Chapter 4 Physics|Short Question 9th||tutoronlines

Chapter no 4

Turning Effect of Forces

The Table of Content:

Number Name
1 Resultant vector
2 Torque
3 Centre of mass and gravity
4 Like & unlike parallel
5 Equilibrium
Q.1 Can a small child play with a fat child on the see-saw? Explain how?

Ans. Yes, Fat child can play with small child by adjusting the moment arm.

Q.2 Two children are sitting on the see-saw, such that they cannot swing. What is the net torque in this situation?

Ans. When two children are sitting on the see-saw, such that they cannot swing. In this case, net torque zero because second condition of equilibrium is satisfied.

Q.3 A ladder leaning at a wall is in equilibrium. How?

Ans. Ladder is in equilibrium because it’s satisfies second condition of equilibrium.

Q.4 The weight of the ladder produce an anticlockwise torque. The wall pushes the ladder at its top end thus produce a clockwise torque. Does the ladder satisfy second condition for equilibrium?

Ans. Yes, its satisfies second condition of equilibrium because both torques are equal in magnitude but opposite in direction.

Q.5 Does the speed of a ceiling fan go on increasing all the time?

Ans. Speed of ceiling fan does not increase all the times. At acquires maximum speed it moves with uniform speed.

Q.6 Does the fan satisfy second condition for equilibrium when rotation with uniform speed?

Ans. No, it does not satisfy the second condition of equilibrium. Because it neither in the state of rest nor moving with uniform velocity.

Exercise short Questions

Q.1 Define the following. Resultant vector, torque, centre of mass, centre of gravity?
Ans. Resultant vector:

A resultant vector is a single vector that has the same effect as the combined effect of all the vectors to be added.

Torque:

The turning Effect of Forces acting on the body is called torque.

Mass of Centre
:

Centre of mass of a system is such a point where an applied force causes the system to move without rotation.

Centre of gravity:

A point where the whole weight of the body appears to act vertically downward is called center of gravity of a body.

Q.2 Difference the following.(i) like and unlike parallel force (ii) torque and couple (iii) stable and neutral equilibrium.
Ans.(i) like and unlike parallel force
Like parallel force:

Like parallel force are the force that are parallel to each other and have the same direction.

Unlike parallel force:

Unlike parallel force are the force that are parallel but have the direction opposite to each other.

(ii) torque and couple
Torque:

The turning effect of force acting on the body is called torque.

Couple:

A couple is formed by two unlike parallel force of the same magnitude but not along the same line

(iii) stable and neutral equilibrium.
Stable equilibrium:

A body is said to in stable equilibrium if after a slight tilt it return to its pervious position.

Neutral equilibrium:

If a body remains in its new position when disturbed from its previous position it is said to be in a state of neutral equilibrium

Q.3 How head to tail rule helps to find the resultant of force?

Ans. In head to tail rule, resultant force is found by joining its tail with tail of the first force and its head with head of the last force.

Q.4 How can a force be resolved into its rectangular components?

Ans. See the text book.

Q.5 When a body to be in equilibrium?

A body is said to be a equilibrium when both condition of equilibrium are satisfied. If no net force act on it. A body in equilibrium remains at rest or moves with uniform velocity.

Q.6 Explain the first condition of equilibrium?

Ans. See on text book

Q.7 Why there is need of second condition for equilibrium if a body satisfies first condition for equilibrium?

Ans. Two equal and opposite (unlike) force having their different lines of action form couple, which produce angular acceleration so body cannot be in equilibrium. First condition of equilibrium satisfied. So there is need of second condition.

Q.8 What is second condition of equilibrium?

Ans. A body satisfies second condition of equilibrium when the resultant torque acting on it is zero.

Q.9 Give an example of a moving body which is in equilibrium?

Ans. (i) A body moving with uniform velocity in straight line is the equilibrium.

(ii) A paratrooper coming down with terminal velocity is in equilibrium.

Q.10 Think of a body which is at rest but not in equilibrium?

Ans. A ball thrown vertically upward become at rest at the top. At this state it is not in equilibrium although it is at rest.

Q.11 Why a body cannot be in equilibrium due to a single force acting on it?

Ans. A single force acting on a body is not balance by any other force and produces acceleration. Therefore, in the presence of a single force body cannot be in equilibrium.

Q.12 Why the height of vehicles is as low as possible?

Ans. Vehicle made heavy at the bottom. This lowers their centre of gravity and helps their stability and avoid the chance of toppling down.

Q.13 Explain what is meant by stable equilibrium, unstable equilibrium and neutral equilibrium. Give one example in each case.
Stable equilibrium:

A body to in stable equilibrium if after a slight tilt it return to its pervious position.

Example:

A book is lying on the table. The book return to its pervious position when let free after a slight tilt.

Unstable equilibrium:

If a body does not return to its pervious position when sets free after a slightest till, it is said to be in unstable equilibrium.

Example:

Pencil just balance at its tip with centre of gravity at the highest position. Pencil topples over cause by the torque of its weight acting

Neutral equilibrium:

If a body remains in its new position when disturbed from its previous position it is said to be in a state of neutral equilibrium.

Example:

A ball placed on a horizontal surface. The ball remains in its new displaced position.

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