Intro to Physics

2nd Semester Topics

Momentum and Law of Conservation of Momentum

A truck rolling down a hill has more momentum than a roller skate with the same speed. But if the truck is at rest and the roller skate moves, then the skate has more momentum.

8.1 Momentum

A small-massed object with a large speed can have the same momentum as a large-massed object with a small speed.

Momentum is affected by the mass of the object and its velocity (or speed).

8.1 Momentum

Momentum is mass in motion!

Whenever objects collide in the absence of external forces, the net momentum of the objects before the collision equals the net momentum of the objects after the collision.

8.5 Law of Conservation and Collisions

Motion of the cue ball Motion of the other balls

The momentum before firing is zero. After firing, the net momentum is still zero because the momentum of the cannon is equal and opposite to the momentum of the cannonball.

8.4 Conservation of Momentum

Velocity cannon to left is negative

Velocity of cannonball to right is positive

(momentums cancel each other out!)

Impulse

• Momentum is mass in motion (p = mv)• An impulse transfers momentum (I = Ft).• Impulse remains the same, but time of transfer and

impact force will vary inversely.

• Impulse = change in momentumFt = ∆mv

More time, less force Less time, more force

Same momentum, same impulse

Momentum and impulse

Collisions

a. A moving ball strikes a ball at rest.

8.5 Examples of Elastic Collisions when the objects have identical masses

Momentum of the first ball was transferred to the second; velocity is identical

8.5 Inelastic Collisions

Start with less mass, end up with more mass

Notice how speed changes to conserve momentum (more mass, less speed—inverse relationship!)

Example of an elastic collision with objects same speed but different masses

What happens to the speed of the smaller car after the elastic collision with the more massive truck? (the car’s speed increases to conserve momentum)Notice that the car has a positive velocity and the truck a negative velocity. What is the total momentum in this system? (40,000 kg x m/s)

Work vs. Power

Work = force × distance

•Did the weightlifter do work on the barbell and weights?

•How?

•Is the weightlifter currently doing work on the barbell and weights?

Why or why not?

•Explain two ways that the work done by the weightlifter be increased.

9.1 Work1) Increase the weight on the ends

of the barbell

2) Increase the distance over which the weightlifter pushes the barbell and weights.

Work has the same units as energyJoules Newton x meter

J N x m

9.1 Work

• One joule (J) of work is done when a force of 1 N is exerted over a distance of 1 m (lifting an apple over your head).

Power is the rate at which work is done.

9.2 Power

The unit of power is the joule per second, also known as the watt.

One watt (W) of power is expended when one joule of work is done in one second.

One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one million watts.

P = w/t

Power

100 W incandescent light bulb

How much electrical energy per second?

100 joules per second.

Jet engine vs. lawn mower engine

Both receive ½ gallon of fuel (same energy, same work)•A high-power jet engine does work rapidly, uses ½ gallon in 1 second.•The low-powered lawn mower engine does work slowly, using ½ gallon in 30 minutes.

9.2 Power

vs.

KE vs. PE

• KE• Energy of motion

• PE• Energy of position or

stored energy

If an object is moving, then it is capable of doing work. It has energy of motion, or kinetic energy (KE).

• The kinetic energy of an object depends on the mass of the object as well as its speed.

9.5 Kinetic Energy

Kinetic Energy

KE increases with speed

Gravitational Potential Energy•Energy is stored in an object as the result of increasing its height. •Work is required to elevate objects against Earth’s gravity. •Example: Water in an elevated reservoir and the raised ram of a pile driver have gravitational potential energy.

9.4 Potential Energy

Elastic Potential Energy—potential to do work•Energy stored in a stretched or compressed spring or material. •When a bow is drawn back, energy is stored and the bow can do work on the arrow. •These types of potential energy are elastic potential energy.

9.4 Potential Energy

CHEMICAL POTENTIAL ENERGY

• Energy due to the bond position between molecules (stored during bonding).

• Potential chemical energy is released from chemical reactions (burning, for example).

• Fuels, Food, Batteries, for example.

Law of conservation of Energy

When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.

