Work, Power, and Machines Chapter 14

Chapter Pretest- Assess your Prior Knowledge According to Newton’s first law, if no net force acts on an object, the object continues in motion with constant ________. Velocity Force Acceleration A

Chapter Pretest- Assess your Prior Knowledge A horizontal force on an object can be broken down into these components: 5N north 5N east If no other forces act on the object, in what direction will the object move? Northeast

Chapter Pretest- Assess your Prior Knowledge Newton’s second law of motion states that the net force acting on an object equals the product of what two variables? Mass Acceleration

Chapter Pretest- Assess your Prior Knowledge A machine produces an output force of 12.3N when an input force of 8.6N is applied. What is the ratio of the machine’s output force to its input force? 1.4

Chapter Pretest- Assess your Prior Knowledge A person exerts 22N on a box. If a frictional force of 3N opposes this force, what is the net force acting on the box? 19N

Chapter Pretest- Assess your Prior Knowledge A machine has an output force of 57.3N when a force of 32.6N is used to operate the machine. What is the percentage increase in the force? 176% Output/Input

Chapter Pretest- Assess your Prior Knowledge A small wheel has a radius of 32cm, and a large wheel has a diameter of 128cm. What is the ratio of the diameters of the large wheel to the small wheel? 4 2 0.25 0.5 B

14.1 Work and Power I will describe the conditions that must exist for a force to do work on an object I will calculate the work done on an object I will describe and calculate power I will compare the units of watts and horsepower as they relate to power

What is Work? Work When is work done? The product of force and distance When a force acts on an object in the direction the object moves Example: Work is done by the boy in the green hat when he exerts a horizontal force to push the cart down the road

Work Requires Motion NO movement NO Work is Done For a Force to Work on an Object NO movement NO Work is Done Some of the force must act in the same direction as the object moves Example: Weightlifter WORK done when he exerts an upward force to raise the barbell over his head NO WORK done as he stands with barbell over his head (NO MOVEMENT!)

Work Depends on Direction Direction of force Direction of motion Work Depends on: When force and motion are in the same direction the work done is MAXIMIZED! ANY part of a force that does NOT act in the direction of motion does NO WORK on an object

A C B Demonstrating Work FORCE A Force + Motion = same direction WORK MAXIMIZED! B ONLY horizontal part of the force does work to move suitcase to the right C Lifting force NOT in the direction of motion therefore the force does NO work on the suitcase C FORCE DIRECTION OF MOTION This force does work B Force FORCE This force does NO work

Press Object Against Wall DEMO Drop Object Force = gravity (downward) Force + motion = same direction So, WORK DONE Press Object Against Wall Force = person pushing object towards wall NO motion NO WORK Push Object Across Desk Forces = gravity (downward) + person pushing (horizontally) Motion = horizontal WORK DONE HORIZONTALLY ONLY Describe: The force acting on the object The motion of the object Is work being done on the object? Drop Object Press Object Against Wall Push Object Across Desk

Using the Work Formula Work = Force X Distance J = N X m Calculating Work Units of Work Joule (J) = N*m When a force of 1 N moves an object 1 m in the direction of the force, 1 J of work is done Using the Work Formula Work = Force X Distance J = N X m Wdone = 1600 N x 2.0 m Wdone = 3200 N*m = 3200 J

YOUR TURN: How much work does a 25-Newton force do to lift a potted plant from the floor to a shelf 1.5 meters high? F= 25 N d = 1.5 m W done = ? Wdone = F x d Wdone = 25 N x 1.5 m Wdone = 37.5 N*m = 37.5 J

What is Power? Power Doing work at a faster rate requires MORE power The rate of doing work Doing work at a faster rate requires MORE power To increase power: Increase the amount of work done in a given time OR Do a given amount of work in less time

Power Example-Snow Removal Person shoveling Person doing work Slower time Less power Snow blower Machine doing work Faster time More power

Using the Power Formula Calculating Power Units of Power Watts (W)= N*m/s = J/s Equal to one Joule per second Example 40 watt light bulb Requires 40 J per second it is lit Using the Power Formula Power = Work / Time W = J / s W= (72 N x 1.0 m) /2.0 s W= 36 Nm/s = 36 J/s = 36 W

Your Turn: Your family is moving to a new apartment. While lifting a box 1.5m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? F = 200 N t = 1.0 s d = 1.5 m Power = ? Power = work / time Power = ( F x d) / t Power = (200 N x 1.5 m) / 1.0s Power = 300 N*m/s = 300 J/s = 300 W

Ticket In You lift a book from the floor to a bookshelf 1.0m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used?

Work & Power -Extra Credit Must show all work, NO TALKING A student rows a boat across a still ponds, doing 3600 J of work on the oars in 60 s. What is the student’s power output? A truck pulls a trailer at a constant velocity for 100 m while exerting a force of 480 N for 1 minute (60 s). Calculate the power. (hint: 1st calculate the work!)

Horsepower (hp) 1 horsepower (hp) = 746 Watts (W) Compared the power outputs of steam engines to the power output of a very strong horse. Example: Rate of 4 horsepower HORSE DRAWN PLOW GASLOINE POWERED ENGINE (SNOWBLOWER)

Work & Machines Machine Device that CHANGES a force Example Car jack You apply force to jack handle Jack CHANGES this force and applies a MUCH GREATER force to the car Lug Wrench You apply force to wrench handle Jack CHANGES this force and applies a MUCH GREATER force to the lug nut Make work EASIER to do CHANGES SIZE of force needed DIRECTION of a force DISTANCE over which a force acts

Increasing Force When a machine INCREASES the distance over which you exert a force…then it DECREASES the amount of force you need to exert Example: Car Jack Each rotation of the jack handle applies a small force over a long distance Each rotation lifts the car only a short distance

Increasing Distance When a machine DECREASES applied force, but INCREASES the distance over which the force is exerted Example: Boat Oars Act as machines that increase the distance over which the force acts

Changing Direction When a machine CHANGES the direction of the applied force Example: handle of oar Pull back on handle The other end moves in opposite direction

Work Input and Output Because of friction, the WORK done by a machine is ALWAYS LESS then the work done on the machine

Work Input to a Machine Input force Input distance Work input The force you exert on a machine Input distance The distance the input force acts through Work input The work done by the input force acting through the input distance Work Input Input Force Input Distance

Work Output on a Machine Output Force The force exerted by a machine Output Distance The distance the output force exerted through Work Output The work done by the output force acting through the output distance Work Output Output Force Output Distance

Mechanical Advantage MA of a Machine Example: nutcracker The number of times that a machine increases an input force Example: nutcracker A: Force = 7 times greater than the force you exert on the nut cracker MA = 7 B: Force = 3 times greater than the force you exert on the nut cracker MA = 3 A B

Actual Mechanical Advantage AMA The mechanical advantage determined by measuring the actual forces acting on a machine Equals the ratio of the output force to the input force Example: MA of rough surface < smooth surface Greater force is needed to overcome friction Output Force AMA Input force

Ideal Mechanical Advantage IMA The mechanical advantage in the absence of friction Because friction is ALWAYS present, the AMA of a machine is always LESS than the IMA Because friction reduces MA, engineers often design machines that Uses low-friction materials Use lubricants

Calculating Mechanical Advantage Ideal Mechanical Advantage (IMA) Easier to calculate than AMA Neglects effects of friction Input Distance IMA Output distance

Efficiency Work output always LESS than work input Efficiency Because machine must overcome friction Efficiency The % of work input that becomes work output Always LESS than 100% Because always some friction REDUCE FRICTION = IMPROVED EFFICIENCY Work Output Efficiency Work Input X 100%