Quick Review Flashcards - Click to flip and test your knowledge!
Question
How does the effect of a force applied to a perfectly rigid body differ from that applied to a non-rigid body?
Answer
On a rigid body it only causes motion, whereas on a non-rigid body it causes both motion and a change in size or shape.
Question
Quantitatively, how is force defined in terms of linear momentum?
Answer
It is defined as the rate of change of linear momentum, expressed as $\vec{F} = \frac{d\vec{p}}{dt}$.
Question
If mass $m$ is constant, what is the formula for force $F$ in terms of acceleration $a$?
Answer
$F = m a$
Question
What is the relation between the gravitational unit of force 'kilogram-force' ($kgf$) and the SI unit 'newton' ($N$)?
Answer
$1 \text{ kgf} = g \text{ N}$, where $g$ is the acceleration due to gravity.
Question
What is the average value of $g$ used for converting $kgf$ to $N$ on the earth's surface?
Answer
$9.8 \text{ m s}^{-2}$
Question
Define 'translational' or 'linear' motion.
Answer
Motion in a straight path in the direction of the applied force when a body is free to move.
Question
What is 'rotational motion'?
Answer
The motion of a body pivoted at a point that rotates about an axis when a force is applied at a suitable point.
Question
What is the 'moment of a force' (or torque)?
Answer
The turning effect produced by a force on a body about a fixed axis or point.
Question
State the formula for calculating the magnitude of the moment of a force.
Answer
$\text{Moment of force} = \text{Force} \times \text{Perpendicular distance of the line of action of force from the axis of rotation}$
Question
Which two factors determine the turning effect of a force on a body?
Answer
The magnitude of the force applied and the perpendicular distance of the line of action of the force from the axis of rotation.
Question
Under what condition is the maximum turning effect obtained from a given force?
Answer
When the force is applied at a point where the perpendicular distance from the axis of rotation is maximum.
Question
What is the SI unit of the moment of force?
Answer
Newton $\times$ metre ($N m$)
Question
What is the CGS unit of the moment of force?
Answer
dyne $\times$ cm
Question
How many $dyne cm$ are equivalent to $1 N m$?
Answer
$10^7 \text{ dyne cm}$
Question
Why is the unit of moment of force ($N m$) not written as joule ($J$)?
Answer
Because torque is a vector quantity, while work or energy (measured in joules) is a scalar quantity.
Question
By convention, what is the sign assigned to an anticlockwise moment?
Answer
Positive ($+$)
Question
By convention, what is the sign assigned to a clockwise moment?
Answer
Negative ($-$)
Question
What is the direction of the vector representing an anticlockwise moment relative to an observer?
Answer
Outwards, towards the observer.
Question
What are the two ways to change the direction of rotation of a body?
Answer
By changing the point of application of the force or by changing the direction of the force.
Question
Why is a long handle provided for a spanner used to tighten a nut?
Answer
To increase the perpendicular distance, thereby producing a large moment with a small applied force.
Question
Define a 'couple' in physics.
Answer
Two equal and opposite parallel forces not acting along the same line.
Question
What is the term for the perpendicular distance between the two forces forming a couple?
Answer
Couple arm
Question
Formula: Moment of couple
Answer
$\text{Either force} \times \text{perpendicular distance between the two forces (couple arm)}$
Question
Give three practical examples where a couple is applied for rotation.
Answer
Turning a water tap, tightening the cap of an inkpot, and turning a door key.
Question
What is the resultant force of a couple?
Answer
Zero, as the two forces are equal and opposite.
Question
When is a body said to be in a state of equilibrium?
Answer
When a number of forces acting on it produce no change in its state of rest or of linear or rotational motion.
Question
Define 'static equilibrium'.
Answer
When a body remains in a state of rest under the influence of several forces.
Question
Define 'dynamic equilibrium'.
Answer
When a body remains in the same state of motion (translational or rotational) under the influence of several forces.
Question
Why is a rain drop falling with a constant velocity considered to be in dynamic equilibrium?
Answer
The weight of the drop is balanced by the sum of the buoyant force and the force due to friction (viscosity) of air.
Question
What are the two necessary conditions for a body to be in equilibrium?
