Turning Effect of Forces
9th Class Physics Chapter 4 Exercise Short Questions with Answers
Q 4.2 : define the following :
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 .”
” The turning effect of a force is called torque or moment of the force .”
Symbol : t
Formula : t = r F
Unit : Newton metre (Nm)
Centre of Mass :
” Centre of mass of a system is such a point where an applied force causes the system to move with rotation .”
→ The Centre of mass of a system moves as if its entire mass is confined at that point .
Centre of Gravity :
” A point where the whole weight of the body appears to act vertically downward is called Centre of gravity of a body .”
Q 4.3 : Differentiate the following :
Like and Unlike parallel forces
Torque and couple
Stable Equilibrium and Unstable Equilibrium
|Stable Equilibrium||Neutral Equilibrium|
|” A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position .”||” 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 .”|
|A book laying on a table .||Rolling Ball|
Q 4.4 : How head to tail rule helps to find the resultant of forces ?
Head to tail Rule :
” It is a graphical method used to find the resultant of two or more forces is called Head to Tail Rule .”
→ In this method , vectors are added graphically by following steps / method :
- The tail of 2nd vector coincide with the head of 1st vector and so on .
- The resultant of all vectors is obtained by coinciding the tail of 1st vector with the head of last vector .
Q 4.5 : How can a force be resolved into its rectangular components ?
Resolution of a Force :
” A force can be resolved inti its perpendicular components by drawing its two mutually perpendicular components .”
- Consider a force F is making an angle θ with x-axis .
- This force can be resolved into its rectangular components by drawing its two mutually perpendicular components .
- The magnitude of these rectangular components is :
Fx = F cos θ
Fy = F sin θ
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Q 4.6 : When a body is said to be in equilibrium ?
A body is in equilibrium if it satisfies both conditions of equilibrium .
- 1st Condition : A body is in equilibrium if net force acting on it is zero .
ΣF = 0
ΣFy = 0
ΣFx = 0
- 2nd Condition : A body is said to be in equilibrium if the resultant torque acting on it is zero .
Σt = 0
Q 4.7 : Explain the first condition for equilibrium .
1st Condition for Equilibrium :
” A body is said to satisfy first condition for equilibrium if the resultant of all the forces acting on it is zero .”
F1 + F2 + F3 + . . . . . .. . . + Fn = 0
ΣF = 0
Q 4.8 :Why is there a need of second condition for equilibrium if a body satisfies first condition for equilibrium ?
- In above example , two equal and opposite forces are acting on a body but a long different lines .
→ This body is in equilibrium according to 1st Condition .
- But body can rotate , Therefore , we need another condition of equilibrium to satisfy that body is in complete equilibrium condition .
Q 4.9 : What is second condition for equilibrium ?
2nd Condition for Equilibrium :
” A body satisfies second condition for equilibrium when the resultant torque acting on it is zero .”
Σt = 0
Q 4.10 : Give an example of moving body which is in equilibrium .
- A Paratrooper coming down with terminal velocity (constant velocity ) is in equilibrium .
- All the forces acting on it are equal to zero , So it satisfies 1st Condition for Equilibrium .
Q 4.11 : Think of a body which is at rest but not in equilibrium .
→If we throw a ball upward , at its highest position , It is at Rest for a moment .
- But force of gravity is also acting on it to produce acceleration .
Thus , the ball is at rest but not in equilibrium .
Q 4.12 : Why cannot a body be in equilibrium due to single force acting on it ?
Because a single force is not balanced . It produces acceleration in the body .
→ Thus, under the influence of single force , a body cannot be in equilibrium .
Q 4.13 : Why the height of vehicles is kept as low as possible ?
Height of vehicle is kept low and their bottom is made heavy .
- To lower their centre of gravity .
- To increase their stability .
Q 4.14 : Explain what is meant by stable , unstable and neutral equilibrium . Give one example in each case .
Stable Equilibrium :
” A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position .”
Example : A book laying on a table .
Unstable Equilibrium :
” If a body does not return to its previous position when sets free after a slight tilt , it is said to be in un-stable equilibrium .”
Example : A pencil standing on its point .
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 : Rolling Ball .
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