Lesson 13.2: Energy is transferred when work is done

Lesson Focus

Work isn't just about effort; it's how energy moves from one place to another. This lesson explores how doing work transfers energy, causing changes in an object's motion and position.

Learning Objectives

• Recognize that doing work on an object is the process of transferring energy to it.
• Learn to calculate an object's mechanical, kinetic (motion), and potential (stored) energy.
• Explain the law of conservation of energy, which states that total energy never changes.
• Analyze experimental data to observe how a rolling ball's energy transforms from one form to another.

When a force does work on an object, it transfers energy to that object. Think of throwing a ball: your arm does work, giving the ball your energy. Because work is the transfer of energy, both are measured in the same unit, the Joule (J). So, doing work on something increases its energy.

Lifting an object against gravity gives it stored energy, called gravitational potential energy (GPE). Its position gives it the potential to fall and do work. We calculate it using the formula GPE = mgh, where 'm' is mass, 'g' is gravity's acceleration, and 'h' is height. The higher it is, the more GPE it has.

Any moving object has kinetic energy (KE), the energy of motion. We calculate it using the formula KE = 1/2mv², where 'm' is mass and 'v' is velocity. Because velocity is squared, doubling an object's speed quadruples its kinetic energy. This means speed has a much greater effect on KE than mass does.

An object can have energy from its position (PE) and its motion (KE) simultaneously. This total is called mechanical energy (ME). We find it by adding the two energies together with the formula ME = PE + KE. For example, a flying bird has both height and speed, giving it significant mechanical energy.

The law of conservation of energy states that energy cannot be created or destroyed, only transformed. As a skater rolls down a ramp, her potential energy converts into kinetic energy, but her total mechanical energy stays the same. The energy at the top (all PE) equals the energy at the bottom (all KE).

Why does a swinging pendulum eventually stop? Friction and air resistance do work on it, converting its useful mechanical energy into thermal energy (heat). The energy isn't lost; it just changes into a different form. This transformation explains why the ME seems to decrease, but the total energy in the system is conserved.