Center of Gravity

Throwing a wrench into parabolic motion

Center of gravity

• Definition: The point located at the object’s average position of weight.

• For a symmetrical object, this is the geometric center of the object.

• For a non-symmetrical object (such as a wrench), the center of gravity is towards the heavier end.

Motion of a non-symmetrical object

• For an object that is non-symmetrical, or has varying density that is in motion, the center of gravity will follow the path as discussed earlier this semester.

• The rest of the object will rotate about the center of gravity.

Location of the CG

• The location of the CG does NOT have to be a location on the object.

• Example

Toppling

• Activity: we are going to hang a plumb bob from a box and increase the incline of the surface the block sits on.

• Where is the location of the plumb bob in reference to the base of the box when it topples?

Leaning tower of Pisa

Activity

• Stand exactly two foot lengths away from a wall and place a stool between you and the wall.

• Bend over with a straight back and let your head lean against the wall.

• Lift the stool off the floor while your head is STILL on the wall.

• Then attempt to straighten up without bending your knees.

Tails

• What is the purpose of a tail for an animal?• It allows for an animal to shift its center of

gravity and increase its stability

CG of people

• When standing up straight, a human’s average CG is about 2-3 cm below our belly buttons.

• It is slightly lower for women because women tend to have smaller shoulders

• It is slightly higher for children because of larger heads compared to their shorter legs at that age.

CG for people

• When you change your position, your CG changes as well.

• Your CG is always somewhere above your support base.

• That is why you are more stable with your feet shoulder width apart than standing on one leg.

Learning to walk

• When learning to walk, a baby needs to learn to coordinate the position of their CG with their supporting foot when making a step.

• Pigeons bob their head to make up for this.

High Jump

• Doing the “Fosbury flop” in high jump, jumpers change their CG to below the bar while their body is able to pass over the bar.