10th Physics |Unit 1| Bookback questions Notes
Define inertia. Give its classification.
The inherent property of a body to resist any change in its state (rest or motion), unless it is influenced by an external unbalanced force.
Classification of Inertia :
Inertia of rest ❖ Inertia of motion ❖ Inertia of direction
Classify the types of force based on their application.
Types of Forces:
If a 5 N and a 15 N forces are acting opposite to one another. Find the resultant force and the direction of action of the resultant force.
Given , 𝐹1 = 5𝑁 𝐹2 = 15 𝑁 Two forces acting opposite to one another. Therefore, Resultant force is,
𝐹𝑛𝑒𝑡 = 𝐹2 − 𝐹1
𝐹𝑛𝑒𝑡 = 15 − 5 = 10 𝑁.
∴ The magnitude of resultant force is 10 N and its direction is along 15 N force.
Differentiate mass and weight.
5. Define moment of a couple.
The rotating effect of a couple is known as moment of a couple.
It is the product of any one of the forces and the perpendicular distance between the line of action of two forces.
𝐌 = 𝐅 × 𝐒
Its SI unit is Nm.
6. State the principle of moments.
- When a number of like or unlike parallel forces act on a rigid body and the body is in equilibrium, then the algebraic sum of the moments in the clockwise direction is equal to the algebraic sum of the moments in the anticlockwise direction.
Moment of clockwise direction = Moment of anticlockwise direction
(OR)
- At equilibrium, the algebraic sum of the moments of all the individual forces about any point is equal to zero.
7. State Newton’s second law. [MDL – 19]
The force acting on a body is directly proportional to the rate of change of linear momentum of the body and the change in momentum takes place in the direction of the force. It is also called law of force.
F = M x A
8. Why a spanner with a long handle is preferred to tighten screws in heavy vehicles?
The turning effect of a body is more when the perpendicular distance between the line of action of the applied force and the axis of rotation is more.
Thus, when the handle of the spanner is long, the force required to turn the body is less.
Hence, the spanner has a long handle is preferred to tighten screws in heavy vehicles.
9. While catching a cricket ball the fielder lowers his hands backwards. Why?
He pulls back his hand while catching the ball,
To increase the time of contact.
To reduce the impulse.
To decrease the pain.
10. How does an astronaut float in a space shuttle?
Astronauts are not floating but falling freely around the Earth due to their huge orbital velocity.
Since the space station and astronauts have equal acceleration, they are under free fall condition.
1. What are the types of inertia? Give an example for each type.
Types of Inertia :
Inertia of rest
Inertia of motion
Inertia of direction
Inertia of rest:
The resistance of a body to change its state of rest is called Inertia of rest.
Ex : When we vigorously shake the branches of a tree, some of the leaves and fruits are detached and they fall down.
Inertia of motion :
The resistance of a body to change its state of motion is called inertia of motion.
Ex : An athlete runs some distance before jumping. Because, this will help him jump longer and higher.
Inertia of direction :
The resistance of a body to change its direction of motion is called Inertia of direction.
Ex : When we make a sharp turn while driving a car, we tend to lean side waysNewton's First law :
Every body continues to be in its state of rest or the state of uniform motion along a straight line unless some external force acts upon it.
Newton's second law :
The force acting on a body is directly proportional to the rate of change of linear momentum of the body and the change in momentum takes place in the direction of the force.
Newton's third law :
- Newton's second law of motion :
The force acting on a body is directly proportional to the rate of change of linear momentum of the body and the change in momentum takes place in the direction of the force
Derivation for the equation of force:
Let, m → mass of a moving body
F → unbalanced external force u → initial velocity
v → final velocity after a time interval of 't'
Initial momentum of the body 𝑃𝑖 = 𝑚𝑢 , Final momentum of the body 𝑃𝑓 = 𝑚𝑣
Change in momentum, ∆𝑃 = 𝑃𝑓 − 𝑃𝑖
∆𝑃 = 𝑚𝑣 − 𝑚𝑢
Law of Conservation of momentum:
There is no change in the linear momentum of a system of bodies as long as no net external force acts on them.
Proof:
Let two bodies A and B having masses m1 and m2 move in a straight line.
Let initial velocities of A and B be u1 and u2 respectively such that u1 > u2.
During an interval of time ‘t second’, they tend to have a collision.
After collision, A and B move along the same straight line with a velocity v1 and v2 respectively.
Principle: Law of conservation of linear momentum and Newton’s III law of motion.
When the rocket is fired, the fuel in the propellant tank is burnt and a hot gas is ejected with a high speed from the nozzle of the rocket, producing a huge momentum.
To balance this momentum, an equal and opposite reaction force is produced in the combustion chamber, which makes the rocket project forward.
In motion, the mass of the rocket gradually decreases, until the fuel is completely burnt out.
There is no net external force acting on it, and so the linear momentum of the system is conserved.
The mass of the rocket decreases with altitude. This increases the velocity of the rocket and reaches escape velocity, which is sufficient to just escape from the gravitational pull of the Earth.
6. State the universal law of gravitation and derive its mathematical expression.
Newton's Universal Law of gravitation :
Every particle of matter in this universe attracts every other particle with a force. This force is directly proportional to the product of their masses and inversely proportional to the square of the distance between the center of these masses.
The direction of the force acts along the line joining the masses.
Mathematical Expression of Universal Law of gravitation :
Where Universal gravitational constant, G = 6.674 × 10−11Nm2kg−2
- It is used to measure the dimensions of heavenly bodies.
- Helps to calculate the following with a higher accuracy, mass of the earth, radius of the earth, acceleration due to gravity, etc.
- Helps in discovering new stars and planets.
- Helps to predict the path of astronomical bodies.
- Helps to explain the germination of roots (geotropism).
- It helps to maintain the motion of all the planets around the sun and moon around the earth.
- It helps to maintain the water flow in the rivers and seas
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