12 Grade challenge/physics

PHYSICS Sample Q&A

Q1. A race car is moving at a constant speed of 35m/s. A security car was moving at a speed of 5m/s as the race car passes by it and was accelerating at constant rate of 5 m/s². What was the speed of the security car when it took over the race car ?

To find the speed of the security car when it overtakes the race car, we can set up equations for the motion of both cars.

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Q2. Which one of the following statements is correct regarding the motion in a plane?

A. When a body moves in a horizontal circle, its velocity is constant

B. When a body moves in a vertical circle, its speed is constant

C. The centripetal force for a body moving in either vertical or horizontal circle is toward the center

D. In projectile motion the horizontal component of the motion is uniformly accelerated motion.

The correct statement regarding the motion in a plane is:

C. The centripetal force for a body moving in either vertical or horizontal circle is toward the center.

Explanation:

• In circular motion, centripetal force is always directed towards the center of the circle. It is responsible for keeping the body moving in a curved path.
• Option A is incorrect because in circular motion, the direction of velocity changes continuously, even if the speed remains constant.
• Option B is incorrect because when a body moves in a vertical circle, its speed varies due to the influence of gravity.
• Option D is incorrect because in projectile motion, the horizontal component of motion is uniform, not uniformly accelerated. The vertical component experiences acceleration due to gravity.

Q3. Which of the following statement is correct about errors in measurement?

A. Experimental error is the same as mistake

B. Errors make measured values same as accepted value

C. Errors occur in every scientific investigation

D. Errors can be avoid by using modern measurement instruments.

The correct statement about errors in measurement is:

C. Errors occur in every scientific investigation.

Explanation:

• Option A is incorrect because experimental error refers to the difference between a measured value and the true value, while a mistake refers to an error resulting from human action or judgment.
• Option B is incorrect because errors can cause measured values to deviate from the accepted or true value.
• Option D is incorrect because while modern measurement instruments may reduce some types of errors, they cannot completely eliminate errors. Proper calibration, handling, and understanding of limitations are crucial despite the use of advanced instruments.

Q4.  What can be concluding when the number of significant figures increases?

A. True reading increases

B. Precision increases

C. Uncertainity increases

D. Accuracy increases

When the number of significant figures increases:

B. Precision increases.

Explanation:

• Precision refers to the level of detail in a measurement, often indicated by the number of significant figures. An increase in the number of significant figures implies a more precise measurement, as it provides more detailed and accurate information about the quantity being measured.
• Option A is incorrect because the true reading of a measurement does not necessarily increase with the number of significant figures, as it depends on the accuracy of the measuring instrument.
• Option C is incorrect because increasing the number of significant figures generally reduces uncertainty by providing more specific and accurate data.
• Option D is incorrect because while precision may increase with more significant figures, accuracy is determined by the proximity of the measurement to the true value, which can be affected by systematic errors.

Q5. A body that is released from the top of a tall building reaches the ground after six seconds. What is the ratio of the kinetic energy acquired by it at the end of the first three seconds to the increment in the kinetic energy during the next three seconds?

A. 3/2

B. 1

C. 1/3

D. 2

The problem can be solved by analyzing the motion of the body.

The time of descent is split into two parts: the first three seconds and the next three seconds. During free fall, neglecting air resistance, the body’s

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Q6. A thin rod when rotated about an axis at the end of the rod has a moment of inertia 1/3ML². The rod has a mass of 1.5kg and length of 1.0 m. if the rod initially at rest gains a rotational kinetic energy of 1000.0 J, at what speed does it rotate?

10 rod/s

18.8 rod/s

The rotational kinetic energy of the thin rod is given by the formula:

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Q7. If in a given rotational system of a body the angular momentum is increased with time, which one of the following statements is not correct?

A. The radius of rotation may increase

B. There is no torque exerted on the system

C. The moment of inertia may increase

D. There is a torque exerted on the system

The correct answer is:

B. There is no torque exerted on the system.

Explanation:

• When the angular momentum of a system is increased with time, it implies that there is a torque acting on the system, causing the angular momentum to change. This makes option B incorrect.
• Option A is correct because the radius of rotation may increase, leading to a change in angular momentum.
• Option C is correct because if the mass distribution of the system changes or if the shape of the rotating body changes, the moment of inertia may increase, affecting the angular momentum.
• Option D is correct because the change in angular momentum indicates the presence of a torque that causes the rotation to change.

Q8. Which of the following is a correct statement about the first condition of equilibrium?

A. The sum of all torques must be zero

B. Clockwise moments should balance counter clockwise moments

C. The net force on an object must be zero

D. The torque produced by a force should be perpendicular to the force.

The correct statement about the first condition of equilibrium is:

A. The sum of all torques must be zero.

Explanation:

• The first condition of equilibrium states that the sum of all torques acting on an object must be zero for the object to be in rotational equilibrium.
• Option B refers to the concept of balancing moments, but it doesn’t encompass all cases of equilibrium.
• Option C refers to the first condition of translational equilibrium, not rotational equilibrium.
• Option D is a definition of torque and its relationship with force but is not directly related to the first condition of equilibrium.

Q9. For a constant force F, if the impulse imparted to the system is large, then what can you conclude about the collision?

