# Why do we lean when carrying heavy objects

1. Sep 4, 2010

### zorro

When we lift heavy objects, why do we tend to lean to the opposite side?

2. Sep 4, 2010

### Danger

Counter-balance. We try to maintain a centre of gravity that lessens the strain on our muscles and skeletons.

3. Sep 4, 2010

### zorro

I need a more detailed explanation in terms of Torque, centre of mass etc

4. Sep 4, 2010

### Staff: Mentor

Sounds like a homework question to me. Why don't you attempt to describe how it works?

5. Sep 4, 2010

### zorro

No, this is not a homework question.
I read in my book that "The torque of the contact forces about the centre of mass should be zero to maintain the equilibrium. This may happen only if the vertical line through the centre of mass cuts the bas surface at a point withing the contact area or the area bounded by the contact points. That is why a person leans in the opposite direction when he or she lifts a heavy load in one hand."
The second sentence is not clear to me. I need a more detailed explanation.

6. Sep 4, 2010

### Danger

Sorry, man... it's beyond me. It's just an instinct to lean away from the load. I don't know from math.

7. Sep 4, 2010

### Staff: Mentor

The only way to have torque equal to zero is if the net upward force from the base is exactly in line with force of gravity. Gravity acts through the center of mass, so if a line from the center of mass falls outside of the base, there's no way for the force on the base to line up and cancel the force of gravity. This is the basis of balance and stability.

See: http://dev.physicslab.org/Document.aspx?doctype=3&filename=RotaryMotion_CenterMass.xml"

Last edited by a moderator: Apr 25, 2017
8. Sep 4, 2010

### zorro

Thanks, I got the concept :)