Physics Test - 4

Concept:

Force(F):

• It is Push or Pulls required to displace the object from one place to another place.
• Force is an external agent capable of changing the state of rest or motion of a particular body.
• It has a magnitude and a direction.
• The direction towards which the force is applied is known as the direction of the force.
• The application of force is the point where force is applied.
• S.I unit of force is Newton or kg m s^-2
• It is also given by-product of mass and acceleration.

Explanation:

• Newton's second law of motion states that force is directly proportional to mass times acceleration.
• In other words, more is the mass more force is required and less is the mass less is the force required.
• Force can make a body that is at rest to move.
• It can stop a moving body or slow it down.
• It can accelerate the speed of a moving body.
• It can also change the direction of a moving body along with its shape and size.

So, Force can only change the speed, shape, and Direction of the motion but is not able to change the mass.

Additional Information

There are two types of forces based on their applications:

1. Contact Force - It is a force applied to a body by another body that is in contact with it.

• eg: Muscular Force, Mechanical Force, Frictional Force

1. Non-Contact Force - It is a force that acts on an object without coming physically in contact with it.

• eg: Gravitational Force, Electrostatic force, Magnetic force

The correct answer is option 2) i.e. 10 g

CONCEPT:​

• The time period of a simple pendulum is given by the equation:

Where L is the length of the string of pendulum and g is the acceleration due to gravity.

• The time period of a spring-mass system is given by the equation:

Where m is the mass and k is the spring constant.

CALCULATION:

The time period of a simple pendulum = 2 s

As the pendulum is replaced by the spring-mass system, it should have the same time period for correct functioning.

Hence, T = 2 s

CONCEPT:

• Converging lens or convex lens: A lens in which light rays enter into it parallel to its axis and converge at a single point on the opposite side.

EXPLANATION:

• When light passes through a convex lens, the light is converged at a point. This is why it is known as a converging lens (not diverging lens).
• As shown in the figure, the convex lens is thicker at the center and thinner at the edges.
• For converging lenses or convex lenses, the focal length is always positive (not negative).
• So the correct answer is option 2.

​Additional Information

The Sign convention for a convex lens is as shown:

CONCEPT:

• Circular motion: When a body moves in such a way that its force is always perpendicular to the velocity, the motion will be circular motion.
• Centripetal force: The force that is perpendicular to the velocity and directed towards the center is known as centripetal force. or
  • The force that makes a body move in a circular motion is known as centripetal force.
  • The direction of this force is always perpendicular to the direction of the velocity.

The centripetal force acting on a body of mass 'm' revolving with radius 'r' is:

EXPLANATION:

Given that applied force is uniform and perpendicular to the motion.

• From the definition of the centripetal force, if a force is perpendicular to the motion, it is a centripetal force and this force makes the body move in a circular motion.

• Since F is uniform, so v will also be uniform. So speed (magnitude of velocity) will not change.
• In a circular motion, a body moves in a circle, and in a circle the direction of motion changes at every point.

• At point A, the direction of the velocity is in the upward direction. 
• At point B, the direction of the velocity is in the leftward direction,
• At point C, the direction of the velocity is in the downward direction.

• The body comes in the initial direction after one complete circle.
• So If the force on a moving body is always uniform and perpendicular to its motion, then Speed remains the same, but direction changes.
• Hence the correct answer is option 4.

The Correct Answer is Option (3) i.e Ultrasound.

CONCEPT:

• Ultrasonic waves are the sound waves with frequencies higher than the upper audible limit of human hearing.
  • They have a short wavelength.
• SONAR: It a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, communicate with or detect objects on or under the surface of the water
  • SONAR stands for ‘Sound Navigation and Ranging.’
  • SONAR is based on the principle of echo.
  • Sonar is often used as a measurement of acoustic location and echo characteristics of the target object in the water.

EXPLANATION:

• The ultrasonic waves have short wavelengths and high frequencies.
• Due to the short wavelengths, it will travel greater distances with less diffraction. But the ordinary sound (audible sound) can't travel long distances without diffraction.
•  Also, the ultrasonic waves can't be heard by humans so there will not be any effect of noise also. 

