Mechanical Energy and Work

1. Define mechanical energy.

• Position
• Motion

2. Correctly use the SI unit of energy. Joule = kg (m² / s²)

3. Define work 2 ways. Note: W =Fd is a special case in which force is constant. It's not an actual definition.

• In words: mechanical energy transfer done by a force (F) acting through a displacement (S) in its same dimension 
• Graphically: work is the area under a force (F) vs. displacement (S) curve. Note: "S" is traditionally used to represent a generic displacement that could occur in the x, y, or z dimensions.

4. State whether work and mechanical energy are vectors or scalars.

5. State 2 requirements for work to be done by a force.

• Motion
• Non-zero force component in same dimension as motion

6. Calculate the work done by a constant force acting through a displacement at an angle.

   W = F ∙ d , the right side of the equation is called a dot product and has the meaning shown below

   W = F(d) cos q , where q is the angle between the force and displacement

Homefun (formative/summative assessment): Read sections 10.1

Formative Assessment: Physics Investigation

Follow up Questions

 

The Nature of Kinetic Energy

7. Define kinetic energy 2 ways:

• In words: the work needed to accelerate an object from rest to its current velocity. (The energy an object possesses due to its motion)
• Mathematically: K ≡ 1/2 mv²

Power

11. Define power 2 ways:

    Mathematically: Power = (work) / (time)

    In words: power is the rate of using energy or the rate of doing work.

12. Solve power problems

    for constant force and velocity, Power = Fv

Homefun (formative/summative assessment):Section Review Problems 15 to 18; page 265

Simple Machines--Levers

Relevance: Simple machines are everywhere. Some knowledge of how to use them can help with all kinds of tasks.

16. Define mechanical advantage. Note: this term is commonly found in textbooks.

    MA = (force out) / (force in)

17. Define motion advantage. Note: this term is not commonly found in textbooks.

    MoA = (r_out) / (r_in)

18. Calculate the theoretical MA of ideal levers using torques.

    

    (Torque in) = (Torque out)

    F_in(r_in) = F_out(r_out)

    r_in / r_out = F_out / F_in

    MA = r_in / r_out

19. Estimate MAs and MoVs for levers such as the jaw and forearm and explain why they favor certain patterns.
20. State that mechanical advantage is the reciprocal of motion advantage. In other words, a simple machine can increase force or increase motion but not both at the same time.

    MA = 1 / MoA

21. Calculate the MA and MoA for an inclined plane.
22. Explain why even an ideal or perfect simple machine cannot do more output work against a load than the work input to the machine by the user or input power source.
23. Calculate the efficiency of simple machines. Efficiency of real machines is always less than 100% due to deflection, stretching or bending of parts and due to friction.

    Efficiency = (work out) / (work in)

Homefun (formative/summative assessment): problems 29, 31, page 273

Review of Objectives 1- 19 (1-3 days)

Formative Assessments:

1. Work review problems at the board

2. Work practice problems.

Metacognition Problem Solving Question: Can I still work the problems done in class, several hours or days later? Some amount of repetition on the exact same problems is necessary to lock in learning. It is often better to thoroughly understand a single example of a problem type than to work example after example understanding none of them completely.

Relevance: Good test preparation is essential to performance in physics class.

Homefun (formative/summative assessment): turn in on the day stapled to the back of the test.

Summative Assessment: Unit exam objectives 1-23