9th Physics Chapter 4 long questions
9th Physics Chapter 4 long questions.
4.1 Explain the principle of moments with an example.
Principles of moments states that when a body is in balanced position, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that point.
A child weighing 200 Newton sits 2 meter from the pivot on the left side. Another child sits on the right side at a distance of 4 meter from the pivot.
Anticlockwise moment = 200 x 2 = 400 Nm
So, the force on the right side is
F x 4 = 400
F = 100
Hence a child of 100 N sitting 4 m away balances the seesaw.
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4.2 Describe how could you determine the centre of gravity of an irregular shaped lamina experimentally.
Suspend the lamina from a point using a string and a weight to make a plumb line. Draw the line along the lamina’s surface. Repeat by suspending the lamina from different points. The intersection of these lines will give the centre of gravity of the lamina.
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4.3 State and explain two conditions of equilibrium.
First condition of equilibrium
A body is said to be in translational equilibrium only if the vector of sum of all the external forces acting on it is equal to zero.
By Newton’s second law of motion, F=ma if the body is in translational equilibrium, the a=0, therefore net force should be zero.
Mathematical form:-
$${\textstyle\sum_{}}F=0\\or\\{\textstyle\sum_{}}{\textstyle{\scriptstyle F}_x}{\textstyle=}{\textstyle0}{\textstyle\;}{\textstyle a}{\textstyle n}{\textstyle d}{\textstyle\;}{\textstyle\sum_{}}{\textstyle{\scriptstyle F}_y}{\textstyle=}{\textstyle0}$$
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Second condition of equilibrium
The vector sum of all the torques acting on a body about any point must be zero.
It implies to rotational equilibrium which means that the body should not rotate under the action of the forces.
If first condition is satisfied, the system may still not be in complete equilibrium.. If the forces are forming a couple, they will rotate the object.. Hence for complete equilibrium both the conditions must satisfy.
Mathematical form:-
$${\textstyle\sum_{}}\tau=0$$
Hence for complete equilibrium
$$\textstyle\sum_{}F_x=0\;and\\\sum_{}F_y=0\;and\\\sum_{}\tau==0$$
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4.4 How the stability of an object can be improved? Give a few examples to support your answer
Stability of an object can be improved by lowering its centre of gravity. A car with low frame has lower centre of gravity.
By increasing the base area of the object its stability can be improved. A table with wide legs is more stable than a table with narrow legs.
Stability of an object can be improved by keeping the weight evenly distributed. A tall bookshelf fixed to a wall.
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