physics chapter 10 and 11

Chapter 10

Work, Power &

Simple Machines

Work

• Work – product of force exerted on an object and the distance the object moves in the direction of the force.

• W = F (d) Units: N*m = 1 Joule

• W = F (d) cos θ

Power

• Power – the rate of doing work; rate at which energy is transferred

• P = W / t Units: J/s = 1 Watt

Machines

Machines –

ease the load by changing the magnitude or direction of a force, but it does not change the work done.

2 Parts of the Machine

• Effort Force (Fe) - force exerted by you

• Resistance Force (Fr) - Force exerted by the machine

• Win = Fe (de)

• Wout = Fr (dr)

Mechanical Advantage vs. Ideal Mechanical Advantage

• Mechanical Advantage (MA) is the ratio of resistance force to effort force

• MA = Fr / Fe

• Ideal Mechanical Advantage (IMA)

assumes transfer of all energy (No friction)

• IMA = de / dr

Efficiency of a Machine

• Efficiency of a machine is the ratio of output work to input work.

• Efficiency = (Wout / Win) x 100

• Efficiency = (MA / IMA) x 100

6 Simple Machines

1. Lever

2. Pulley

3. Wheel and Axle

Simple machines in the inclined plane family

4. Inclined Plane

5. Wedge

6. Screw

1st Class Lever

• The pivot is between the effort and the resistance (load)

• Ex: Catapult, See-saw

2nd Class Lever

• Pivot is on one end and resistance force is in the middle

• Ex: Wheelbarrow

3rd Class Lever

• Pivot is on one end and effort force is in the middle

• Ex: Broom, Hockey Stick, Shovel

Load

Effort

Pivot

Chapter 11

Potential and Kinetic Energy

Energy

• Energy – the ability to change an object or its surroundings

• There are several types of energy, but we will focus on these two.

• Potential Energy – energy stored in an object because of its position or state

• PE = mgh

• Kinetic Energy – energy due to the motion of an object

• KE = 1/2mv2

Work Energy Theorem

• The net work done on an object is equal to its change in kinetic energy.

• Work = ∆KE = KEf – KEi

• If the net work is positive (Force is in the same direction as motion) then KE increases; if the net work is negative (Force in the opposite direction as motion) then KE decreases.

• F d = KEf – KEi

The Law of Conservation of Energy

• Within a closed, isolated system energy can change form, but the total amount of energy is constant.

• KEi + PEi = KEf + PEf

• Mechanical Energy = KE + PE

Energy of a Pendulum

• Where is the potential energy the greatest?

• 1 and 5• Where is the kinetic

energy the greatest?• 3• If the pendulum has 10 J

of kinetic energy at point 3, how much mechanical energy does it have at point 4?

• 10 J

Why is the first hill the highest on a roller coaster?

• It has to store the most potential energy so it can be converted to kinetic energy to finish the ride.

The energy conversions in the pole vault

• Kinetic on the runway• Elastic when the pole

bends• Potential and Kinetic as

he moves off the ground• All potential at bar

clearance• Kinetic as he hits the mat• Thermal after the landing

Elastic and Inelastic Collisions

• Perfectly Elastic Collision (Bounces) – A collision where the kinetic energy is conserved

• Inelastic Collision (Sticks) – A collision where some of the kinetic energy is changed into other forms (usually heat)