Work and Kinetic Energy

Why do we need to do energy-based analysis when we already learned force-based analysis?

• Hard to keep track of all the forces in the real world

• Newton's laws don't apply in atomic and subatomic worlds

• Energy & work are scalars (easy to work with)

• All of the above

Tags

NGSS.HS-PS3-4

When wind is blowing directly at your face, in which of the following directions is it easier to walk?

• sideways (perpendicular to direction of wind)

• facing the wind

• at a 45 degrees angle

Tags

NGSS.HS-PS2-1

What is the SI unit of Work Done?

• Joule

• Newton

• Watt

• kg m/s

Answer explanation

1 Joule = 1 N m = 1 kg m²/s²

How can you calculate work done?

• Graphically by determining the area under the curve - Fcos(theta) vs. displacement

• Numerical integration using computer or calculator

• Analytically using Integral Calculus when Force is known as a function of position

• All of the above

Answer explanation

In this class, we will mostly use Analytical method (Integral Calculus)

What is Hooke's law?

• Net work done = change in kinetic energy

• KE = 1/2 m v²

• Spring force (F_s) = - kx

• Every action has an equal and opposite reaction

Answer explanation

Also known as Spring equation or Restoring force exerted by spring

Which of the following is true in a closed system?

• the total energy (sum of all types of energy) before the process = after the process

• the total kinetic energy before the process = after the process

• net work done on an object before the process = after the process

Tags

NGSS.HS-PS3-1

Which of the following is true?

• W = Fdcos(theta)

• W = 1/2 kx² for a spring

• KE = 1/2 mv²

• W_net = Change in Kinetic Energy

• All of the above

Tags

NGSS.HS-PS2-1

You carry a 20 pound weight at a constant height and at a constant speed for a distance of 2 miles. How much work do you do on the weight?

Answer explanation

There is no component of lifting force in the direction of 2 mile displacement.

Then, why you get tired? That has more to do with the physiology of muscles and the contraction that happens, which requires chemical energy.

If you push against a wall and the wall does not move, then how much work do you do on the wall? (Write on a separate sheet of paper - why do you become tired?)

1. You slowly lift a bowling ball from the floor and put it on a table. The initial and final kinetic energies of the bowling ball are the same (namely zero). Hence from the work-kinetic energy theorem, we know that no work is done on the bowling ball. On the other hand, you certainly feel that you have done some work! Explain this apparent contradiction.