Question

Can a turkey only roll if the net force acting on it is unbalanced?
Can the turkey roll when the net force is balanced?
Explain the motion of the turkey when forces are balanced:
If the turkey isn’t moving what would be required to get it to move?

Answer 1

Short answer: No — rolling (rotation) does not require an unbalanced net force. A turkey can roll even when the vector sum of all forces is zero, provided there is a nonzero net torque (moment).

Explanation:
- For a rigid body you must consider translation and rotation separately:
- Sum of forces = m · a_cm (controls acceleration of the center of mass).
- Sum of torques about the center of mass = I · α (controls angular acceleration).
- If the net force is zero the center of mass has no acceleration (it either stays at rest or moves with constant velocity). That does not force α = 0. If the forces form a couple (equal and opposite forces whose lines of action do not coincide), they produce zero net force but a nonzero net torque and so will cause pure rotation about the center of mass.
- Example: two people push on opposite sides of the turkey in opposite directions but not along the same line — the turkey will spin even though the sum of forces is zero.
- If both net force and net torque are zero the turkey is in static equilibrium and nothing changes: no translation and no rotation.

If the turkey isn’t moving, what’s required to get it moving?
- To translate the turkey’s center of mass you need an unbalanced force (a resultant force ≠ 0). Practically, the applied force must also overcome static friction with the ground.
- To make it rotate from rest you need a nonzero net torque (a moment) about its center of mass. That can be a pure couple (two equal, opposite, noncollinear forces) or a single force applied off the center so it both accelerates the center and produces torque.
- In short: either an unbalanced force (for translation) or an unbalanced torque (for rotation) — or both — will start motion.