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Chapter 36: Power

- We can measure the power when work is done over a long period of time.

- The power P is the result of the two values: P and Fv.

- It is important to be able to rewrite any unit in terms of the fundamental units of meters, kilograms, seconds, and coulombs.

- The next to last step in the power unit analysis clearly shows the units of force times velocity.

- A bucket is raised from a well in 2.5 seconds.
- The bucket is not moving at the beginning or end of the lift.

- Energy can be seen through an object's motion.

- An object starts at rest and gains 2 J of energy.
- Determine the speed of the object.

- When calculating the net work done by all external forces, the gain or loss of energy W is seen as a change in the object's speed.

- The object is moving at 10 m/s.

- We are told that it is 10 m/s.

- The object lost a lot of movement energy as it slowed down.
- The work is done by friction.

- We could calculate the force required to stop the object if we knew the distance.

- Potential energy is energy stored within a system between particles that are bound by forces.
- The conservative forces are related to potential energy.
- There are many different types of potential energy.
- We are only concerned with three for the purposes of the AP exam.
- We are only concerned with the changes in potential energies.

- The more energy is stored in the system, the better.
- The relationship between the mass that is interacting with Earth and the planet's surface may be changed by changing its height.

- The greater the potential energy stored in the system, the more springs are compressed or stretched away from their equilibrium point.
- The PE elastic is 1/2 kx 2 and is stored in the stretched or compressed spring.
- See the examples.

- The closer two charges are held, the greater the potential energy.
- The electrical potential energy is given by the equation.
- See the examples.

- The mechanical energy is conserved if conservative forces are present.
- The final mechanical energy must be equal to the initial mechanical energy according to the law.

- A rock falls in a vacuum.

- We are dealing with potential energy due to gravity.

- The direction of vf is downward.
- At the beginning of the problem-solving, the mass m canceled out completely.

- An arrow is shot from the roof of a building at an angle of 45 degrees.

- A mass sliding along a floor at 3 m/s hits a spring and bounces back.

- We are dealing with the potential energy of a spring.

- When the spring is at maximum compression, the quotient is 0 m/s.

- The work done by nonconservative forces must be accounted for when calculating the final energy levels.

- During the fall, the air friction does -39 J of work.

- Julia is trying to calculate the final speed of a dropped ball.

- She thinks the ball dropped over a vertical distance of D y.

- Julia knows the answer is wrong.
- Explain what she should do to correct her mistake.

- Julia's work has included gravity twice.
- The work was determined by gravity.
- She included potential energy in her term.

- Work is a force over a distance and a transfer of energy.

- The rate at which work is done is called power.

- Net work done by all forces to an object is the same as the change in energy of that object.

- The work done by conservative forces is path independent.

- Internal conservative forces store potential energy within a system.

- The total mechanical energy is conserved if there are only conservative forces.

- In an isolated system, missing mechanical energy can be found in the form of internal energy.

- The work could be going into any of these.

- If all forces are treated as external forces, the net work is going into the energy of the object.

- The mechanical energy can be described as potential energies if the only external forces are gravity and springs.

- Students assume that all the joules of energy are to be found in KE and PE in a problem.
- They will overlook the nonspring force in the problem that takes some energy out of KE and PE and puts it into IE.

- A problem can be solved with either energy or kinematics.
- The more powerful tool is energy.
- If the acceleration is constant, you can double-check your answer.

- A pendulum consisting of a mass m attached to a light string of length is displaced from its rest position, making an angle th with the vertical.

- The conveyor belt machine has an engine.

- A mass m is moving along the floor.
- The mass encounters a part of the floor that has a coefficient of friction.

- Two mass are dropped at the same time.

- The maximum vertical displacement of the pendulum is 17 cm above its rest position.

- The spring has been compressed by 0.04 m and a mass rests on top of it.

- A box is pulled along a smooth floor by a force F.

- A sphere is dropped through a column of liquid.
- The sphere has a speed of 5 m/s when it has fallen a distance of 2.0 m.

- A mass is attached to a massless spring by a light string that passes over a pulley.
- When the mass is released, the spring has a force constant of 500 N/m.

- A block is on an incline.
- The mass is connected to a massless spring by means of a light string.
- The force constant is 100 N/m.
- The spring is un stretched after the block is released.
- The block comes to rest at a distance of 16 cm.

- A car is travelling up a steep incline at a speed of 25 MPH.

- If you divide 1 N by 2, you get 1 kg m/s 2.

- You can verify which expression has units of joules per second to get the answer.

- gravity is the only conservative force that can do work if there is no friction.

- The average force must be halved if the velocity is doubled and the power is constant.
- The average speed of the belt must be halved.

- The work is being taken from the beginning.
- Since the motion is horizontal, N is equal to f.

- The velocity changes will be the same regardless of the amount of mass.

- The work needed to raise the mass a height is provided by this energy.

- h is equal to 0.54 m.

- The value decreases with angle.
- The work decreases as well.

- As long as the same mass is raised to the same height, the work done to run up the stairs remains the same.

- If we assume that the starting energy for the system is zero, the loss of potential energy is balanced by a gain in elastic potential energy for the spring.
- The mass displacement is the same as the spring's.

- v is 1.7 m/s.

- The work done by gravity down the incline is affected by the work done by friction.
- The net work is applied to stretching the spring by an amount equal to the displacement of the mass.

- The coefficient of friction is given a value of 0.036.

- The motion of an object is relative to the frame of reference.
- We can imagine a second frame moving with the object in which it appears to be at rest, if an object is moving relative to one frame.
- The object has energy because of its relative motion.
- In the second frame, the object appears to be at rest.

- The force of gravity causes your arm muscles to strain.
- You feel tired because this requires energy from your body.

- We use our knowledge of inclined planes to determine how much force the car must exert in order to cancel out the downward force of gravity.

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