A. Linear momentum is conserved

B. The contact or impact time is large

C. Kinetic energy is conserved

D. The change in velocity is small

The correct option is:

A. Linear momentum is conserved.

Explanation:

• Impulse is defined as the change in momentum of an object when a force is applied for a certain amount of time. A large impulse implies a large change in momentum.
• Option A is correct because a large impulse indicates that linear momentum is conserved, meaning that the total momentum before the collision is equal to the total momentum after the collision.
• Option B is incorrect because a large impulse doesn’t necessarily imply a large contact or impact time.
• Option C is incorrect because even if the impulse is large, it doesn’t necessarily mean that kinetic energy is conserved. In most collisions, kinetic energy is not conserved due to the conversion of kinetic energy into other forms of energy, such as heat or sound.
• Option D is also incorrect because a large impulse generally implies a significant change in velocity, not a small change.

Q10. A satellite moves at a constant speed in a circular orbit about the center of the earth at an altitude half the radius of the earth above its surface. If g is the gravitational acceleration at the surface of the earth and R its radius, then what is the speed of the satellite?

To find the speed of the satellite, we can apply the concept of centripetal force in circular motion.

The centripetal force required to keep the satellite in circular motion is provided by the gravitational force between the satellite and the Earth.

The centripetal force is given by:

Q11.An object of mass 1kg moving with a speed of 2m/s was acted on by a force that produces an acceleration of 2m/s². What is the change in kinetic energy of the object if the force is exerted on the object for 2seconds in its direction of motion?

A. 1 J

B. 16 J

C. 8 J

D. 4 J

None of the above

To find the change in kinetic energy, we can use the equation:

Change in kinetic energy=Work done by the forceChange in kinetic energy=Work done by

 the force

The work done by the force is given by the formula:

Q12. An electric immersion water heater is rated at 400w. how long will it take to heat one kilogram of water from 10 to 30 ? (the specific heat of water is 4.2 J/g k.)

A. 45 min

B. 1 min

C. 15 min

D. 3.5 min

To find the time it takes to heat the water, we can use the formula:

Energy=Power×TimeEnergy=Power×Time

Given that the power of the heater is 400 W, we need to convert this to Joules per second (J/s).

1 watt = 1 joule/second

So, the power is 400 J/s.

The energy needed to heat the water can be calculated using the formula:

Energy=mass×specific heat×change in temperatureEnergy=mass×specific heat×change in temperature

The specific heat of water is 4.2 J/g°C. The mass of water is 1 kg, and the change in temperature is 30°C – 10°C = 20°C.

Let’s plug in the values:

400×Time=1×4.2×20400×Time=1×4.2×20

400×Time=84400×Time=84

Time=84400Time=40084

Time=0.21 secondsTime=0.21seconds

As the time is in seconds, we need to convert it to minutes:

0.21 seconds=0.21/60 minutes=0.0035 minutes0.21seconds=0.21/60minutes=0.0035minutes

So, the time it takes to heat one kilogram of water from 10°C to 30°C with the 400 W heater is 0.0035 minutes, which is approximately 3.5 seconds. None of the provided options match the calculated time.

Q13. A beaker with water resting on a scale weighs 40 N. a block suspended on a hanging spring weighs 20 N. the spring scale reads 15 N when a block is fully submerged in the water. What is the reading of a scale on which the beaker with water rests, while the block is submerged in the water after detached from the hanging spring?

A. 25 N

B. 60 N

C. 55 N

D. 45 N

Let’s consider the forces acting on the block when it’s fully submerged in the water.

The weight of the block is 20 N, and the apparent weight of the block when fully submerged in water is the difference between the weight in air and the upthrust, which is 15 N. This means the upthrust on the block is 20 N – 15 N = 5 N.

The upthrust is caused by the buoyant force, which is equal to the weight of the water displaced by the block.

Using Archimedes’ principle, we can deduce that the weight of the water displaced is equal to the upthrust, which is 5 N.

Now, the beaker with the water is resting on a scale and weighs 40 N. When the block is fully submerged, the scale will register the weight of the beaker, the water, and the additional weight of the water displaced by the block.

The weight of the water displaced is 5 N, and the weight of the block in the water is 15 N. Therefore, the reading on the scale would be:

40 N+5 N+15 N=60 N40N+5N+15N=60N

So, the correct answer is B. 60 N.

Q14. An object with a mass of 150 kg and a volume of 0.75 m³ is floating in a liquid of density 0.8 g/cm³. What percentage of the objects volume will be submerged below the surface of the fluid?

A. 100 %

B. 50 %

C. 75 %

D. 25 %

First, let’s convert the density of the liquid from grams per cubic centimeter (g/cm³) to kilograms per cubic meter (kg/m³) as follows:

Density of the liquid = 0.8 g/cm³ × 10³ kg/g = 800 kg/m³

We have the mass and volume of the object, and we know that when an object floats, it displaces its weight in the fluid.

The weight of the liquid displaced is given by the formula:

Weight of displaced liquid = Volume of the submerged part × Density of the liquid × gravitational acceleration

The weight of the object is given by:

Weight of the object = Mass of the object × gravitational acceleration

For the floating object, the weight of the object is equal to the weight of the displaced liquid:

Mass of the object × gravitational acceleration = Volume of the submerged part × Density of the liquid × gravitational acceleration

Since the gravitational acceleration cancels out, we have:

Mass of the object = Volume of the submerged part × Density of the liquid

Solve for the volume of the submerged part:

Volume of the submerged part = Mass of the object / Density of the liquid = 150 kg / 800 kg/m³ = 0.1875 m³

The percentage of the object’s volume submerged below the surface of the fluid is:

(0.1875 m³ / 0.75 m³) × 100% = 25%

So, the correct answer is D. 25%.

Good Luck!

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