Answer
The resultant of all forces acting on it must be zero, and the algebraic sum of moments of all forces about the pivot must be zero.
Question
State the Principle of Moments.
Answer
In equilibrium, the algebraic sum of moments of all the forces acting on the body about the axis of rotation is zero.
Question
According to the principle of moments, what is the relationship between clockwise and anticlockwise moments in equilibrium?
Answer
Sum of anticlockwise moments $=$ Sum of clockwise moments.
Question
Name one common device that works on the principle of moments.
Answer
A physical balance (or beam balance).
Question
Define 'Centre of Gravity' ($C.G.$) of a body.
Answer
The point about which the algebraic sum of moments of weights of all the particles constituting the body is zero.
Question
Where does the entire weight of a body act according to the concept of Centre of Gravity?
Answer
At the Centre of Gravity ($C.G.$).
Question
On what factor does the position of the centre of gravity of a body of a given mass depend?
Answer
Its shape, which determines the distribution of mass (particles) within it.
Question
Is it necessary for the centre of gravity to always be within the material of the body?
Answer
No, it can lie where there is no material, such as the centre of a ring or a hollow sphere.
Question
Where is the centre of gravity of a uniform rod located?
Answer
At its mid-point.
Question
Where is the centre of gravity of a solid or hollow sphere located?
Answer
At its geometric centre.
Question
State the position of the centre of gravity for a solid cone.
Answer
At a height of $\frac{h}{4}$ from the base on its axis, where $h$ is the height of the cone.
Question
State the position of the centre of gravity for a hollow cone.
Answer
At a height of $\frac{h}{3}$ from the base on its axis, where $h$ is the height of the cone.
Question
Where is the centre of gravity of a triangular lamina located?
Answer
At the point of intersection of its medians (centroid).
Question
Where is the centre of gravity of a circular ring located?
Answer
At the centre of the ring.
Question
How can a body be balanced using a knife edge?
Answer
By placing the knife edge exactly below the body's centre of gravity.
Question
Briefly describe how to locate the centre of gravity of an irregular lamina.
Answer
By using a plumb line suspended from multiple holes near the edge; the point where the lines intersect is the $C.G.$
Question
For the stable equilibrium of a body, where should the centre of gravity be positioned relative to the base?
Answer
As low as possible and above the base near the geometric centre of the body.
Question
Define 'uniform circular motion'.
Answer
When a particle moves with a constant speed in a circular path.
Question
Why is uniform circular motion considered 'accelerated motion' despite having constant speed?
Answer
Because the direction of motion changes continuously at every point, causing a continuous change in velocity.
Question
What is the direction of velocity at any instant for a particle in uniform circular motion?
Answer
Along the tangent drawn at that point of the circular path.
Question
How does uniform circular motion differ from uniform linear motion regarding acceleration?
Answer
Uniform linear motion is unaccelerated (velocity is constant), whereas uniform circular motion is accelerated (velocity is variable).
Question
Define 'centripetal force'.
Answer
A force acting on a body moving in a circular path, directed towards the centre of the path.
Question
What provides the centripetal force for an electron moving around the nucleus in an atom?
Answer
The electrostatic force of attraction on the negatively charged electron by the positively charged nucleus.
Question
What provides the centripetal force for a planet moving around the sun?
Answer
The gravitational force of attraction on the planet by the sun.
Question
What provides the centripetal force for a stone whirled in a circular path at the end of a string?
Answer
The tension in the string.
Question
Define 'centrifugal force'.
Answer
A fictitious force assumed by an observer moving with the body, acting in a direction away from the centre of the circular path.
Question
Why is centrifugal force called a 'fictitious' or 'virtual' force?
Answer
It is not a real force arising from an interaction; it is only assumed to explain observations from a rotating frame of reference.
Question
Is centrifugal force the reaction force of centripetal force?
Answer
No, because action and reaction forces do not act on the same body, whereas these are considered for the same body.
Question
What happens to a stone whirled in a circle if the string suddenly breaks?
Answer
The stone flies off tangentially in the direction of its velocity at that instant.
Question
In a uniform circular motion, which quantity remains constant?
Answer
Speed
Question
In a uniform circular motion, which quantities are variable?
Answer
Velocity and acceleration.