Physics Class-9, Unit-4 Turning Effects of Force Solved Exercise MCQs

i. Two equal but unlike parallel forces having different line of action produce

(a) torque
(b) couple
(c) equilibrium
(d) neutral equilibrium

... Answer is (b) couple
When Forces are equal and line of action are same then "NO Motion". When Forces are unequal and line of action same then there will Translational motion. When Forces are equal but the line of action are not the same they produce Couple.

ii. The number of forces that can be added by head to tail rule are:

(a) 2
(b) 3
(c) 4
(d) any number

... Answer is (d)  any number
According to head to tail rule we can add two or more vectors. So among four option "any number " is the correct answer.

iii. The number of perpendicular components of a force are:

(a) 1
(b) 2
(c) 3
(d) 4

... Answer is (b) 2
The process of splitting of vector into its components is called Resolution of a vector. A Vector can split into two perpendicular components. Force is vector quantity So, The number of perpendicular components of a force are two. F = Fx + Fy

iv. A force of 10 N is making an angle of 30⁰ with the horizontal. Its horizontal component will be:

(a) 4 N
(b) 5 N
(c) 7 N
(d) 8.7 N

... Answer is (a) 8.7 N
Given: F = 10 N and θ = 30⁰ The Horizontal Components of Force Fx = F cos θ Putting values we get F= 10 cos 30⁰ = 10 N x 0.866 = 8.66 N = 8.7 N

v. A couple is formed by:

(a) two forces perpendicular to each other
(b) two equal and opposite forces in the same line
(c) two like parallel forces
(d) two equal and opposite forces not in the same line

... Answer is (d) two equal and opposite forces not in the same line
When Forces are equal and line of action are same then "NO Motion". When Forces are unequal and line of action same then there will Translational motion. When Forces are equal but the line of action are not the same they produce Couple.

vi. A body is in equilibrium when its:

(a) acceleration is uniform
(b) speed is uniform
(c) speed and acceleration are uniform
(d) acceleration is zero

... Answer is (d) acceleration is zero
According to the First Condition for Equilibrium "A body is said to be in equilibrium if the resultant of all forces acting body is zero" i.e. ΣF = 0. When ΣF = 0 mean that acceleration is also zero.

vii. A body is in neutral equilibrium when its centre of gravity: :

(a) is at its highest position
(b) is at its lowest position
(c) keeps its height if displaced
(d) is situated at its bottom

... Answer is (c) keeps its height if displaced
If a body remains in its new potion when disturbed from its previous position, it is said to be in a state of neutral equilibrium. In neutral equilibrium the center of gravity remains at the same height.

viii. Racing cars are made stable by:

(a) increasing their speed
(b) decreasing their mass
(c) lowering their centre of gravity
(d) decreasing their width

... Answer is (c) lowering their centre of gravity
A body will be in stable equilibrium if the center of gravity is at the lowest position. So vehicles are made heavy at the bottom. This lower their center of gravity and helps to increase their stability.

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