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Unit 4 Review - Energy


written by

Erin Brzusek

erin brzusek

published on august 5, 2020

Last updated on August 4, 2020

Here is an outline for AP Physics 1, Unit 4: Energy, created in accordance with the 2019 Course and Exam Description published by the College Board. 

👉Check out all the Fiveable guides, slides, and videos for AP Physics 1

Energy 💥

Read: AP Physics 1Unit 4 Overview

  • Energy is defined as the capacity of matter to perform work (such as causing motion) ⚡️

In Unit 4, students are introduced to the idea of conservation and work as the agent of change for energy. Students will use old and new models and representations to analyze physical situations.

These models will now incorporate force or energy as major components. Complete knowledge of these energy models will give students the capability to make predictions and justify claims with evidence about physical situations. 


Open and Closed Systems 💭

Read: AP Physics 1Open and Closed Systems

  • A system is defined as an object or a collection of objects that are treated as having no internal structure 💫

  • In all systems energy, charge, linear momentum, and angular momentum are conserved.

For an isolated or closed system, conserved quantities are constant. An open system exchanges any conserved quantity with its surroundings.

An exchange can be either a force exerted by objects outside the system or a transfer with objects outside the system. The boundary between a system and its environment is decided by the person considering the situation.


Work 💪

Read: AP Physics 1 - Work and Mechanical Energy

  • Work occurs when a force exerted on an object moves that object parallel to the direction of that force ➡️ 

  • This is a scalar quantity, (+) indicates work parallel to displacement and (-) indicates work anti parallel to displacement
  • The area under a Force vs. Displacement graph (as indicated in the picture below) is work.
  • Work is positive when the graph showcases an object speeding up, 🏃‍♂️ and negative when the graph showcases an object slowing down🚶‍♂️

Key equation: W = D x F, where D is the distance parallel to F, the force applied. The SI units are Joules for Work, Newtons for Force, and Meters for distance. 


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Mechanical Energy 🏃

Watch: AP Physics 1 - 🎥 Kinetic Energy

  • Mechanical Energy is the sum of a system’s potential and kinetic energy.

  • If TME is not conserved (stays the same between two points in a problem) it is ‘lost’ to a form of internal energy 🌬
  • TME (Total Mechanical Energy) = PE (Potential Energy) + KE (Kinetic Energy)

Key equation: PE (Gravitational Potential Energy) = mgh, where m is an object’s mass, g is the acceleration due to gravity, and h is the vertical height of an object. The SI units are Kilograms for M, 9.8 /s/s or 10 m/s/s for G, and Meters for H. 

Key equation: PE (Spring Potential Energy) = ½ kx^2, where k is the spring constant and x is the stretch or compression. The SI units are Newtons/Meters for K and Meters for X. 

Key equation: KE (Kinetic Energy) = ½ mv^2, where m is an object’s mass, and v is its velocity. The SI units are Kilograms for Mass, and Meters/Second for Velocity. 

Key equation: KE (Rotational Kinetic Energy) = ½ Iω^2, where I is the rotational inertia and ω is angular velocity. The SI units are Radians/Second for ω and Kilogram X Meter^2 for I. 


Conservation of Energy 💤

Read: AP Physics 1 - Conservation of Energy, Work-Energy Principle, and Power

  • The law of conservation of energy states that the initial mechanical energy must equal the final mechanical energy 🌗

  • Initial Mechanical Energy + Nonconsersative Forces (ex. friction) = Final Mechanical Energy

Key equation: KEi + PEi + Wext = KEf + PEf, where i is initial, f is final, KE is kinetic energy, PE potential energy, and Wext is work external. 


The Work-Energy Principle ➕

  • The work-energy theorem states that the work done by the sum of all forces acting on an object or system equals the change in the kinetic energy of the object or system. 

Key equation: W = ΔKE, where W is the total work and ΔKE is the change in kinetic energy (KEf - KEi). The SI units are Joules for W and ΔKE. 


Power 💨

  • When work is completed over a period of time, we measure the power of the work 💪

Key equation: P = W/T, where P is power, W is work, and T is time. The SI units are Watts for Power, Joules for Work, and Seconds for Time.


Be sure to review the past FRQ prompts published by the College Board (Conservation of Energy 2018, Conservation of Energy 2017, and Conservation of Energy 2016) and check out this quizlet to look over some important terms and key concepts from this unit. You can do it!  

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