Work Power and Machines Chapter 14 Chapter Pretest

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 ____. A. Velocity B. Force C. Acceleration A

Chapter Pretest. Assess your Prior Knowledge • A horizontal force on an object can be broken down into these components: – 5 N north – 5 N 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. 3 N when an input force of 8. 6 N 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 22 N on a box. If a frictional force of 3 N opposes this force, what is the net force acting on the box? 19 N

Chapter Pretest. Assess your Prior Knowledge • A machine has an • What is the percentage output force of 57. 3 N increase in the force? when a force of 32. 6 N is used to operate the 176% machine.

Chapter Pretest. Assess your Prior Knowledge • A small wheel has a radius of 32 cm, and a large wheel has a diameter of 128 cm. • What is the ratio of the diameters of the large wheel to the small wheel? A. B. C. D. 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 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 Work Depends on: • Direction of force • Direction of motion • 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

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

• Drop Object DEMO Describe: 1) The force acting on the object 2) The motion of the object Is work being done on the object? Drop Object Press Object Against Wall Push Object Across Desk – 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

Calculating Work • Using the Work Formula – Work = Force X Distance • J=NXm • Wdone = 1600 N x 2. 0 m • Wdone = 3200 N*m = 3200 J 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

YOUR TURN: 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 • How much work does a 25 Newton force do to lift a potted plant from the floor to a shelf 1. 5 meters high?

What is Power? 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

Calculating Power • 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 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

Your Turn: 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. 0 s Power = 300 N*m/s = 300 J/s = 300 W • Your family is moving to a new apartment. While lifting a box 1. 5 m 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?

Ticket In • You lift a book from the floor to a bookshelf 1. 0 m 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: 1 st 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 – 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 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 Force Output Distance

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

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 – 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 X 100% Efficiency Work Input