- 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 PULLED ACROSS A TABLE AT CONSTANT SPEED Friction Force of table on block Pull on the string Weight

Friction FORCES ON A BLOCK PULLED ACROSS 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 ENERGY TO LEAVE YOUR SYSTEM Energy is conserved, but it can change from measured potential and kinetic energy into heat and sound. Potential energy = Weight × height Kinetic energy = 0 Rolling ball - not much friction Potential energy = 0 Kinetic energy = ½mv 2 = weight × initial height Potential energy = Weight × height Kinetic energy = 0 Sliding box - lots of friction Potential energy = 0 Kinetic energy = ½mv 2 < weight × initial height energy lost to heat

Think back to the example of the floating barge photo by Bill Blevinsphoto 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 energy from one form to another INCLINED PLANE Lifting a mass 1 m Pushing the same mass 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 × distance is equal for both

Simple Machines help change energy from 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 energy from one form to another Machines can change the amount of force or 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 will apply 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