Chapter 13 Wor k and Ene rgy Physical

Chapter 13: Wor k and Ene rgy Physical Science Mr. King

Objectives for Today: • Define work and power • Calculate the work done on an object and the rate at which it is done.

Lesson 1: Work, Power, and Machines What is work? Work is done only when a force causes a change in the position or motion of an object in the direction of the applied force. Equation for Work: Work= force • distance or w= fd

Work is Measured in Joules • Because work is calculated as force times distance, we will end up with a Newton multiplied by a meter, or N • m, as our unit. This unit is called a joule (J). 1 N • m= 1 Joule= 1 kg • m 2/s 2

Work is Zero When an Object Does not Move • If you are trying to lift a car or heavy weight, you might apply a large force. If it does not move then no work has been done.

Math Skills: Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2. 0 m and exerts an average force of 190 N?

A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?

An apple weighing 1 N falls through a distance of 1 m. How much work is done on the apple by the force of gravity?

The brakes on a bicycle apply a 125 N of frictional force to the wheels as the bicycle travels 14. 0 m. How much work have the brakes done on the bicycle?

While rowing in a race, Pompeo uses his arms to exert a force of 165 N per stroke while pulling the oar 0. 800 m. How much work does he do in 30 strokes?

A mechanic uses a hydraulic lift to raise a 1200 kg car 0. 5 m off the ground. How much work does the lift do on the car?

Power • Running up a flight of stairs doesn’t require more work than walking up slowly, but it is more exhausting. • Power is how much work is done in a given amount of time. Power = work or time p= w or t

Power is measured in watts (W). • A watt is the amount of power required to do 1 J of work in 1 s. • Don’t confuse “W” for work and “W” for watts. Read the context to figure it out.

Math Skills While rowing across the lake during a race, John does 3960 J of work on the oars in 60. 0 s. What is his power output in watts?

Using a jack, a mechanic does 5350 J of work to lift a car 0. 5 m in 50. 0 s. What is the mechanic’s power output?

Machines and Mechanical Advantage • Which is easier, lifting a car yourself or using a jack? • The jack makes the job easier but it is still the same amount of work. • Machines help us do work redistributing the work that we put into them.

• Machines can change the direction of an input force. • Machines can also increase or decrease the force by changing the distance over which the force is applied

Mechanical Advantage Tells Us How Much a Machine Multiplies Force or Increases Distance Mechanical Advantage Equation Mechanical advantage = output force = input distance input force output distance A machine with a mechanical advantage greater than 1 multiplies the input force. Such a machine can help you move or lift heavy objects, such as a car or a box of books. A machine with a mechanical advantage less than 1 does not multiply force, but increases distance and speed.

Calculate the mechanical advantage of a ramp that is 5. 0 m long and 1. 5 m high. Given: input distance= 5. 0 m output distance= 1. 5 m Unknown: mechanical advantage Mechanical advantage = input distance output distance Mechanical advantage= 5. 0 m = 3. 3 1. 5 m

Determine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N. Mechanical advantage = output force = input distance input force output distance

A sailor uses a rope and pulley to raise a sail weighing 140 N. The sailor pulls down with a force of 140 N. What is the mechanical advantage of the pulley? Mechanical advantage = output force = input distance input force output distance

Colby pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5. 2, how much force is exerted on a nail in the claw? Mechanical advantage = output force = input distance input force output distance

While rowing in a race, John pulls the handle of an oar 0. 80 m on each stroke. If the oar has a mechanical advantage of 1. 5, how far does the blade of the oar move through the water on each stroke? Mechanical advantage = output force = input distance input force output distance

Before we begin lesson 2, the formulas so far are…

Lesson 2: Simple Machines • The most basic machines are called simple machines.

There six types of simple machines Lever Pulley Wheel and Axle Inclined Plane Wedge Screw

The Lever Family • All levers have a rigid arm that turns around a point called a fulcrum. • Levers are divided into three classes according to the location of there fulcrum and of the input and output forces.

Levers are divided into three classes: First Class levers- have a fulcrum located between the points of application of the input and output. (Like a see saw)

• Second Class Levers: the fulcrum is at one end of the arm and input force is applied to the other end. (Wheel barrow)

• Third Class Levers: multiply distance rather than force. Their MA is less than one. Human body contains many third class levers.

Pulleys are modified levers • You may have seen pulleys to raise a flag on a flag pole, raise up a sail on a boat, or to string up a deer. • The pulley acts as the fulcrum and the rope acts as the rigid arm.

A wheel and axle is a lever or pulley connected to a shaft. One example is the steering wheel of a car. When a small input force is applied, a large output force is applied to the axle to turn the car.

The Inclined Plane Family • A wedge is a modified inclined plane. The head of an axe, the tip of a nail are examples. • A screw is another example of a modified inclined plane wrapped around a cylinder.

Compound Machines • Many devices you use today are compound machines, they consist of more than one simple machine. • A pair of scissors joins two first class levers at a common fulcrum.

Lesson 3: What is Energy? • We experience energy everyday from light and sound energy from thunder and lightning, to nuclear energy, and electrical energy. Some things that are just sitting there have energy to be released. • When you stretch a slingshot you are doing work and applying energy to the elastic band. When you release it that energy is transferred to the ammo that you have in the sling.

One way to define energy is the ability to do work. So, energy and work are the same units, joules.

Potential Energy: stored energy • When you stretch a rubber band, energy is stored as potential energy. • An apple will fall if the stem breaks off of the branch. The energy that could potentially do work on the apple results from its position above the ground. This is called Gravitational Potential Energy.

Gravitational Potential Energy Equation grav. PE = mass x free fall acceleration x height Most of the time you will measure from the object to the ground. Sometimes though it could be relative to the object. What if a bird’s nest was 3 branches below the apple? You would measure the distance between the nest and apple for your height.

For the PE formula: PE= mass x gravity x height mass x gravity = (from last chapter) So, if you are given an amount in kilograms, you must take the mass (kg) and multiply it by gravity (m/s 2) and you get kg x m/s 2, which is what? So, if you are just given N to start the problem, the formula is just PE = WH

• When an apple starts to fall from the branch of a tree, it has the ability to do work. v. The energy that an object has because of its motion is called kinetic energy. • A falling apple can do more work than a falling cherry because kinetic energy depends on mass.

Kinetic Energy Equation Kinetic energy= ½ x mass x speed 2 KE = ½mv 2

Math Skills: Potential Energy 1. Calculate the gravitational potential energy in the following systems: 1. A car with a mass of 1200 kg at the top of a 42 m hill 2. A 65 kg climber on top of Mt. Everest (8800 m high) 3. A 0. 52 kg bird flying at an altitude of 550 m.

Math Skills: Kinetic Energy 1. Calculate the kinetic energy in joules of a 1500 N car moving at the following speeds: 1. 29 m/s 2. 18 m/s

A 35 kg child has 190 J of kinetic energy while sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill? KE = ½mv 2

Formulas for the Test KE= ½mass x velocity 2 Kinetic Energy Work Mechanical Advantage Given Force PE= mass x gravity x height Gravitational Potential Energy N= kg x m/s 2 Power Mechanical Advantage Given Distance J= kg x m 2/s 2

grav. PE = mass x free fall acceleration x height Page 384 390 399 410 #1 -2 #1 -5, 7 -8, 28 Write complete sentences.