ELI5. How do we measure or equate something like “effort” in a task? When is carrying a light object over a longer distance equal to carrying a heavier object over a shorter distance?

Question

Here’s kind of what I’m thinking: A strong man carries an object weighing 100kg 15 meters from the living room to the shed. It takes him a couple of minutes. A distance runner carries a backpack weighing 5kg 10,000 meters from one town to the next and it takes him 45 minutes. Which task is harder?

Where I’m stuck is, if Work is Force X Distance, and if it takes no force to keep an object moving at a steady rate, then was any “work” done (ignoring the initial lifting of the object)? Because clearly tremendous effort was required for either task, but I don’t know how to measure “effort” because it’s not exactly “work”.

In: 0

Answer 1

Keeping an object moving with 0 work requires a system with 0 friction. There obviously is friction in the real world, so there is work being done to move both objects horizontally. If you could assume 0 friction, you would need neither your strongman nor your distance runner, because you could just slide the packages along your icy ground.

You can determine the amount of energy used by taking your power (wattage) over the duration * the duration.

So a bike rider riding at 100 Watts for 30 minutes uses the same amount of energy as a rider at 200 Watts for 15 minutes.