9th Class Physics New Book 2025 Chapter 4 Answer Question
Class 9th Physics New Book 2025 Chapter 4 Answer Question.
9th Class Physics New Book 2025 Chapter 4 Constructed Response Questions
4.1 A car travels at the same speed around two curves with different radii. For which radius the car experience more centripetal force? Prove your answer.
If same car travels with same speed around two curves with different radii, for smaller radius car will experience greater centripetal force. Because mass and speed will be constant and radius of the circle will be inversely proportional to the centripetal force.
$$F\;=\;\frac{m\;v^2}r\\If\;m\;and\;v\;are\;cons\tan t\\then\\F\;\propto\frac1r$$
============
4.2 A ripe mango does not normally fall from the tree. But when the branch of the tree is shaken, the mango falls down easily. Can you tell the reason?
It happens due to inertia. When branch moves, mango want to stay in its original state due to inertia. Due to this, stem of the mangoes experiences a force and the mango falls down. This is an example of Newton’s first law of motion.
============
4.3 Discuss the concepts of stability and centre of gravity in relation to objects topping over. Provide an example where an object’s centre of gravity affects its stability, and explain how altering its base can influence stability.
Stability means after tilting how likely the object is to return to its original position. Centre of gravity of an object is a point where the entire weight of the object appears to be acting. A cupboard in a room is has high centre of gravity. So a small push can tilt it. When it tilts, centre of gravity can move outside the base, causing the cupboard to topple over. If we increase the base, the vertical line through the centre of gravity still falls inside the base, so cupboard will not topple.
============
4.4 Why an accelerated body cannot be considered in equilibrium?
An accelerated body cannot be considered in equilibrium because acceleration means forces are unbalanced. A body is said to be in equilibrium if all the forces acting on it are balanced and its state of motion does not change.
============
4.5 Two boxes of the same weight but different heights are lying on the floor of a truck. If the truck makes a sudden stop, which box is more likely to tumble over? Why?
The taller box will more likely tumble over due to its higher centre of gravity
============
9th Class Physics New Book 2025 Chapter 4 Question Answer
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.
============
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.
============
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:-
$${\textstyle\sum_{}}F=0\\or\\{\textstyle\sum_{}}{\textstyle{\scriptstyle F}_x}{\textstyle=}{\textstyle0}{\textstyle\;}{\textstyle a}{\textstyle n}{\textstyle d}{\textstyle\;}{\textstyle\sum_{}}{\textstyle{\scriptstyle F}_y}{\textstyle=}{\textstyle0}$$
============
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
$$\textstyle\sum_{}F_x=0\;and\\\sum_{}F_y=0\;and\\\sum_{}\tau==0$$
============
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.
Some important links are given below
