Homework : (55 points) 92 review problems

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What is the difference between longitudinal and transverse waves? Give an example of each How much power is dissipated by the circuit to the right. ( Eq : P = I 2 R = I V) What is the current through the 2-ohm resistor below? . Homework : (55 points) 92 review problems. Types of Waves.

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What is the difference between longitudinal and transverse waves? Give an example of each

• How much power is dissipated by the circuit to the right. (Eq: P = I2 R = I V)
• What is the current through the 2-ohm resistor below?

Homework:

(55 points) 92 review problems

Types of Waves

Transverse wave:

medium vibrates at right angles to the direction the energy moves

λ

Compression wave:

(longitudinal wave)

medium vibrates in the same direction as the direction the energy moves

Electromagnetic Waves
• Mechanical waves require a mediumin order to travel.
• examples:
• electromagnetic waves do not require a medium

water,

earthquakes,

and sound

2. How much power is dissipated by the circuit below?

Given

Equation 1

Equation 2

R = 200 Ω

V = I R

P = I V

V = 100 V

3. What is the current through the 2-ohm resistor below?

Equation

V = I R

Given

R1 = 2 Ω

V = 10 V

Parallel circuit:

Voltage is equal at both resistors

How much power is dissipated by the circuit to the right. (Eq: P = I2 R = I V)

• What is the current through the 2-ohm resistor below?

Homework:

(55 points) Finish Review Problems

Quiz!
• Grab a pencil!

GO!

Momentum
• What is the difference between kicking a:
• stationary ball?
• ball travelling towards you at 30 mph?
• Newton solution: moving inertia
Momentum

vector

(direction is important)

• Units:

kgm / s

Momentum
• Let’s try it:
• momentum of a 50kg person walking at 2 m/s
• momentum of a speeding bullet

which would you stand in front of?

Danger
• Why does bullet have more effect?
• energy of a walking person
• energy of a speeding bullet

energy!

Physics of “Soft”
• Falling can have different results
• Hard landing:
• Soft landing: able to walk away
• What’s the difference?

Broken bones, pain…

Phew!

How you change the momentum

Changing Momentum
• Let’s connect the force to momentum:
• Substitute with

impulse

• Impulse is the change in momentum
Changing Momentum
• Physics of “soft” refers to how momentum is reduced
• For example: two 50kg kids jump off a 12 ft (~4 m) building.
• Kid 1 lands with straight legs
• Kid 2 tucks and rolls when landing
Changing Momentum
• Which kid hits the ground faster?
• both land with same speed:
• At the bottom, they both have the same momentum:
Changing Momentum
• In order to stop, their impulse will have to be:
• Kid 1 has a really short landing, 0.05 s
• Kid 2 makes the landing last longer, 1.0 s
• We can use the impulse to find the force each kid feels
Changing Momentum
• Kid 1
• Kid 2
• A pound is about 4Newtons, so
• Kid 1 feels about 2,000lbs
• Kid 2 feels about 100lbs

(broken bones)

(piggy back ride)

Changing Momentum
• Kid 1
• Ft
• Kid 2
• Ft
• You can always make a soft change if the impulse time is long enough
Time to Practice

Go to pg. 506

Physics of Catastrophe
• Catastrophic event:
• collisions
• explosions
• In order to know the velocity after, you need to know the momentum before
Conservation of Momentum
• Main reason Newton used
• It is conserved!
• So, all the total momentum before something explodes
• Is thesame after it explodes!
Conservation of Momentum
• But initially it was not moving!
• Since momentum is a vector:
• All of the xvectors add to zero
• All of the y vectors add to zero
Conservation of Momentum
• Same for crashes
• All the momentum before the crash
• Is the same after the crash
• Let’s try one…

Example

A 65 kg swimmer runs with a horizontal velocity of 5.6 m/s off a dock

He jumps into a 15 kg rubber raft that is drifting towards him with a velocity of 1.0 m/s

What is the velocity of the swimmer and raft after the impact? (assume no friction or resistance due to air or water)

Example

• A 65 kg swimmer runs with a horizontal velocity of 5.6 m/s off a dock
• He jumps into a 15 kg rubber raft that is drifting towards him with a velocity of 1.0 m/s
• What is the velocity of the swimmer and raft after the impact?
• Start by drawing a diagram for before and after

vf

vf

or

Diagram
• Before:
• After:

notice the subscripts

How will v1f compare to v2f?

Set up conservation equation

only one vf

which direction is vf?

Try this out!