Torque Review Question

To tighten a bolt, you push with a force of 80 N at the end of a wrench that is 0.25 m from the axis of the bolt. (a) What torque are you exerting? (b) If you move your hand inward to be only 0.10 m from the bolt, what force do you have to exert to achieve the same torque?

Chapter 8 – Rotational Motion

Center of Mass and Center of Gravity

Center of Mass vs. Center of Gravity

• Center of Mass– The average position of the mass of an object.

The CM moves as if all the external forces acted at this point

• Center of Gravity– The average position of weight or the single point

associated with an object where the force of gravity can be considered to act.

• Refers to the same point of an object; terms can be used interchangeably

Center of Gravity

• Multiple-flash photograph shows a top view of a wrench sliding across a smooth horizontal surface.

• The center of mass, indicated by the white dot, follows a straight line path.

• The wrench rotates around that point

StabilityWhy doesn’t the Leaning Tower of Pisa fall over?

Because it’s center of gravity lies above the base

support

As long as the center of gravity of any object is

above it’s support, it will not fall over!

Fun with your center of gravity…

• Touch your toes

• Pick up the chair

• Knock over the marker

Example #1

Where is this man’s center of gravity relative to his hands?

Example #2

Why is it dangerous to slide open the top drawers of a fully loaded file cabinet that is not secured to the floor?

Center of Gravity Review Questions

1. Where is the center of mass of a baseball? Where is its center of gravity? Where are these centers for a baseball bat?

2. How far can an object be tipped before it topples over?

3. Toss a stick into the air and it appears to wobble all over the place. Specifically, what place?