9.7 Conservation of Energy

9.7 Conservation of Energy

Same energy transformation applies

The 2 J of heat can be called non-useful work (work that is not part of the object’s total mechanical energy).

10 J of PE does 8 J useful work on the arrow and 2 J of non-useful work on the molecules that compose the bow and string and arrow. The arrow has 8 J of KE.

Everywhere along the path of the pendulum bob, the sum of PE and KE is the same. Because of the work done against friction, this energy will eventually be transformed into heat.

9.7 Conservation of Energy

Non-useful work can also be called non-useful energy!

Watch how KE and gravitational PE transform

Where is the KE at the maximum?

Where is the PE at the maximum?

How is PE stored?

Watch the change in height vs. the change in speed!

How does the change in height affect KE and PE?

What happens to KE and TME when the brakes are applied? What work is being done?

Watch the transfer of KE and PE.

What happens to the PE when the skier moves down the hill?

What happens to the KE and TME when the skier travels over the unpacked snow?

What work is done?

Same work, more force, less displacement (from left to right)

Simple Machines and Mechanical Advantage

• Mechanical advantage measures how many times your input force is multiplied

• This makes work easier, but does not reduce the amount of work done.

• Same work, but different mechanical advantage, input force and input distance

• No machine is 100% efficient because of friction

• Which lever would have the highest mechanical advantage?

a

b

c

Simple Machines

• Two families

Lever Inclined plane--Lever

--Pulley

--Wheel and axle

--Ramp

--Wedge

--Screw

Pulley

Fixed pulley

1 support rope

MA = 1

Pulleys

MA = 2

Two supporting ropes

Pulleys

MA = ?

2

PulleyHow many support ropes?

4

What is the mechanical advantage?

4

Wheel and Axle

• Wheel connected to a shaft

Thermal Energy

• Thermal energy is the energy of the molecules that compose matter (they are in motion)

• Kinetic Theory– All matter is made of molecules that move

randomly– The faster the molecules move, the greater the

average kinetic energy of the molecules– The higher the average kinetic energy, the higher

the temperature

Increasing Avg. KE

Increasing Temp.

solid

liquid

gas

Particle speed is increasing

Matter is changing state

melting

evaporation

condensation

freezing

1. Heat is the quantity of thermal energy transferred 2. Heat always flows from a substance with a higher

temperature to a substance with a lower temperature.3. Heat flows only when there is a difference in temperature.4. Heat units are calories or joules.

21.2 Heat

Heat and Heat flow

What causes heat to flow?

21.2 Heat

A difference in temperature between objects in thermal contact.

Energy flow and phase change

If you heat a solid sufficiently, it will melt and become a liquid. If you heat the liquid, it will vaporize and become a gas. The change in the internal energy of a substance causes the change of phase.

23.8 Energy and Changes of Phase

Three ways thermal energy is transferred

Heat from the flame causes atoms and free electrons in the end of the metal to move faster and jostle against others. The energy of vibrating atoms increases along the length of the rod.

22.1 Conduction

Convection occurs in all fluids. a. Convection currents transfer heat in air. b. Convection currents transfer heat in

liquid.

22.2 Convection

Most of the heat from a fireplace goes up the chimney by convection. The heat that warms us comes to us by radiation.

22.3 Radiation

Specific Heat Capacity

A substance with a high specific heat capacity can absorb a large quantity of heat before it will raise in temperature (water has a high specific heat).

A substance with a low specific heat requires relatively little heat to raise its temperature (copper has a low specific heat).

21.6 Specific Heat Capacity

21.6 Specific Heat Capacity

highest

lowest

Generation of Sea BreezesDay

Land • low specific heat• heat and cools rapidly• less resistant to temperature change

Sea• high specific heat• heats and cools slowly• more resistant to temperature change

Sea breeze

Air above the land heats more rapidly and rises

Air above the sea remains cooler and moves on land to replace the land air that rose

Convection

Generation of Sea BreezesNight

Air above the water is warmer than the air above the land and rises

Air above the ground is cooler than the air above the water and moves over the sea to replace the sea air that rose

Land • low specific heat• heat and cools rapidly• less resistant to temperature change

Sea• high specific heat• heats and cools slowly• more resistant to temperature change

Land Breeze