Physics, Energy

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Romar M. Cabinta
EXERCISES 15
WORK, ENERGY, AND POWER

A. CONCEPTUAL QUESTIONS

1. Is work done when you move a book from the top of the desk to the floor? Why?

Yes. It is because the displacement of the book from the top of the desk to the floor and the force that is applied to the book is parallel with one another.

2. State the law of Conservation of Mechanical Energy in two ways?

The law of conservation of energy states that energy may neither be created nor destroyed. Therefore the sum of all the energies in the system is a constant.

TMEinitial=TMEfinal

3. Explain the basic ideas that govern the design and operation of a roller coaster.

A roller coaster is operated and designed through the application of Physics. The law of Conservation of Energy governs the changes in a coaster's speed and height. Simply put, the higher an object is off the ground, the more potential energy it has - that is, potential to gain speed as it falls. As it falls toward the ground, that potential energy changes to kinetic energy, or energy of motion. The sum of the two types of energy is constant, but a roller coaster must maintain an adequate balance of potential and kinetic energies to deliver a thrilling ride.

4. An inefficient machine is said to “waste energy”. Does this mean that energy is actually lost? Explain.

Energy is never lost. An inefficient machine wastes energy by converting it to an unproductive state. A machine, such as a motor car engine has the primary task of converting the energy in the fuel to motion of the car. It is unproductive because a large proportion of the fuel's chemical energy is dissipated in the form of noise, heat, vibration etc. so that only a small proportion is actually used for its prime purpose.

5. Is it possible for a simple machine to multiply both force and speed at the same time? Why?

It is impossible for a simple machine to multiply both force and gain speed at the same time. It is because the gain in speed of a machine is the result of an exertion of a lot more force and therefore do not take place at the same time. One best example is a bicycle crossing a steep hill requires a greater force to be exerted to be able to gain speed.

B. PROBLEMS

1. Starting from rest, 5-kg slides 2.5 m down a rough 30° incline. The coefficient of kinetic friction between the block and the incline is 0.4. Determine the work done by (a) the force of gravity; (b) the friction between the block and incline; (c) the normal force; and (d) the net force on the block.

W=5kg9.8kgs2
W=49 N

a.) W=Fd W=Wsin30°(2.5m)
W=49sin30°(2.5m)
W=61.25 J

b.) W=-Fd
W=-μkNd
W=-(0.4)(42.44N)(2.5m)
W=-42,44 J

c.) W=0
Normal force does not exert work because
it is perpendiuclar with the displacement.

d.) WT=49Nsin30°2.5m-0.442.44N2.5m+0
WT=18.81 J

2. Car A has twice the mass of car B, but only half as much kinetic energy. When both cars increase their speed by 5m/s, then they have the same kinetic energy. What were the original speeds of the two cars? CAR A CAR B mass=2mB mass=mB KEA=12KEB KEB =KEB

VA=5ms VB =5ms VA=2KEAmA KEA=KEB VA=2(12KEB)2mB 12mAv=12mBv
12122mB5=12mB5 VA=KEB2mB 10mB4=5mB2 VB=KEBmB 5mB2=5mB2

3. A 400-g bead slides on a curved frictionless wire, starting from rest at point A. Find the speed of the bead at point B and point C.

400g×1kg1000g=0.4 kg
PEA=mgh
PEA=(0.4 kg)(9.8)(5m)
PEA=19.6 J
PEB=(0.4)(9.8)(0)
PEB=0 J...
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