1. MONDAY(10/06): Introduction to Chap. 6, Work and Energy,

Power, and Gravitational Potential Energy. HW: Read pages 148-159 and

solve prob. 1, 3, 4, 5, and 10 on page 174.

2. TUESDAY(10/07): Conservation of Energy and the Work-Energy

Theorem. HW: Read pages 159-173 and solve prob. 14, 17, 27, 29, and

35 on pages 175-6.

3. WEDNESDAY(10/08): The 6 Simple Machines, Mechanical Advantage,

and Efficiency. HW: Solve prob. 37, 47, 57, 59, and 64 on pages 176-8.

4. THURSDAY(10/09): No School - Fall Holiday. HW: Continue to work

on assigned problems.

5. FRIDAY(10/10): Lab Experiment on Pulley Systems. HW: Process lab

data and solve application problems.

6. MONDAY(10/13): Post-Lab Discussion and Problem-solving. HW: Write

 abstract for Lab Report, due Tuesday.

7. TUESDAY(10/14): Ch.7 - Impulse and Momentum. Elastic/Inelastic

Collisions. HW: Read pages 182-193 and solve prob. 10, 11, 21, and 22

on pages 201-202.

8. WEDNESDAY(10/15): PSAT session from 8:30 AM to 12:00 Noon.

HW: Continue to solve all assigned homework problems.

9. THURSDAY(10/16): Collisions in Two Dimensions. HW: Read pages

193-199 and solve prob. 25, 30, 32, 35, 37, and 47 on pages 202-3.

10. FRIDAY(10/17): Review I for Ch.6 & 7 - Work, Power, Energy, Impulse,

and Momentum. HW: Complete all handouts started in class.

10. MONDAY(10/20): Review II for Ch.6 & 7 - Work, Power, Energy, Impulse,

and Momentum. HW: Complete all handouts started in class.

11. TUESDAY(10/21): TEST on Ch.6&7 - Work, Energy, Impulse,

and Momentum. HW: Go to web-site for notes on Ch.8 - Rotational

Kinematics.

1. Work is done in physics when a force is applied to an object and it

undergoes a displacement in the direction of the force.

2. This definition allows us to calculate Work = Force x Displacement or

W = F·Δx ,and then it is measured in Newton meters, Nm, or Joules.

3. Work is a scalar quantity, even though Force and Displacement are

vectors, and can be computed as a Dot-Product of the two vectors.

4. This unit was named after James Prescott Joule (1818-1889), the

Scottish physicist who determined a method to perform mechanical work

with heat. Recall 4.19 J = 1 cal.

5. We usually do work against friction when sliding an object, and against

gravity when lifting, W = mgΔy.

6. When applying a force at an angle, we use the cosine of the angle

to compute the amount of work done, W =(Fcosθ)Δx.

7. Energy is the ability to do work, and there are two kinds of energy,

kinetic and potential.

8. Kinetic energy is due to mass and velocity, K = ½mv² .

9. Potential energy is due to an objects position, U_GRAV = mgΔy ,

gravitational.

10. Potential energy can also be elastic, U_ELAS = ½kx² , with k being the

spring constant.

11. Work can also be computed by finding the area under a Force vs

Displacement curve. For you Calculus fans, this also means that we can

use Integral Calculus to calculate Work, W = ∫F(x) dx.

12. Work can also be explained as the transfer of energy by mechanical

means. Mechanical energy is the total kinetic and potential energy

present in a given situation.

13. There is a conservation law for energy which states that "energy can

change in form but can never be created or destroyed", or K_i + U_i = K_f + U_f.

14. Neglecting friction, mechanical energy is conserved, so that the total

amount remains constant.

15. The net work done on or by an object is equal to the change in the

kinetic energy of that object. This means that W = ΔK = K_f - K_i .

16. Power is the rate of doing work, P = W/Δt , or P = F·Δx/Δt , or even

P = F·v_avg and is measured in Watts.

17. The Watt was named after James Watt (1736-1819) from Scotland,

who perfected the steam engine and made it practical to use.

18. Power is also the rate at which energy is transferred, with 1000 watts

being, of course, a kilowatt, kw.

19. Machines with the same power ratings in watts do the same amount of

work in different time intervals.

20. There are six simple machines: pulley, inclined plane, wheel and axle,

jackscrew, lever, and wedge. For each we can compute the Mechanical

Advantage, Ideal Mechanical Advantage, Work Output, Work Input, and

an Efficiency. We will derive the equations for these.

Before going on, lets review some important concepts that you need to

remember from the previous discussion:

(i) the energy associated with the motion of an object is kinetic energy,

K = ½mv² .

