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Understanding Work and Energy in Physics: Calculations and Concepts

This chapter focuses on the concepts of work and energy in physics. It explains how to calculate work using the formula W = FΔx, where W is work, F is force, and Δx is the distance moved. The chapter also discusses positive and negative work based on the direction of force and motion. Sample problems illustrate how to compute work for lifting and lowering masses, as well as the net work when forces act on an object. The kinetic energy (KE) of moving objects is defined and calculated using the formula KE = ½mv², with practical examples for clarity.

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Understanding Work and Energy in Physics: Calculations and Concepts

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  1. Adv Physics Chapter 5 Sections 1 & 3

  2. Work • Force applied over a given distance • If force is constant and body moves in a straight line W = F Δx where W = work (scalar quantity) F = force in direction of motion Δx = distance moved and [W] = Nm = Joule, J = ft lb

  3. Sample Problem Compute the work required to lift a 1 kg mass to a height of 5 m if it doesn’t accelerate.

  4. Sample Problem Compute the work required to lower a 1 kg mass a distance of 5 m if it doesn’t accelerate.

  5. Work cont’d. • Work is positive if the force and motion are in the same direction • Work is negative if the force and motion are in opposite directions • Net work – sum of the work done by all the forces acting on an object, ΣW

  6. Sample Problem Compute the net work done in raising the 1 kg mass 5 m and then returning it to its initial position.

  7. Sample Problem Compute the work required to slide a 1 kg mass at constant speed along a horizontal surface having a coefficient of kinetic friction of 0.3 for a distance of 5 m.

  8. Sample Problem A girl pulls a wagon with constant velocity along a level path for a distance of 45 m. The handle of the wagon makes an angle of 20 degrees with the horizontal and she exerts a force of 85 N on the handle. Find the amount of work the girl does in pulling the wagon.

  9. Energy • Having the ability to do work • Kinetic energy – energy due to motion KE = ½ m v2 where KE = kinetic energy (scalar quantity) m = mass v = speed [KE] = J

  10. Sample Problem A baseball has a mass of 0.14 kg and is thrown with a speed of 26 m/s. What is its kinetic energy?

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