• The SONAR techniques consist of a transmitter and a receiver. The transmitter produces ultrasonic waves of Ultrasound. So option 3 is correct.
• Infrasonic Waves are the longitudinal waves whose frequency is below 20 kHz. Example: Earthquake.
• Electromagnetic waves can't be produced by SONAR.

The correct answer is option 4) i.e. λ_X < λ_V < λ_M

CONCEPT:

• Electromagnetic spectrum: It is the arrangement of electromagnetic radiations according to their frequency or wavelength.
  • ​The electromagnetic spectrum has a range of frequencies, wavelengths, and photon energies.
  • Typically, electromagnetic waves travel at the speed of light in a vacuum. 
  • The electromagnetic spectrum is as follows:

EXPLANATION:

From the electromagnetic spectrum, it is inferred that the wavelength of X-rays is lesser than that of microwaves and visible light.

Hence, the correct order is λ_X < λ_V < λ_M.

The correct answer is option 2) i.e. The weight of the luggage is perpendicular to the distance moved

CONCEPT:

• Work is said to be done by an object when a force acting on it causes the object to displace.

Mathematically it is denoted by:

W = F.x

Where F is the force acting on the object and x is the displacement caused.

• Work depends on two factors: Force and displacement.
  • Positive work: If the force applied and displacement are in the same direction, work done is said to be positive.
  • Negative work: If the displacement is in a direction opposite to the direction of applied force, work done is said to be negative.
  • Zero work: If the direction of displacement and force are mutually perpendicular, work done is said to be zero.

EXPLANATION:

• In the given case, the force acting on the luggage is its weight i.e. acting downwards due to gravity.
• As the porter moves from one end of the platform to the other end, the distance moved is perpendicular to the direction of the force.

Thus, the work done on the luggage will be zero as the weight of the luggage is perpendicular to the distance moved.

Ans: Option: 1

CONCEPT:

Coulomb's law: This law states that the electrostatic force of attraction between two charges is directly proportional to the product of their magnitude and inversely proportional to the square of the distance between them.

CONCEPT:

• Voltage in AC: In an AC voltage source, the voltage of the source keeps changing with time and is defined as:

V = V₀ sin ωt

where V is the voltage at any time t, V0 is the max value of voltage, and ω is the angular frequency.​​

CALCULATION:

Given that Z = 20Ω and 

V = 100 sin 10t 

compare it with the standard equation. V = V₀ sin ωt

Peak voltage V₀ = 100 Volt

ω = 10

Peak Current = Peak voltage / Z

Peak Current = 100 / 20 = 5 A

So the correct answer is option 2.

CONCEPT:

• Newton’s Second law of motion: The rate of change of momentum of any object is directly proportional to the applied force on the body.

Where Δ P = Change in momentum and Δ t = change in time taken

⇒ Δ_P = P₂ – P₁

Where P₂ = final momentum and P1 = initial momentum of the system

• So change in momentum is

⇒ ΔP = Force (F) × time taken (t)

• Impulse: When a large force works on a body for a very small time interval, it is called impulsive force.
  • The impulse caused by a force during a specific time interval is equal to the body's change of momentum during that time interval.
  • Impulse, effectively, is a measure of the change in momentum. It is denoted by J or I.

J or I = Δp = F × Δt = Δ (m v) = m (Δ v)

Where Δp = change in momentum, F = force, and Δt = change in time

CALCULATION:

Given that:

Time taken (Δt) = 0.1 sec

Mass (m) = 150 g = 0.15 kg

Initial speed (u) = 20 m/s

Final velocity (v) = 0 (since after catching, the ball will come in rest)

Change in momentum (Δp) = m Δ v = 0.15 × (Final velocity - initial velocity) = 0.15 × (0 - 20) = - 3 kg m/s

Since impulse (J) = change in momentum (Δp) = force (F) × time taken (Δ t) 

Force (F) = J/Δ t = 3/0.1 = 30 N.

So option 2 is correct.