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3.1Work, Energy, and Power

Translational Kinetic Energy — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 14 1 quiz

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Lesson summary

3.1 Translational Kinetic Energy — AP PHYSICS 1 notes: Describe translational kinetic energy in terms of mass and velocity, and use K = ½mv2.Translational motion = straight-line motion without rotation, so every point has the same velocity.Translational kinetic energy = the energ…

  • Uniform circular motion
  • Frame dependence
  • Ground observer
  • Car driver
  • Truck driver
  • Work–energy theorem

Section 1 of 8Translational kinetic energy

Translational kinetic energy
Translational motion
Translational motion straight-line motion without rotation, so every point has the same velocity.
Translational kinetic energy
Translational kinetic energy the energy an object has because of its translational motion.
K
½mv2
  • K in J, m in kg, v in m/s; 1 J = 1 kg·m2/s2.
  • Rearrangedv = √(2K/m).
Scalar
  • K depends on speed not direction and is never negative.
System
  • System total kinetic energy is the sum of the individual kinetic energies.

Section 2 of 8Proportionality

ChangeEffect on K
Double mass×2
Double speed×4
Halve speed×¼
Triple speed×9
Rule
  • K is proportional to m and to v2.
Braking
  • Braking stopping distance for the same braking force is proportional to v2.
Uniform circular motion
  • K stays constant although the velocity changes.

Section 3 of 8Reference frames

Frame dependence
  • Frame dependence velocity is relative, so kinetic energy is relative to the observer.
Ground observer
  • Ground observer sees a 1500 kg car at 5 m/s with K = 18 750 J and a 4500 kg truck at 10 m/s with K = 225 000 J.
Car driver
  • Car driver sees the truck at 5 m/s, so K = 56 250 J.
Truck driver
  • Truck driver sees the car at −5 m/s, so K = 18 750 J, because v is squared.
Passenger
  • Passenger has zero K relative to the train but large K relative to the ground.

Section 4 of 8Work and kinetic energy

  • Work–energy theorem
    • Wnet = ΔK; external work changes the system's kinetic energy.

Section 5 of 8Comparing kinetic energies

1
Write K = ½mv2 for each object or state.
2
Form the ratio K2/K1.
3
Cancel the ½ and any common quantities.
4
Substitute the mass ratio and the square of the speed ratio.
5
State the factor with a justification in words.

Section 6 of 8More from the specification

More from the specification
Kinetic energy across reference frames
  • Kinetic energy across reference frames translational kinetic energy half m v squared depends on velocity relative to chosen inertial reference frames.
Planet
  • A large body that orbits the Sun and whose gravity clears its orbit.
Stationary
  • Not moving.
Potential energy
  • Energy stored by the configuration of a system.
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
Gravitational field
  • Region where mass experiences gravitational force.
Field strength
  • G = Fg/m.
Reference frame
  • Coordinate system used by an observer.
Spring constant
  • Stiffness k in N/m.
Elastic
  • Returns to original shape when force is removed.
Satellite
  • A body that orbits a larger one; the Moon is the Earth's natural satellite.

Section 7 of 8Key terms

TermMeaning
Kinetic energyK = ½mv2, energy of motion.
ScalarQuantity with magnitude only.
JouleUnit of energy, 1 kg·m2/s2.
Work–energy theoremWnet = ΔK.
Translational motionMotion in which all points move with the same velocity.

Section 8 of 8Exam technique

1
Forgetting to square the speed.
2
Giving kinetic energy a direction or a negative value.
3
Using a speed in km/h or a mass in g without converting.
4
Assuming K is the same for every observer.

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