Chapter 11 Energy

Quiz 11

Chapter 11 Objectives

• Use a model to relate work and energy • Calculate the kinetic energy of a moving

object • Determine how to find the gravitational

potential energy of a system

Chapter 11 Objectives

• Identify ways in which elastic potential energy is stored in a system

• Solve problems using the law of conservation of energy

• Analyze collisions to find the change in kinetic energy

Energy

• What is energy? What are the different types of energy?

Energy

• Working definition is the ability to do work– It takes energy to lift a cheeseburger up to your

mouth– It takes energy to walk up stairs

• Work is the transfer of energy– When work is done on an object, the energy of

the object is increased by the amount of work done on it

Potential Energy

• Energy by virtue of location– Often think of gravitational potential energy

Potential Energy

• But also exists for electromagnetics

Potential Energy

• For gravity is described as – PE = mgh• M is mass in kg• G is gravity field strength• H is height in m

– A 1 kg object at height 1 m above the ground has 9.8 J of Potential energy

• How much work is required to lift a 250 g cheeseburger 0.30 m to your mouth?

Potential Energy

– Other examples• Chemical• Elastic (spring)• Nuclear

Kinetic Energy

• When we give an object potential energy (ie lift an object away from earth), when it falls back to earth, it regains the energy it lost as Kinetic Energy– Falling objects convert potential energy into

kinetic energy• Kinetic Energy is energy by virtue of its speed

Kinetic Energy

• Kinetic Energy is described by the equation KE = ½ mv2

– M is mass in kg– V is speed in m/s

• An object is dropped from a 10 m high building. What will be the speed of the object right before it hits the ground? Ignore air resistance.

Roller Coaster

• A roller coaster of mass 1200 kg goes over the first drop. The coaster drops 60 m. What is the speed of the coaster at the following heights (relative to ground)– 45 m– 30 m– 15 m– 0 m

http://images.google.com/imgres?imgurl=http://www.glenbrook.k12.il.us/gbssci/Phys/Class/energy/u5l2b21.gif&imgrefurl=http://www.glenbrook.k12.il.us/gbssci/Phys/Class/energy/u5l2bb.html&h=240&w=472&sz=5&hl=en&start=100&usg=__WTX0GbtordsDmJ01IisI9H3XsUU=&tbnid=ZtN_tk1dKkN_eM:&tbnh=66&tbnw=129&prev=/images?q=roller+coaster+potential+kinetic+energy&start=80&gbv=2&ndsp=20&hl=en&sa=N

Net Work

• Work can be + or – (review)• Sometimes there is no equal and opposite• A mountain climber slides down a rope that is

12 m tall. The climber started at rest and hits the ground at a speed of 2.0 m/s. Discuss and calculate changes in energy.

Work

• When you lift an object, you do positive work on the object– You give the object energy

• Positive work = work in the direction of motion

• Gravity does negative work on the object (against the motion) when lifted against gravity– But then does positive work on the object when it

is released

Work

• Friction always does negative work, because it is always opposing motion

• A man pushes a piano up a U-Haul ramp. Describe the signs of work from the 3 forces.

Work (review)

• Work as a transfer of energy can give objects kinetic energy – Gravity doing positive work on a falling object

increases its speed– Your car engine does positive work on the car– Your car brakes do negative work on the car

• Work is described by the equation W = FD– Force is Newtons, D is meters

Work Questions

• If an object is moving it possesses• a) kinetic energy.• b) potential energy.• c) work.• d) power

Work Questions

• Which of the following statements is true for a pine cone falling with acceleration less than g?

• a) It is converting potential and kinetic energy into heat.

• b) It is converting potential energy into kinetic energy and heat.

• c) It is converting potential energy into heat only.• d) It is converting potential energy into kinetic

energy only.

Work Questions

• A light car and a heavy truck have the same kinetic energy.

• a) They both have the same speed.• b) The car is going faster than the truck.• c) The car is going slower than the truck.

Work Questions

• A car is traveling twice as fast as a truck. The car has half the mass of the truck.

• a) Both have the same kinetic energy.• b) The car has more kinetic energy than the

truck.• c) The truck has more kinetic energy than

the car.

Work Questions

• Car A has twice the mass of car B. Both are initially at rest. Equal forces act on each car for equal distances. At the end:

• a) Car A has more kinetic energy than car B.• b) Car A has less kinetic energy than car B.• c) Car A is traveling faster than car B.• d) Car A is traveling slower than car B.• e) Cars A and B are traveling at the same speed.

Work Questions

• The form of energy produced by sliding friction is

• a) kinetic.• b) potential.• c) gravitational.• d) thermal.

Work Questions

• An object’s kinetic energy change is equal to• a) its loss of potential energy.• b) the total work done to it.• c) the heat produced by its motion.

Work Questions

• A car of mass 1,400 kg is traveling at 20 m/s. If the engine pumping out 8000 J per second, what is the drag force (air resistance) on the car?

Archery

• An arrow of mass 0.23 kg is fired with a velocity of 45 m/s. The arrow penetrates a target to a depth of 5.0 cm.

• A) How much kinetic energy did the arrow have in flight?

• B) How much work did the target do on the arrow?

• C) What force did the target apply?