ch 7 Physics Work and Energy Concepts
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How much work is done on the lawn mower if he exerts a constant force of 75.0 N at an angle of 35 degrees below the horizontal and pushes the mower 25.0 m on level ground?

1340.0 J

What is the kinetic energy of an object at the top and bottom of the ramp if a 15 kg block, initially at rest, comes to the bottom with a speed of 7 m/s?

Top: 0 J, Bottom: 343.0 J

How much work is required to stop a 1500 kg car moving at speed of 25 m/s?

468750 J

How much work is required to lift a 40 kg crate 20 m high?

<p>7840 J</p> Signup and view all the answers

What is the final speed of the roller coaster if it starts from rest at the top of the 20.0 m hill and the work done by frictional forces is negligible?

<p>20.0 m/s</p> Signup and view all the answers

In which of these scenarios is useful mechanical work being done? (Select all that apply)

<p>Dave climbing a ladder</p> Signup and view all the answers

Which of the following affects the work you do in lifting a book? (Select all that apply)

<p>The mass of the book</p> Signup and view all the answers

How much work is required to stretch a spring from 25 cm beyond its natural length to 85 cm beyond its natural length given a spring constant of 500 N/m?

<p>3000 J</p> Signup and view all the answers

What is the power output for a 60.0-kg woman who runs up a 3.00 m high flight of stairs in 3.50 s, starting from rest but having a final speed of 2.00 m/s?

<p>171.43 W</p> Signup and view all the answers

How much work is required to lift a 20 kg crate 200 m above ground in 5 minutes?

<p>3920 J</p> Signup and view all the answers

What is the average power exerted on a 20 kg crate lifted 200 m into the air in 5 minutes?

<p>13.07 W</p> Signup and view all the answers

How long will it take a 2.5 kW motor to lift an 80 kg block 30 m high?

<p>6.0 s</p> Signup and view all the answers

What is the cost of running a 0.200-kW computer 6.00 h per day for 30.0 days if the cost of electricity is $0.120 per kWh?

<p>$0.432</p> Signup and view all the answers

Which of the following statements accurately defines work in a scientific context?

<p>Work is the transfer of energy when a force causes displacement.</p> Signup and view all the answers

What happens to the energy of a briefcase when it is carried up stairs at a constant speed?

<p>Energy is transferred into the briefcase, allowing it to do work.</p> Signup and view all the answers

How are work and energy measured in SI units?

<p>Both are measured in joules.</p> Signup and view all the answers

Which scenario illustrates no work being done, despite the application of force?

<p>Pushing a stationary wall.</p> Signup and view all the answers

What factor does NOT affect the amount of work done on an object?

<p>The mass of the object.</p> Signup and view all the answers

When lowering a briefcase, what type of work is being done on the briefcase?

<p>Negative work is done, removing energy from the briefcase.</p> Signup and view all the answers

Which of the following equations correctly represents the work done on an object?

<p>W = Fd cos θ</p> Signup and view all the answers

What is the relationship between work and energy during the performance of work?

<p>Work done on an object transfers energy to or from it.</p> Signup and view all the answers

What is the definition of net work in relation to external forces?

<p>Total work done by all external forces acting on an object.</p> Signup and view all the answers

Which equation represents the work-energy theorem?

<p>W_net = m(v_f^2) / 2 - m(v_i^2) / 2</p> Signup and view all the answers

What type of energy does the quantity mgh represent?

<p>Potential energy associated with the object's position.</p> Signup and view all the answers

If a 15 kg block is at the top of a frictionless incline, which statement is true regarding its kinetic energy?

<p>Its kinetic energy is zero at the top and maximum at the bottom.</p> Signup and view all the answers

What happens to gravitational potential energy as an object is released from a height?

<p>It decreases and is converted into kinetic energy.</p> Signup and view all the answers

How can the gravitational potential energy of an object be increased?

<p>By lifting the object higher in a gravitational field.</p> Signup and view all the answers

What unit is used for measuring gravitational potential energy?

<p>Joules</p> Signup and view all the answers

To determine the work done on an object being lifted, which of the following factors is NOT relevant?

<p>The speed at which the object is lifted.</p> Signup and view all the answers

Which of the following is an example of a conservative force?

<p>Gravitational force</p> Signup and view all the answers

Which statement reflects the relationship between kinetic energy and net work done on an object?

<p>Net work done on an object is equal to its kinetic energy change.</p> Signup and view all the answers

How is the potential energy of a spring calculated according to Hooke's law?

