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Note to self: next year do a better job of separating conservation and non-conservation. There was too much bleeding over this year - and a few occasions of back and forth.
#Goals: SWBAT...
1. Categorize forces as internal or external
2. Understand how internal vs. external forces affect the TME
2. Answer basic energy conservation questions
QUIZ FIRST :-) Then complete the warm-up & #042A
Warm-Up (4min): Internal vs. External Forces
- Categorize the following as either internal or external forces.
- Fa, Ff, Fair, Ftension, FN, Fg, Fspring
- Read, then complete the fill-in activity. For your warm up, you can just write the "fill in the blank" words.
- If two parts of a system exert forces on each other, those forces are said to be internal forces for the system.
- An external force is a force between a part of a system and something outside the system.
- So whether a force is internal or external depends on what you define as your system.
- Examples:
- 1) A baseball. The forces holding the particles of the baseball together are internal forces; the force of a bat hitting the ball is an _________ force on the ball; so is gravity.
- 2) A cat and dog fighting in a sack. The two are killing each other, but considered as one system the forces they exert on each other are all internal.
- 3) The cat and dog considered separately. Considering the cat as a separate system, forces between the cat and the dog are now ________ forces.
- 4) A car. The parts of the car exert forces on each other: the pistons push the rods, the axle pushes the wheels, etc. All these forces are _______ to the car as a whole. But the friction that the ground exerts on the tires is an ________ force. So is the weight of the car, and the force the ground exerts to hold up the car.
CLASSWORK
1. #042A: So, What Does Work Do??? Notes
- If I apply a force to an object, do I change the amount of energy it possesses? In other words, if I push something, do I change it's energy? Think about it.
- What if I do work on an object? Does that change it's energy?
- Talk it over with your neighbor, then prepare to share.
- TAKE NOTES ON THE FOLLOWING
- NON-CONSERVATIVE FORCES:
- When net work is done upon an object by an external force, the total mechanical energy (KE + PE) of
- that object is changed. If the work is positive work, then the object will gain energy.
- If the work is negative work, then the object will lose energy.
- The gain or loss in energy can be in the form of potential energy, kinetic energy, or both.
- Under such circumstances, the work that is done will be equal to the change in mechanical energy of the object.
- Because external forces are capable of changing the total mechanical energy of an object, they are sometimes referred to as nonconservative forces.
- CONSERVATIVE FORCES:
- When the only type of force doing net work upon an object is an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant.
- In such cases, the object's energy changes form.
- For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remains constant. This is referred to as energy conservation.
- When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical is conserved.
- Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy (TME), they are sometimes referred to as conservative forces.
2. 042B: How do Internal Forces Affect KE and PE?
- When the only forces doing work upon the objects are internal forces - gravitational and spring forces - TME stays the same. Thus, energy is transformed from KE to PE (or vice versa) while the total amount of mechanical energy is conserved. Read each description for the practice, and indicate whether energy is transformed from KE to PE or from PE to KE.
- Questions are here: LINK
- First question looks like this:
- A ball falls from a height of 2 meters in the absence of air resistance.
3. #042C: Positive vs. Negative Work and Energy Change
- answer the six scenario questions
At Home Learning (HW)
1. Friday we'll be solving some pretty complex TME problems. Prepare by looking at all three examples here: LINK
2. #042C: TME Practice with Flipping Physics: Watch/take notes/complete edpuzzle questions on the following:
- (8:58) Introductory Work due to Friction equals Change in Mechanical Energy Problem - EDpuzzle
3. EXTRA CREDIT: For credit, you'll need to show all work. Due by friday.
- A. A wagon is pulled by a force of 38.0 N exerted on the handle at an angle of 42.0° with the horizontal. If the wagon is pulled in a circle of radius 20.0 m, how much work is done?
- B. A 120-kg lawn tractor, shown in Figure 10-17 (pg 279 --> LINK), goes up a 21° incline that is 12.0 m long in 2.5 s. Calculate the power that is developed by the tractor.
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