Questions to think about before… • What does work mean to you??? • List some examples of work:
Work & Science • Now...think about work in terms of science...it probably means something very different than what you listed above.
14.1: Work and Power • What is work? • Recall...From Chapter 12 • Question: How does an stationary object begin moving?
Answer… • Answer: When an unbalanced force acts on it. • Work: the product of force and distance • Work is done when a force acts on an object in the direction the object moves.
Work Requires Motion Question: Does a weight lifter do work on the barbell to lift it over his head?
Stationary Objects • Question: Is the weight lifter doing work while he holds the barbell stationary over his head?
ANSWER • Answer: NO, the barbell is stationary • For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is NO movement, NO work is done!!!
Work Depends on Direction • The amount of work done on an object, if any, 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.
Is work being done???? • The force acts upward and to the right. • The suitcase only moves to the right. • 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 • Units of Work • SI unit for force is newtons • SI unit for distance is meters
JOULE • The SI unit for work is newton-meter or the JOULE (J) • When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.
Practice Problem • Imagine the weight lifter. The weight lifter lifts a 1600 newton barbell over his head. Assume the barbell is lifted to a height of 2.0 meters. What is the work done? • Work = Force x Distance
Practice Problem Answered Work = 1600 N x 2.0 m Work = 3200 N m = 3200 J
What is Power? • Power: the RATE of doing work • Doing work at a faster rate requires more power. 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
Answer: YES, because the shovel is moving in the same direction as the force being applied
Answer: YES, but because the snow blower does the work in less timeit has more POWER!!!
Calculating Power • Power = Work / Time • Work is in joules (J) • Time is in seconds (s) • The SI unit for POWER is the watt (W) = one joule per second • Thus, a 40-watt light bulb requires 40 joules each second that it is lit.
Practice Problem • You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
Practice Problem Answered Power = work / time OR can be written as: Power = (Force x Distance) / Time (72 N x 1.0 m)/ 2.0 s = 36 J/s = 36 Watts
Horsepower • Horsepower (hp): common unit for power. One horsepower is equal to about 746 watts. • FYI...Interesting side note: Horsepower is literally based on the power output of a very strong horse!!!
14.2 Work and Machines • Machine = a device that changes a force • Machines make work easier to do. They can: • Change the size of the force needed • The direction of a force • The distance over which the force acts • However… They can’t do work for us!
Ex: a car jack • Each rotation of the jack applies a small force over a large distance and the car is lifted a small distance • Tradeoff = total distance traveled is much greater
Ex: oars of a boat • You move oars a small distance and the end in the water moves a large distance • Tradeoff = increased travel of the oar requires you to exert a greater force
Ex: pulley • You pull down on the rope and the load moves up
Work Input • Because of friction, the work done by a machine is always less than the work done on the machine! • The force you exert on the machine is called input force. The distance the input force acts through is called the input distance. • Work input = IF X ID
Work Output • The force exerted by the machine is called the output force. The distance the output force is exerted through is the output distance. • Work output = OF X OD
Mechanical Advantage = the number of times that the machine increase an input force • AMA = load force/effort force • Q: Using a lever, a person is able to lift a 100N object using only 20N of force. Calculate the MA of this machine
A: AMA = 100N/20N = 5 • In other words, this machine has multiplied the effort force 5 times.
Ideal Mechanical Advantage = MA without friction • IMA = Input Distance/Output Distance • Q: A woman drives her car onto a ramp. She drives 1.8 meters along the ramp to raise it 0.3m off the ground. Calculate IMA
Efficiency • Because some of the work input to a machine is always used to overcome friction, the work output is always less. • Efficiency = the percentage of the work input that becomes work output • Always less than 100% due to friction • Efficiency = W.output/W. input X 100%
14.4 Simple Machines • The six types of simple machines are: • Lever • Wheel and axle • Inclined plane • Wedge • Screw • Pulley