<p>PE_s = 1/2 kx^2</p> Signup and view all the answers

Which factors contribute to the work done in lifting an object?

<p>The mass of the object</p> Signup and view all the answers

What happens to the total mechanical energy when only conservative forces are acting on a system?

<p>It remains constant</p> Signup and view all the answers

Which statement best describes gravitational potential energy?

<p>Energy due to the position of an object</p> Signup and view all the answers

In the context of energy, which process represents the conversion of potential energy?

<p>Falling from a height</p> Signup and view all the answers

What principle explains the constant total mechanical energy in a system with only conservative forces?

<p>Conservation of Mechanical Energy Principle</p> Signup and view all the answers

What is the potential energy at a height of 10m for an object with a mass of 5kg (use g=9.81 m/s²)?

<p>49.05 Joules</p> Signup and view all the answers

Which of the following statements accurately describes nonconservative forces?

<p>They can create thermal energy that dissipates.</p> Signup and view all the answers

In a scenario where a rock is dropped onto the ground, what happens to its mechanical energy?

<p>It is dissipated as thermal energy and sound.</p> Signup and view all the answers

If the work done by nonconservative forces is negative, what effect does it have on the mechanical energy of a system?

<p>It decreases the mechanical energy.</p> Signup and view all the answers

What happens to the work done by nonconservative forces when a person pushes a crate up a ramp?

<p>It adds to the mechanical energy of the system.</p> Signup and view all the answers

In which scenario is mechanical energy conserved?

<p>A spring compressing when a mass is dropped onto it.</p> Signup and view all the answers

Which of the following correctly describes the relationship between work done by nonconservative forces and mechanical energy?

<p>Work done adds or removes mechanical energy from a system.</p> Signup and view all the answers

What role does friction play in the context of mechanical energy?

<p>It converts mechanical energy into thermal energy.</p> Signup and view all the answers

What does the law of conservation of energy state about total energy in a process?

<p>Total energy is constant and may change in form or transfer.</p> Signup and view all the answers

Which form of energy is produced from oxidation of fuels such as gasoline and food?

<p>Chemical energy</p> Signup and view all the answers

How is the efficiency of an energy conversion process defined?

<p>Useful energy output divided by total energy input</p> Signup and view all the answers

What is the SI unit for power, and what does it measure?

<p>Watt; the rate of work done or energy expended</p> Signup and view all the answers

Which process involves the conversion of light energy into chemical energy?

<p>Photosynthesis</p> Signup and view all the answers

What represents the concept that not all energy input results in useful work output?

<p>Some energy is transformed into forms that are not usable.</p> Signup and view all the answers

Which form of energy is considered stored energy derived from sunlight?

<p>Radiant energy</p> Signup and view all the answers

What defines power in the context of energy transfer?

<p>The rate at which work is done</p> Signup and view all the answers

Study Notes

Work

  • Work is done when a force causes a displacement.
  • The work done by a constant force is the product of the force's component in the direction of motion and the distance the force acts over.
  • Work is measured in joules (J): 1 J = 1 N.m = 1 kg.m2/s2.

Kinetic Energy

  • Kinetic energy (KE) is the energy of motion.
  • The KE of an object is proportional to its mass (m) and the square of its speed (v): KE = 1/2 * mv2.
  • The work-energy theorem states that the net work done on a system equals the change in its kinetic energy.

Gravitational Potential Energy

  • Gravitational potential energy (PEg) is the energy stored in an object due to its position relative to a gravitational field.
  • For an object raised vertically, PEg = mgh, where h is the height above the reference level.
  • PEg can be converted to other forms of energy, such as KE.

Conservative Forces

  • A conservative force is one where the work done by or against it depends only on the starting and ending points of a motion, not the path taken.
  • Gravitational force is a conservative force.

Potential Energy of a Spring

  • The potential energy stored in a spring is proportional to the square of its displacement from its equilibrium position: PE = 1/2 * kx2, where k is the spring constant.
  • Like gravitational force, the force exerted by a spring is conservative, meaning the work done depends only on the initial and final stretch/compression.

Conservation of Mechanical Energy

  • The principle of conservation of mechanical energy states that the total energy of a system remains constant if only conservative forces are acting. This is expressed as: KEi + PEi = KEf + PEf.

Nonconservative Forces

  • A nonconservative force is one where the work done depends on the path taken.
  • Friction is a nonconservative force.
  • Nonconservative forces can add or remove mechanical energy from a system.

