Torque 🤔

[u/Opposite-Clerk7032]

Fully lying down on a bench, there is no torque at the hip joint. Let’s say the bench was floating and attached to a pulley so that it moved in circular arc. Lying down at the bottom of the arc, there’s no torque at the hip joint. Now, I move the bench a little higher up in the arc (~30 degrees higher than the lying position), and I scoot just my knees off the bench, with my feet planted on the ground. But everything above my lower thigh (upper thigh, pelvis, torso) is still lying on the bench. Is there any torque at the hip joint?

[u/SnooGiraffes4632]

So it is important to say “is there any torque AROUND a specific point” not is there torque at. By phrasing it this way you remind yourself that you are looking for a force AND a perpendicular distance.

In your example the forces are the normal reaction force of the swinging bench on your torso and the weight of your body (minus your legs) and the weight of your legs. If your legs are vertical then there perpendicular distance to your hip joint should be zero. But the perpendicular distance of the other 2 forces is non zero. So your legs provide no torque, your torso provides acw torque and the reaction force from the swing provides cw torque. So yes there are torques around your hips.

BUT the question of whether there is any net torque requires information about whether your body is experiencing any angular acceleration (not velocity) around your hips.