Work and Energy Flashcards
How much work is done on a 2 kg object raised to a height of 3 m along a frictionless ramp, with an incline angle of 30°, versus a frictionless ramp with an incline angle of 60°?
The same amount of work. Work is a state function when a conservative force is acting upon the system, and the amount of work done to change the elevation of this particular object is independent of the path taken to do so.
Which of the following angles of incline offer the greatest mechanical advantage in pushing a pallet of medical supplies up to a platform 3 m off the ground? A ramp with a 30, 45, 60, or 90 degree angle?
30, For ramps, the ideal mechanical advantage can be calculated as follows: mechanical advantage = length of incline / height of incline. Intuitively we can picture that a ramp with a minimal angle of incline is going to have a very large length of incline to achieve a certain height of incline. The more we increase the angle of incline, the more we shorten the length of incline and decrease the overall mechanical advantage.
How would you determine mechanical advantage for inclines?
incline length/ incline height
Work
Work is the amount of force over distance and it is measured in joules (or N*m). Work=F*d, also W=Fdcostheta
Work with gases
W=PdeltaV
Mechanical work
W=F*d=Fdcostheta
Mechanical Advantage
MA=Fout/Fin, it must be greater than 1 otherwise its not advantageous
Why is it that when using a tool that provides mechanical advantage, the work done is the same with or without the tool?
Win=Wout, Fin*din=Fout*dout
Why is it that when using a tool that provides mechanical advantage, the work done is the same with or without the tool?
Win=Wout, Fin*din=Fout*dout
Power
Power is work done over time, P=J/s or the WATT!
What will happen to the gravitational potential energy of a red blood cell traveling through the venous system back up to the heart if its speed decreases by half?
GPE will continue to increase. The red blood cell is traveling up the body, back to the heart, so its height from the ground is increasing. Height is directly proportional to gravitational potential energy as shown with PE = mgh. The amount of increase in gravitational potential energy is not dependent on the cell’s velocity, so it will increase by some amount independent of changes in velocity.
True or false: when calculation work, you don’t have to worry about the path it takes
false, work is a vector. If you start and stop in the same place then work net =0
What is energy?
the ability to do work, also measured in Joules
What is the work energy theorem?
Work=deltaKE or KEf-KEi
Is work - or + when there is work done ON object BY environment
+
How would you determine work given a pressure volume graph? Pressue is on the y axis and volume is on the x axis
Caluclate the area under the curve
The area enclosed by the four steps shown in Figure 2 represents what?
the work done by the engine during one complete cycle.
Recall that the area in a PV diagram represents the work done
The graph below shows the heat cycle for an engine. If you increase the efficiency of the engin, what will happen to the area within the heat cycle?
Area will increase because efficiency is Wout/Win, and the area within the heat cycle is the amount of work performed so it would increase.
Jeff rides a fourth roller coaster as shown below. What is the minimum ramp height H if the ride at the top of the loop maintains the minimum speed needed to stay on the track throughout the loop?
At the top of the loop, the gravitational and the normal forces (if any) point downward toward the center of the loop. Therefore, the net force causing centripetal acceleration is the sum of the gravitational force and the normal force.
When the centripetal force is the minimum amount needed for the ride to stay on track, the normal force is zero. Note: At the top, the normal force can be zero but the gravitational force cannot. Let’s designate the initial height where the ride starts as H. At the very beginning, the energy of the ride is the gravitational potential energy. Therefore, Ei = mgH.
At the top of the loop, the ride includes both gravitational and potential energy. Therefore, Ef = mgH + ½ mv2 = mg (2r) + ½ mv2. The net acceleration at top (centripetal acceleration) = v2/r. Since we concluded that the normal force at the top is zero, the net acceleration at the top is the gravitational acceleration, g. Therefore, v2 = gr. Substituting this equation into the ‘Ef’ equation, we obtain the following:
Ef = 2mgr + ½ mgr = (5/2) mgr
To obey conservation of energy, Ei = Ef. Therefore: mgh = (5/2) mgr. We conclude that H is 5r/2. Since r is 8 m,
H is 5(8 m)/2 = 20 m.