How Nonconservative Forces Affect Mechanical Energy

  • If 𝑊nc represents the work done by nonconservative forces, the conservation of mechanical energy equation expands to: KEi + PEi + 𝑊nc = KEf + PEf.
  • If 𝑊nc > 0, mechanical energy increases, such as when pushing an object uphill against friction.
  • If 𝑊nc < 0, mechanical energy decreases, such as when friction brings an object to a stop.

Conservation of Energy

  • The law of conservation of energy states that the total energy in a closed system remains constant, though it might change form or be transferred.
  • Energy can exist in various forms, including electrical, chemical, radiant (electromagnetic), nuclear, and thermal.

Transformation of Energy

  • Energy can be transformed from one form to another.
  • For example, chemical energy in food is converted to thermal energy during metabolism, and light energy is converted to chemical energy during photosynthesis.

Efficiency

  • The efficiency of an energy conversion process is the ratio of useful energy output to total energy input.
  • Efficiency is less than 100% because some energy is always lost or converted to unusable forms.

Power

  • Power is the rate at which work is done.
  • Power is measured in watts (W): 1 W = 1 J/s.
  • Power can also be expressed as the rate at which energy is expended.

Power and Energy Consumption

  • The energy consumed over a time period is E = Pt.
  • Electricity bills often use kilowatt-hours (kW.h) as the unit of energy consumption.

Work

  • Work is done when a force is exerted and there is displacement in the direction of the force.
  • Work is calculated as the product of the force component in the direction of motion and the distance.
  • Work and energy are measured in joules (J) where 1 J equals 1 N.m.

Kinetic Energy and the Work-Energy Theorem

  • Net work is the sum of work done by all external forces, or the work done by the net external force.
  • The work-energy theorem states that the net work done on a system equals the change in its kinetic energy.
  • Kinetic energy (KE) is the energy of motion and is calculated as 1/2 * m * v^2.
  • KE is measured in joules (J).

Gravitational Potential Energy

  • The work done to lift an object against gravity is stored as gravitational potential energy (PEg).
  • PEg is calculated as m * g * h.
  • PEg is measured in joules (J).

Conservative Forces and Potential Energy

  • Potential energy is energy stored due to an object's position, shape, or configuration and is completely recoverable.
  • A conservative force is one where the work done depends only on the starting and ending points, not the path taken.
  • Gravitational force is a conservative force.
  • The potential energy of a spring is calculated as 1/2 * k * x^2, where k is the spring constant and x is the displacement from its equilibrium position.

Conservation of Mechanical Energy

  • Mechanical energy is the total kinetic and potential energy of a system.
  • In a system with only conservative forces, mechanical energy is conserved.
  • The conservation of mechanical energy principle is described by the equation: KEi + PEi = KEf + PEf.

Nonconservative Forces and Friction

  • A nonconservative force is one where work depends on the path taken.
  • Friction is a nonconservative force.
  • Nonconservative forces add or remove mechanical energy from a system.
  • Work done by nonconservative forces is represented by Wnc.

Conservation of Energy

  • The law of conservation of energy states that total energy is constant.
  • Energy can change forms or be transferred but the total remains the same.
  • The total energy of a system is calculated as KEi + PEi + Wnc + OEi = KEf + PEf + OEf, where OE represents other energies.

Other Forms of Energy

  • Electrical energy can be converted to other forms and is widely used.
  • Chemical energy is stored in fuels and can be converted to other forms.
  • Radiant energy includes visible light, infrared, and ultraviolet radiation.
  • Nuclear energy is transformed into other forms, including sunlight and electrical energy.
  • Thermal energy is due to the random motion of atoms and molecules.

Transformation of Energy

  • Energy can be transformed from one form to another.
  • For example, chemical energy in food is converted to thermal energy through metabolism.

Efficiency

  • Efficiency is the ratio of useful energy output to total energy input.
  • Efficiency is calculated as (useful energy or work output) / (total energy input), or Wout / Ein.

Power

  • Power is the rate at which work is done.
  • Power is calculated as work / time, or W / t.
  • Power is measured in watts (W), where 1 W equals 1 J/s.
  • High power means a large amount of work or energy is produced in a short time.

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Chapter 7: Work, Energy and Energy Resources PDF

Description

This quiz explores the fundamental concepts of work, kinetic energy, gravitational potential energy, and conservative forces in physics. You will learn about the relationships between force, displacement, and energy types. Test your understanding of these principles with practical examples.

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