of the movement. A force does not have to act entirely in the direction of movement to do work.

14.1 Work Suitcase ex p. 413 Pulling a suitcase the force acts upward and to the right along the handle The suitcase moves only to the right along ground. Horizontal portion of the force acting in the direction of motion Only the horizontal part of the applied force the part in the direction of movement does work Any part of a force that does not act in the direction of motion does no work on an object.

Calculating Work Work = Force x Distance W = F × D Units= Joule (J) Is the SI Unit of work Force =Newtons (N) Distance = meters (m) Newton – meters = Joules

Calculating work sample problem A body builder lifts a 1600 N barbell over his head and is lifted to a height of 2.0 meters. How much work has he done? W = F × D W=1600N × 2m W= 3200N·m=3200J

14.1 What is Power? Power- is the rate of doing work. The amount of work done in a certain amount of time. Need to use more power to do work at a faster rate. To increase power you can increase the amount of work done in a given time or you can do a given amount of work in less time.

14.1 Power Calculate Power Formula: Power = Work p= w time t SI Units= Watts (W) = 1 J/S Horsepower (hp) another common unit 1 hp = 746 watts Power = Watts [W] Work = Joules [J] Time = Seconds [s] Distance= Meters [m]

14.1 Power Sample Problems: 1. How much power does a light bulb contain if it does 600J of work in 5 Seconds? P= WSolving for Power. P = 600J = 120 W = 100 W t 5 s sig. figs 2. If a car has 700W of power and does 2000J of work. How much time was involved? Solving for time. t = W Changed the power formula P t = 2000J = s = 3 s with sig. figs 700W

14.2 Work and Machines Machine – A device that changes a force. Machines make work easier to do -change the size of a force needed -the direction of a force -or the distance over which a force acts.

14.2 Work and Machines 1. Increasing Force Example: Jack handle A small force exerted over a large distance becomes a large force exerted over a small distance. However, if a machine increases the distance over which you exert a force, then it decreases the amount of force you need to exert.

14.2 Work and Machines 2. Increasing Distance Example: oars - decreases the applied force, but increases the distance over which the force is exerted. - However, a machine that decreases the distance through which you exert the force increases the amount of force required.

14.2 Work and Machines 3. Changing Direction Some machines change the direction of the applied force Pulling on an oar – other ends moves opposite

14.2 Work and Machines Work input and work output - because of friction, the work done by a machine is always less than the work done on the machine. Work input of a machine Input force – force you exert on a machine. Input distance – the distance the input force acts through.

14.2 Work and Machines Work Input – the work done by the input force acting through the input distance Work input= input force × input distance Explain perpetual motion page 419 (read on own )

14.2 Work and Machnes Work output of a machine Work output force the force exerted by a machine. Output distance – the distance the output force is exerted through Work output- the output force multiplied by the output distance. Remember you cannot get more work out of a machine than you put in it.

14.2 Work and Machines Explain movement of oar through the water. Newton’s 3 rd Law -fluid friction slows its motion. Output work is always less than input work due to friction.

14.3 Mechanical Advantage and Efficiency The mechanical advantage of a machine is the number of times that the machine increases an input force. The actual mechanical advantage (AMA)- is the mechanical advantage determined by measuring the actual forces acting on a machine. AMA = the ratio of the output force to the input force.

14.3 Mechanical Advantage and Efficiency AMA= output force input force Sample Problem: If you exert 100N on a jack to lift a 10,000N car, what would be the jack’s AMA? (do on board)

14.3 Mechanical Advantage and Efficiency Ideal Mechanical Advantage – of a machine is the mechanical advantage in the absence of friction. To increase the mechanical advantage of a machine you would reduce the friction. This would allow the mechanical advantage of a machine be at its maximum possible value. However, friction is always present and this is why the AMA of a machine is always less than the IMA.

