Phys SAQ

Question 1

How does drawing a line o fbest fit help reduce random error?

Answer 1

The beset fit line balances the deviations of data points on either side of the line, reducing the effects of random errors

Question 2

State 2 ways in which an equation is dimensionally consistent but physically incorrect.

Answer 2

1. The equations may have missing terms or extra terms.
2. The equations may have incorrect coefficients.

Question 3

Why is the path of a mass or charged particle in a uniform electric field or gravitational field the same?

Answer 3

Both particles experience a uniform speed in one direction and uniform force and uniform acceleration in a perpendicular direction. They move in a parabolic path.

Question 4

What are the conditions required for kinematics equations to be valid?

Answer 4

Acceleration must be constant, and the motion must be in a straight line.

Question 5

Why, in the presence of air resistance, the time taken to reach the maximum height is shorter than the time taken to return to the original position?

Answer 5

When the ball is on its way up, air resistance acts downwards, opposing the motion of the object. The resultant force acting downwards is the sum of the air resistance and the weight of the object. On the downward journey, air resistance acts upwards, opposing the motion of the object. The resulstant force acting on the object downwards is its weight minus air resistance. Thus, the acceleration downwards is smaller in magnitude than the deceleratioin in the upward journey. Since both journeys cover the same distance, the time taken for the upward journey is shorter.

Question 6

Explain what the acceleration of a ball at its maximum height is when an object is thrown vertically upwards?

Answer 6

At the maximum height, the obkect is instantaneously at rest, and there will not be any drag force acting on the object. Hence, the only force acting on the object is its weight, and by N2L, ma=mg, thus the acceleration of the ball at its maximum height is 9.81ms-2 downwards

Question 7

Explain why a falling object eventually reaches constant velocity in the presence of air resistance.

Answer 7

When an object falls, there is air resistance acting on the object upwards. Since air resistance increases with speed, as the object accelerates downwards due to gravitity, the air resistance increases, and ma=mg - air resistance. As air resistance increases, the acceleration of the object decreases until the acceleration is zero where the magnitude of the weight of the object equals to the air resistance. This is when the object is said to be at terminal velocity.

Question 8

Conditions for a parabolic motion in absence of air resistance.

Answer 8

1. Horizontal component of velocity is always constant.
2. Vertically, the vertical acceleration is always constant and downwards, with magnitude equal to 9,81ms-2

Question 9

If air resistance is not negligible, how does velocity vary in a projectile motion?

Answer 9

Horizontal component of velocity will decrease to 0.
Vertical velocity decreases at a decreasing rate in the upward journey. It increases at a decreasing rate in the downward journey. If the building is tall enough, the object will eventually reach termnal velocity when the weight of the object is equal to the magnitude of air resistance acting on the object.

Question 10

Why is the path of a ball thrown at an angle different when there is air resistance and without air resistance?

Answer 10

With air resistance, some loss in KE is converted to work done against air resistance. Hence max height is smaller. In the horizontal direction, the horizontal velocity is no longer constant due to work done against air resistance, hence the range is smaller.

Question 11

What does it mean by an inelastic collision between 2 objects with reference to speed?

Answer 11

The relative speed of approach is more than the relative speed of separation.

Question 12

What does it mean by an elastic collision between 2 objects by reference to energy?

Answer 12

The final total kinetic energy after the collision is lower than the total initial kinetic energy before the collision.

Question 13

State if the 2 spheres can be stationary at the same time if the total initial momentum is non-zero

Answer 13

Since the total initial momentum is non-zero, by principle of conservation of momentum, at any point in time, the total momentum of the system cannot be zero. However, if both spheres are simultaneously stationary, the total momentum of the system will be zero which violates the principle of conservation of momentum.

Question 14

By reference to the direction of impulse, explain why for the spheres to move horizontally after collision, the centres of the spheres must be on the same horizontal level.

Answer 14

The direction of the impulses on each sphere are rirghtwards and leftwards horizontally. For the final veloticies to be horizontal, the spheres must undergo a head-on collision in which their centres of mass are collinear.

Question 15

Does the principle of conservation of momentum apply in cases where 2 colliding bodies lose kinetic energy as a result of sticking to one another at the point of collision? Explain your answer with reference to Newton’s third law.

Answer 15

Yes. According to Newton’s Third Law, the two bodies exert equal and opposite forces on each other. Hence the impulse of body A will be equal but opposite to that of body B. Taken as a whole system, the total momentum of the colliding bodies remains unchanged, and the principle applies.

Question 16

State if 2 spheres can be stationary at the same time if their total initial momentum is zero but their total kinetic energy is not, when they undergo an elastic collision.

Answer 16

Since the total initial momentum is zero, by the principle of conservation of momentum, total momentum of the system during collision can be zero. At the instant of collision when both objects undergo. maximum compression, their velocities will be the same and zero. Kinetic energy for an elastic collision is only conserved before and after a collision, not during, as some of the kinetic energy gets converted to elastic potential energy to compress the objects momentarily.

Question 17

An astronaut in a spacecraft orbiting the earth may be described as weightless. Explain why this is so.

Answer 17

The astronaut and spacecraft are both accelerating at the same value towards the Earth as they are both undergoing circular motion. Gravitational force acting on the astronaut just sufficiently provides for the centripetal force on the astronaut. Therefore, no mnormal contact force is experienced by the atronaut and is experiencing weightlessness.

Question 18

Discuss how seat belts and airbags in a car ensure greater safety.

Answer 18

Seat belts and airbags increase the time for which the same change in momentum of the person occurs, thus reducing the decelerating force acting on him. Air bags also reduce the pressure on the driver as a result of the larger area over which the force acts

Question 19

A stone is dropped from a point a few meters above the Earth’s surface. Considering the system of the stone and the Earth, discuss briefly how the principle of conservation of momentum applies before the impact of the stone with the Earth.

Answer 19

The initial total momentum of the system of stone and Earth as it is released is zero. As the stone falls and accelerates towards the Earth, discuss briefly how the principle of conservation of momentum applies before the impact of stone with the Earth.

Question 20

When a man falls from a height and undergoes free fall, his momentum increases. Explain if principle of conservation of momentum is violated.

Answer 20

If the system consists of only the man, the gravitational force on the man by the Earth will be an external force. Since there is a net external force acting on the system, momentum is not conserved. If the system consists of the man and the Earth, gravitational force on the man by the Earth and the gravitational force exerted on Earth by man are internal forces. Since there is no net external net force, momentum is conserved.

Question 21

Two strong magnets are held stationary with the north pole of one pushed against the north pole of the other. On letting go, the magnets spring apart. It is apparent that the kinetic energy of the magnets has increased. Explain how the law of conservation of momentum and law of conservation of energy apply in this case.

Answer 21

Law of conservation of momentum:
No net external force acts on the system of 2 magnets. By the principle of conservation of momentum, since initial total momentum of magnets is zeo, when released, magnets move off in opposite directions with momentum of the same magnitude such that final total momentum of the system is also zero