AS 91524 · 2023 paper · rotational motion
Question 2(a) Achieved
Practises Energy methods for rolling and rotating bodies
Tane works weekends unloading barrels. He rolls an empty barrel of mass 5.50 kg and radius 0.280 m down a ramp 1.34 m high. The linear speed of the barrel at the bottom of the ramp is 3.40 m s⁻¹. Parts (c) and (d) move to Tane on a swivelling stool holding a full bottle of water in each hand: he spins more slowly with his arms outstretched than with his arms in.
| m | 5.50 kg |
|---|---|
| r | 0.280 m |
| h | 1.34 m |
| v | 3.40 m s⁻¹ |
Describe the energy changes that take place as the barrel rolls down the ramp.
Work it out
Try it on paper first. Stuck? Take one hint at a time.
-
1 At the top, what form is the barrel’s energy in?
Gravitational potential energy. -
2 At the bottom it is moving along AND spinning. What forms has the energy taken?
Linear (translational) kinetic energy, ½mv², and rotational kinetic energy, ½Iω². With no losses: Egp → Ek(lin) + Ek(rot).
What the marker wanted
- Achieved The right answer, in words or as an equation.
In our words, from NZQA’s 2023 marking schedule.
✕Common mistake
Saying it all becomes kinetic energy, ½mv². A rolling barrel spins as well, so part of the energy is rotational. NZQA’s 2023 report names energy changes missing either Ek(lin) or Ek(rot) in its Not Achieved, Achieved and Merit lists alike.How did it go?
Got it moves it further out. Review brings it back tomorrow.