Calculating the ideal mechanical advantage It is easier to calculate the actual mechanical advantage because it depends only on the locations of the forces and the distances they act on. IMA = Input distance Output distance Sample Problem: What is the IMA of a 5m long ramp that rises 1m off the ground at its end? (answer on board) If the input distance is greater than the output distance the IMA has to be greater than 1.

14.3 Efficiency Efficiency of a machine is the percentage of the work input that becomes work output. Will always be less than 100% because of friction. Formula: Efficiency = Work output ×100 Work input Sample Problem: What is the efficiency of a machine that has a work input of 40J and a work output of 35J? (answer on board)

14.4 Simple Machines Machines (mechanical devices) are combinations of 2 or more of the 6 different simple machines. The 6 types of simple machines are: 1. the lever2. the wheel and axle 3. the inclined plane 4. the wedge 5. the screw 6. the pulley

14.4 Simple Machines 1. The Lever A rigid bar that is free to move around a fixed point. The fulcrum- is the fixed point the bar rotates around. There are 3 classes of levers based on the locations of the input force, the output force, and the fulcrum.

14.4 Simple Machines- Levers Input arm of a lever is the distance between the input force and the fulcrum. Output arm of a lever is the distance between the output force and the fulcrum. You can calculate the IMA for a lever by dividing the input arm by the output arm.

14.3 Simple machines - Levers 3 Types of Levers 1. 1 st Class Lever- identified by the position of the fulcrum. The fulcrum is always located between the input force and the output force. The mechanical advantage can be greater than 1, equal to 1, or less than 1 depending on the location of the fulcrum. Ex: Screwdriver, seesaw, scissors

14.4 Simple Machines-Levers 2 nd Class Levers- Output force is located between the input force and the fulcrum The mechanical advantage is always greater than 1. Ex: Wheel barrow Handle is the input force, wheel is the fulcrum, and the load lifted is the output force.

14.3 Simple Machines- Levers 3 rd Class Levers- The input force is located between the fulcrum and the output force. The output distance over which it exerts its force is always longer than the input distance you move the lever through. The mech. Advantage is less than 1. Ex: baseball bat, hockey stick, golf club, broom, rake

14.3 Simple Machines Wheel and Axle Consists of two discs or cylinders, each one with a different radius. Input force can be exerted on the wheel or axle depending on the machine. Calculate IMA- divide the radius where the input force is exerted by the radius where the output force is exerted. Mech. Adv. can be greater or less than 1.

14.3 Simple Machines Inclined Planes A slanted surface along which a force moves an object to a different elevation. Distance along ramp is the input distance. The height of the ramp is the output distance. IMA is the distance along the inclined plane divided by its change in height.

14.3 Simple Machines Wedges and Screws Like inclined planes because they have a sloping surface but wedges and screws move. Wedges- used to raise an object or split an object apart. A v-shaped object whose sides are two inclined planes sloped toward each other. Mech. Adv. is greater than 1.

14.3 Simple Machine Thinner wedges have a greater IMA than a thick wedge as long as their widths are the same. Screws An inclined plane wrapped around a cylinder. Screws with threads that are closer together have a greater IMA.

14.3 Simple Machines Pulleys- pull with less force than is needed to lift the load upward. A simple machine that consists of a rope that fits into a groove in a wheel. They produce an output force that is different in size, direction, or both, from that of the input force. The IMA of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted.

14.3 Simple Machines Fixed Pulleys- is a wheel attached in a fixed location. It is only able to rotate in place. Direction of the force is changed, but the size of the force isn’t. IMA is 1. The input and output force are about the same. Examples: Flagpole, blinds

14.3 Simple Machines Moveable Pulley- A pulley is attached to the object being moved not to a fixed location. It reduces the input force needed to lift a heavy object. Examples: sails, platforms (window washing). Pulley system- combining fixed and moveable pulleys. Large mech. Adv., depends on how the pulleys are arranged.

14.3 Simple Machines Compound Machine- A combination of 2 or more simple machines that operate together.