Pg. 189 5F #3-5, SR #1-3

3/13/2013

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One important thing to look for whenever we read any problem are the units that are given - it tells us what variables are given to us.

Practice 5F #3-5. 

#3, we are give  the amount of rain vapor in kg, so we know mass. We also know the amount of power (2.00kW), and distance (2.00 km). However, to solve this problem, we must convert km to m, and kW (kilowatts) to watts - for both of these we can multiply the numbers by 1000. Okay so now we know mass (m), Power (p), and distance (d). we know that P = Fd/t, and we can find F using F = mg (where g = gravity). Then, plug in the known variables in the given formula, and solve for t!

For #4: We know Power (kW), but again we have to convert it to watts. We also know work (Joules). And we know that P = W/t, so we can find t!
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Simple Machines

2/11/2013

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pg. 301 #1-4

2. Remember that percent efficiency = (work out/work in) x 100. It's important to understand that work OUT is the work that the machine is accomplishing, while the work IN is the work physical work that a human must put IN to the machine to accomplish that work. With pulleys, this is the force that must be exerted by you, and the distance that the rope must be pulled by you. 

In this problem, we are given efficiency (73%). So, this tells us that 0.73 = work out/work in, if we rearrange our equation. We also know that work out = Force out (fout) x distance out(dout) , and work in = force in (fin) x distance in (din). So, we can now make our new equations: 0.73 = (fout*dout)/(fin*din). 

We are trying to find the force that would have to be put IN to the machine (force in) if the distance that the rope is pulled is 18.0 m (din = 18.0 m), in order to raise a mass of 58 kg by a height of 3.0 m (dout = 3.0 m). We first have to find out force out value, which is the weight of the mass (fout = 58 kg x 9.81 m/s/s). So, by substituting in our known values, our equation becomes: 0.73 = (58*9.81*3.0)/(Fin*18.0). You are solving for t, so you must rearrange the equation above to do so!


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Ch 6: Momentum Homeworks

3/5/2012

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pg. 211 Practice 6B - all problems use the impulse-momentum equation on pg. 210 (Ft = change in momentum (final momentum -initial momentum))

1. You are given mass, and initial velocity. Final velocity is zero since it is brought to rest. Using time given and impulse-momentum equation, you can find force.

2. You are given mass, initial velocity (zero) and distance - using initial velocity and distance, you can calculate the final velocity of the man just before he hits the water (using kinematic equations: x = 1/2gt^2). Use force-impulse equation and time to find force.

3. You are given mass and initial velocity of the ball (18 m/s north). You must designate a direction as being positive - can use north as positive. Then, the final velocity must be negative since it is kicked in the opposite direction. Using this, you can calculate impulse = Ft by finding the change in momentum. 

4. You are given mass, initial velocity, force, time. Using this, you can find time using impulse-momentum equation. (b) Use the final velocity found in (a) as your initial velocity for this question - using time and force given, you can solve for final velocity. 

pg. 214 SR

1. (a) find the momentum of the bullet (don't forget to convert g -> kg!). Then, set that equal to momentum in p=mv for the baseball to solve for the velocity of the baseball.
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Horsepower Lab

2/22/2012

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First off, to calculate your horsepower:

1. Convert your weight in Lbs to Newtons (1 kg = 2.2 lbs, then multiply by 9.8 to get into N) - this is your applied force. 
2. Find distance by finding height of 1 step, multiply by number of steps. Make sure this number is in Meters. 
3. Find time that it takes you to climb the stairs
4. Power = force x distance / time where, force=Fg, distance = height. 
5. This give you power in watts (J/s) , convert this to hp by dividing by 746 Watts. 

Questions:
1. Calculate your work in Joules: Force due to gravity (in Newtons) multiplied by distance (total ht). Convert to calories (4.185 J = 1 calorie). This gives you the number of calories you exerted climbing one flight of stairs. Use this number to find out how many flights of stairs you would have to climb to exert 550,000 calories (it'll be a high number!)
2. Refer to the types of mechanical energy types we've talked about in class. 
3. self-explanatory
4. think of the big picture in this one - where does ALL energy come from ultimately? What energy transformations occur before YOU get this energy and use it to do work? You may need to refer to your book if you have trouble on this one. 
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Ch. 5: work, power, energy homework

2/13/2012

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SR pg. 186
1. Use position 1 when spring is compressed, position

2 just as mass is released; ME1 = ME2, set y = 0m at position 1 so that ME1 = PEelastic, ME2 = KE + PEgravitational.

Practice 5F pg. 189
1. There are 3 forces that the elevator must counteract: Fg, Ff, and also the maximum load. The net force that the elevator must apply are these forces added together (remember to calculate Fg of load & elevator by multiplying their masses by gravity). Then, use Power = Fv.

3.  You must first find the Fg using mg. Then, plug this into P = Fd/t, where F = Fg. Remember to convert Power to watts from kW, and the distance from km to meters.

4. P = w/t equation

5. First calculate work using the work-energy theorem (find the change in kinetic energy). Then, use this work in part B to find power using P=W/t

SR pg. 189
2. Find time using P = Fd/t; then find work using W = Fd, where F is the force due to gravity of the person.

3. Same thing as #2 except use P = Fv


R&A pg. 193-196
33. Position1 = top of hill (all PE-gravitational), Position2 = bottom of hill (all KE)

39. Use position 1 = running (all KE), position 2 = at top point. Person still has velocity at position 2, so position 2 has KE and PE-gravitational. ME1 = ME2 --> PEg = PEg + KE

43. Position 1 = when spring is compressed (all PE-elastic); Position 2 = at max height (all PE-grav). ME1 = ME2
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Help with homework?

7/1/2011

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If you have a question on homework, feel free to contact me and I will use this blog to post homework problem clarifications. Or you can ask questions about homework assigned using the comment tool below and I will get back to you ASAP.
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