Signs

There was some confusion in the definition of gravitational potential energy used in Unit 11. This is understandable because of the way that the sign of the fall distance was used.

In this course we'll always take the value of g to be +9.8 m/s². Therefore when something falls its acceleration is −g. However, when something falls there's always the option of using the positive sign for the downward direction. That seems to be what was done in Unit 11 where y is used for the fall distance. The result was that gravitational potential energy comes out as

E_pot = −mgy


That's not not exactly wrong, but is probably confusing because almost everywhere else the sign is positive; so you probably remember this:

E_pot= +mgh

I've tried to repair this  confusion by defining the distance of fall as Δy and
as being negative. That way all is more conventional.

Other issues were using the notation E_pot for gravitational potential energy whereas U_g is more usual.

U_g = mgΔy

The textbook uses U^grav which is close. Similarly K is more convenient and usual for Kinetic Energy than E_kin.

I've rewritten the unit's Activity Guide with these changes and I hope it won't be so confusing now. Have a look and let me know what's not clear:

Revised Unit 11