- Review problems with friction and energy in simple machines

- Apply energy principles to levers

- Study the behavior of oscillations in a pendulum and a spring

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 stringWeight

Friction

FORCES ON A BLOCK PULLEDACROSS A TABLE AT CONSTANT SPEED

Force of table on block

Pull on the stringWeight

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 SYSTEMEnergy is conserved, but

it can change from measuredpotential and kinetic energy

into heat and sound.Potential energy = Weight × height

Kinetic 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

Think back to the example of the floating barge

photo by Bill Blevins from Fairport, NY, USA

You determined the barge had a kinetic energy of 5,000,000 Joules the tugboat needed to remove to

stop the barge.In real life, would friction help or hurt the effort?

Would more or less energy need to be removed by the barge?Friction would help; less than 5,000,000 J would be

needed to stop the barge, because the water removes energy too.

Simple Machines help change energyfrom one form to another

INCLINED PLANE

Lifting a mass 1 m

Pushing the samemass to the same height up a ramp

You’ve looked at some simple machines:

Which case stores more energy?

Which requires more force?Which uses more distance?

lifting the mass

pulling the mass up the ramp

neither; force × distanceis equal for both

Simple Machines help change energyfrom one form to another

You’ve looked at some simple machines:PULLEYS

Recall the pulley box; some required

more force, some required more distance of string - but force × distance was the same for all 3

Simple Machines help change energyfrom one form to another

Machines can change the amount of forceor distance, but the energy stays the same; if you lessen the force, you pay for it with

extra distance!

Today you will look at a lever, and you willapply the same principle again.

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

- Energy is always conserved, but it can turn into sound and heat (which are not easily measured).

- Simple machines do not generate new energy, but rather change the force or distance applied to store a fixed amount of energy.

WHAT YOU ARE EXPECTED TO KNOW:

- Review problems with friction and energy in simple machines✓

- Apply energy principles to levers

- Study the behavior of oscillations in a pendulum and a spring

TODAY’S OUTCOMES:FORCE, MOTION AND ENERGY