- Review oscillations and friction

- Study several demonstrations that review the concepts of force, motion and energy

TODAY’S OUTCOMES:FORCE, MOTION AND ENERGY

When the car accelerates forward, the inertia of the map tends to keep it still, unless the force of friction is strong enough to accelerate it at the same rate as the car. If the car accelerates too quickly, the friction force isn’t strong enough to pull the map forward with the car.

1. When Miriam and Harold go on trips, they put the map on the passenger-side dashboard, in case they need to look at it. On their way out of town, the map falls off the dashboard at every stoplight, just after the light turns green.. (A) Why does the map fall off the dashboard at the stoplights? Discuss the role played by any of the laws of motion that are relevant.

(B) Harold says that the maps wouldn’t fall off if Miriam would change her driving style.

What change is he recommending?

If Miriam would lower the acceleration of the car (by letting up on the gas a bit), the force of friction of the dashboard on the map would be strong enough to match the acceleration of the car, and the map would stay put.

FORCES ON A BLOCK PULLEDACROSS A TABLE AT CONSTANT SPEED

Friction

Force of table on block

Pull on the string

Weight

Friction

FORCES ON A BLOCK PULLEDACROSS A TABLE AT CONSTANT SPEED

Force of table on block

Pull on the string

Weight WEIGHT INCREASES

⇒ FORCE OF TABLE INCREASES

⇒ FRICTION INCREASES ⇒ FORCE NEEDED TO PULL THE BLOCK INCREASES

IF SPEED IS CONSTANT, THESE FORCES ARE BALANCED

FRICTION CAUSES ENERGYTO LEAVE YOUR SYSTEM

Energy is conserved, butit can change from measuredpotential and kinetic energy

into heat and sound.Potential energy = Weight

× heightKinetic energy = 0

Rolling ball - not much friction

Potential energy = 0Kinetic energy = ½mv2 = weight × initial height

Potential energy = Weight × height

Kinetic energy = 0

Sliding box -lots of friction

Potential energy = 0Kinetic energy = ½mv2 < weight × initial height

energy lost to heat

OSCILLATIONS

Oscillations can be looked at in terms of force and acceleration, or in terms of

energy.

OSCILLATIONS

Pendulum: Force and Acceleration

tension

weight

weight and tension are NOT equal and opposite

here, so there is a net force, and thus an

ACCELERATION

net force

OSCILLATIONS

Pendulum: Force and Acceleration

tension

weight

At the bottom, tension and weightcancel - no net force

OSCILLATIONS

Pendulum: Force and Acceleration

on the swing upward,forces become unbalancedagain, net force reappears

tension

weight

net force

Acceleration in apendulum is always

toward the central line,or equilibrium

position (where it would hang if stationary.)

OSCILLATIONS

Pendulum: Energy

Potential energy is storedas pendulum is pulled back;there is no motion or kinetic

energy yet

OSCILLATIONS

Pendulum: Energy

At the bottom, the potential energy is gone - but speed and kinetic

energy are highest

OSCILLATIONS

Pendulum: Energy

on the swing upward,the speed and kinetic energy

lower; potential energy is again stored

OSCILLATIONS

SPRING: Force and acceleration

“restoring” force

pull from hand

When the spring is “pulled back”, the pull from your

hand and the restoring force are balanced

equilibrium line

OSCILLATIONS

SPRING: Force and acceleration

“restoring” force

When you release, you remove the force from your hand - forces are no longer balanced - restoring force =

the net force

net force

SPRING ACCELERATES UP

OSCILLATIONS

SPRING: Force and acceleration

When the spring returns to equilibrium position, it is

MOVING quickly, but there is no more restoring force; NO

ACCELERATION at this instant

no net force

OSCILLATIONS

SPRING: Force and acceleration

“restoring” force

When the spring passes the equilibrium line, the restoring

force pulls the other way

net force

SPRING ACCELERATES DOWN

OSCILLATIONS

SPRING: Energy

When the spring is pulled down, POTENTIAL ENERGY

is stored in the spring

OSCILLATIONS

SPRING: Energy

As the spring passes the equilibrium, there is no more

potential energy; but the speed is maximum, along

with kinetic energy

OSCILLATIONS

SPRING: Energy

As the spring reaches the other side, it slows, and

kinetic energy again becomes potential energy

MASS OF SPRING vs. PENDULUM

SPRING

Mass increases, restoring force stays same; so acceleration decreases

Frequency decreases with mass

PENDULUM

Mass increases, restoring force (weight) also increases, so acceleration

stays the same

Frequency does not depend on mass

- Friction can cause energy to decrease in a measured system.

- Energy changes back and forth between kinetic energy and potential energy in an oscillating system

- Increasing mass affects the period of a spring, but not a pendulum

WHAT YOU ARE EXPECTED TO KNOW:

INSTRUCTIONS FOR TODAY:1) Find the station assigned to your group number in your packet. You will do this activity first, answer questions, and present the results at the end of class. You will be given 10 minutes to run through the first activity.2) You will then rotate around the room to try out (and complete as much as possible) the other stations in the room - we will announce a “switch” every 5 minutes.3) When all 11 stations are complete, return to your table and discuss how to summarize your results in class discussion.4) We will then rotate the discussion from group to group as a review.

- Review oscillations and friction✓

- Study several demonstrations that review the concepts of force, motion and energy

TODAY’S OUTCOMES:FORCE, MOTION AND ENERGY

B) Use the Laws of Motion to find the acceleration of the penny. Use the acceleration to find how much time it takes for the penny to stop; from there you can (and should) find the distance the penny moves.

A) This example is an exact analogy to the slowing barge problem you worked on a couple classes ago, except in this case you are given the force, and have to solve for the distance. label the analogous quantities in these 2 problems. Use a “?” for an unknown variable.

For example (do the rest yourself!): mass of barge (100,000 kg) ↔ mass of penny (0.004 kg)initial speed of barge (10 m/sec) ↔ initial speed of penny (0.5 m/sec)

A game you can play is to give a penny a shove so that it slides across a table, trying to get it to stop on a target. You wish to find out how far the penny will go before it stops, if you give it a certain initial speed. Assume you give the penny an initial speed of 0.5 m/sec, and that the force of friction from the table is about 0.002 N. A penny has a mass of about 4 g = 0.004 kg.

Force = mass × acceleration acceleration = Force/mass⇒

force on barge (?) ↔ force of friction on penny (0.002 N)distance in which barge stops (100 m) ↔ distance in which penny stops (?)

final speed of barge (0 m/sec) ↔ final speed of penny (0 m/sec)

acceleration = change in velocity/time time = change in velocity/acceleration⇒

average speed = distance/time distance = average speed × time⇒

(Remember average speed = ½ × initial speed)