FORM 2-PHYSICS- TOPIC 4-FORCES IN EQUILIBRIUM

FORCES IN EQUILIBRIUM

Moment of a Force

Centre of Gravity

Centre of Gravity

Explain centre of gravity

Center of gravity of a body is the point at which the weight of a body appears to be concentrated. OR center of gravity of a body is the point of application of the resultant force due to the earth attraction on the body.The center of gravity of a regular body is found to be at its geometrical center.

Example 1

1. Centre of gravity of a uniform meter rule is at the 50cm mark.
2. Centre of gravity of a circular object is at its center.

The center of gravity of irregular bodies can be found experimentally.

Centre of Gravity of Regular Shaped Body

Determine centre of gravity of regular shaped body

Activity 2

Experiment.

Aim; To determine the center of gravity of a regular body (meter rule)

Materials and apparatus: Meter rule, a known weight and a spring balance.

Procedures

• Balance the meter rule on a fulcrum and mark the position where the meter rule balances horizontally by the letter G.
• Hang a known weight from one end of the meter rule and determine the position where the meter rule balances and mark it X.
• Measure the distance XC and XG(C is the point at the end of the meter rule where an object of known weight, w is attached).

The weight, w of the meter rule always acts at G downwards. Thus it will create a moment to balance the moment due to w.

Results

The mass of the meter rule, w is determined by using the principle of moments.Taking moments about X;

Clockwise moments = Anticlockwise moments

w₁ .XC =w. XG

Centre of Gravity of Irregular Shaped Body

Determine centre of gravity of irregular shaped body

Activity 3

Experiment

Aim: To determine the center of gravity of irregular plate.

Materials and apparatus:Card board, plumb line, inelastic cotton thread, clamp, clamp rod and a marker pen.

Procedures

• Make an irregular shaped piece of card board.
• Make three small holes well-spaced round the edge of the cardboard.
• Suspend the cardboard using a nail or the rod of a clamp or a stout pin in a clamp from one of the holes.
• Suspend the plumb line in front of the card.
• Mark the position of the string of the plumb line on the card by two small crosses.
• Repeat the exercise for the other two holes. Join each pair of crosses with a straight line.

Result and Observation

The three straight lines meets at a certain point. This point is the center of gravity of the body.