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Objective: MCAS review- Kinetic and Potential Energy

Homework: O.R. question

All need to know about KE and PE to do well on MCAS

Conservation of Energy

(Energy is conserved!)

Energy start= Energy end

(may lose some to friction (heat) and air resistance, etc.

P.E K.E.

Potential Energy. – Energy of Position

(how high)

P.E. = mgh

Kinetic Energy-

Energy of motion

(how fast)

K.E. = 1/2mv²

all

Three different boxes are lifted to different heights.

Box X weighs 115 N and is lifted to 15m.

Box Y weighs 210 N and is lifted to 10m.

Box Z weighs 305 N and is lifted to 5m.

Which of the following statementsbest describes the boxes’ change inmechanical energy?

 A.Box X had the greatest change inmechanical energy 

B.Box Z had the smallest change inmechanical energy. 

C.Boxes X and Y had the same changein mechanical energy. 

D.Boxes Y and Z had the same change inmechanical energy.

Mechanical Energy- either P.E. or K.E.

Since boxes being lifted, we need P.E.

Use P.E. equation to solve for each box to see which hs the most change.

P.E. = mgh

Give us weights, so already have mg, just need multiply by h

P.E. box X = 115x15 =1725J; P.E. box Y = 210x10 = 2100J;

P.E. box Z = 305x5= 1525J

2. A 9 kg model airplane flies horizontally at a constant speed. If the plane suddenly dives from its altitude of 50 m and levels off at 20 m, how much potential energy does it lose in the dive?

 A.450 J 

B.1800 J 

C.2700 J 

D.9000 J

Need 2 students to come up to board. One to calculate P.E. at before dive, the other to calculate P.E. at end of dive.

Remember:

P.E. = mgh

Figure out P.E. had at first (at h = 50m), then figure out P.E. had after dive (at h = 20m). Then subtract two answers to get how P.E. lost

A cart at the top of a hill is releasedand rolls down the hill. Which of thefollowing describes the energy of thecart just as it reaches the bottom ofthe hill?

 A.The cart has no energy.  B.The cart has maximum kinetic energy.  C.The cart has maximum gravitational

potential energy.  D.The cart has equal gravitational

potential and kinetic energy.

Still moving so has energy

At lowest point so no P.E.

If no P.E., then P.E. and K.E. cannot be equal.

Yes! Goes fastest at bottom- all P.E. into K.E.

4. Which of the following increases when a metal spring is stretched horizontally?

 A.potential

B.kinetic energy

 C.gravitational energy

 D.electrical energy

Horizontally = across

= change in position

= spring has potential to do something

= change in P.E.

5. The figure below represents a pendulum’s motion with the lowest point of its swing labeled P.

What happens to most of the pendulum’s gravitational potential energy as it reaches the lowest point P?

 A.It is transformed into inertia. 

B.It is transformed into kinetic energy. 

C.It is transformed into thermal energy. 

D.It is transformed into chemical energy.

P.E. K.E.

As get lower, less P.E., but also moves faster, so…

Use what learned from #5 to answer #6.

6. The diagram below shows the path of a student on a sled starting from rest at point W.

The student slides down a frictionless, snow-covered hill past point Z, which is at ground level.

Which of the following statements best describes the energy of the student and sled from point W to point Z?

 A.The total energy at point W is less than at point Z. 

B.The total energy at point W is greater than at point Z.

 C.The potential energy at point W becomes all kinetic energy at point Z.

 D.The kinetic energy at point W becomes all potential energy at point Z.

Energy is conserved!

All P.E. becomes K.E.

Conservation of Energy is E start = E end…

The diagrams below show a man swinging a golf club.

The kinetic energy of the golf club at point Y is more than the potential energy of the club at point X. Which of the following statements best explains why this fact does not violate the law of conservation of energy?

 A.Gravity is constant from point X to point Y. 

B.Air resistance is greater at point Y than at point X. 

C.Acceleration due to gravity is greater at point Y than at point X. 

D.Energy is added by the man to the golf club from point X to point Y.

In order for K.E. to be more than the P.E. started with…

Energy must have been added!

Write in notes

Write the law of conservation of energy in your own words.