(ii) potential energy is associated with the position or configuration of

an object.

(iii) potential energy can be thought of as stored energy that can be

converted to kinetic energy or other forms of energy.

(iv) in working problems involving gravitational potential energy, it is

always necessary to set the gravitational potential energy equal to

zero at some location.

(v) the choice of the zero level is arbitrary, because the important

quantity is the difference in potential energy.

(vi) it is often convenient to use the surface of the Earth as the zero

potential level, or some other level relevant to a particular problem.

21. The gravitational potential energy of a particle of mass m that is

elevated a distance Δy near the Earth's is U = mg·Δy . This is the

same as the amount of work done in lifting the object.

22. The elastic potential energy stored in a spring of force constant k

is U = ½kx² . This is also the same amount of work done in compressing

the spring a distance x .

23. A force is conservative if the work it does on a particle is independent

of the path the particle takes between the two points. This means that it

doesn't matter how you lift an object.

24. Another way to say this is, a force is conservative if the work it does

is zero when the particle moves through an arbitrary closed path and

returns to its original position. So if you lift an object and put it back down,

then no work is done.

25. A force that does not meet any of the above described criteria is non-

conservative, or sometimes called, dissipative. A force of this type is friction.

26. A potential energy function U can be associated only with a conservative

force.

27. If a conservative force F acts on a particle that moves along the x axis

from x₁ to x₂ , the change in the potential energy of the particle equals the

negative of the work done by the force.

28. Mathematically, this means that the change in the potential energy

would be the (for you calculus fans) negative of the Integral, ∫F(x) dx from

x₁ to x₂.

29. The total mechanical energy of a system is defined as the sum of the

kinetic energy and potential energy or, E = K + U.

30. If no external forces do work on a system, and there are no non-

conservative forces, the total mechanical energy of the system is constant

or , K_i + U_i = K_f + U_f . This is a statement of the Law of Conservation of

Energy.

31. The change in total mechanical energy of a system equals the change

in the kinetic energy due to internal non-conservative forces plus the

change in kinetic energy due to all external forces.

32. Internal or external forces can either increase or decrease the kinetic

energy of a system.

WEBSITE NOTES: Ch.7 - Linear Momentum and Collisions.

1. The momentum of an object is the product of the object's mass and

its velocity. We use lowercase p for momentum and this gives the

formula p = mv .

2. Since mass is a scalar and velocity is a vector, momentum is a vector

because the product of a scalar and a vector is a vector.

3. So, anything we have done with vectors previously can be done with

momentum. It can be broken into horizontal and vertical components,

and several momentum vectors can produce a single resultant.

4. The Pythagorean theorem and SOHCAHTOA are still used to solve

momentum problems.

5. The unit for momentum in MKS is the kg m/s , but when working in

CGS we use g cm/s.

6. The change in momentum. Δp , of an object is equal to the impulse

that acts that acts on it. This is known as the Impulse-Momentum

relation.

7. Impulse is the product of the force acting on an object and the time

in which it acts. This gives us the formula for impulse which is J = F·Δt.

The unit of impulse in MKS is the Newton-second or Ns. Impulse can

also be derived using Calculus with J = ∫F(t) dt = Δp, same as area

under a curve.

8. The Law of Conservation of Momentum states that "In a closed,

isolated system, the total momentum of the system does not change".

9. This law means that in all interactions between isolated objects,

momentum is conserved.   p_f = p₀ 

10. Objects can transfer momentum during collisions, but the total

momentum in the system before the collision must equal the total

momentum after the collision.   p_f = p₀

11. During a collision, the change in momentum of the first object is

equal to and opposite the change in momentum of the second object.

12. Collisions can be classified as elastic or inelastic based on

whether or not energy is also conserved.

13. In a perfectly elastic collision, both momentum and kinetic energy

are conserved. p_f = p₀   and   E_f = E₀

14. In a perfectly inelastic collision, two objects stick together and

move as one mass after the collision. This means that momentum is

conserved but kinetic energy is not conserved.  p_f = p₀   and   E_f < E₀

15. In an inelastic collision, kinetic energy is converted into internal

elastic potential energy when the objects deform. Some kinetic energy

is also converted to sound and thermal energy.

16. Few collisions are perfectly elastic or perfectly inelastic.

17. And still, we need these steps to solve any problem in Physics:

(i) read the problem and identify the given variables

(ii) determine what you are asked to solve for

(iii) find the correct vector formula to use

(iv) use Algebra, Trigonometry, and/or Calculus to isolate the unknown

(v) substitute-in the given information and simplify.

View some homework solutions.