# NCERT Physics Class 11 Exemplar Ch 5 Laws of Motion Part 8 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Q. 38. The displacement vector of a particle of mass m is given by

- Show that the trajectory is an ellipse.
- Show that

Answer:

Q. 39. A cricket bowler releases the ball in two different ways

(a) giving it only horizontal velocity, and

Answer:

Speed at ground =

(b) giving it horizontal velocity and a small downward velocity. The speed vs at the time of release is the same. Both are released at a height H from the ground. Which one will have greater speed when the ball hits the ground? Neglect air resistance.

Answer: also is the total energy of the ball when it hits the ground.

So the speed would be the same for both (a) and (b) .

Q. 40. There are four forces acting at a point P produced by strings as shown in Fig. which is at rest. Find the forces

Answer:

Q. 41. A rectangular box lies on a rough inclined surface. The co-efficient of friction between the surface and the box is μ . Let the mass of the box be m.

(a) At what angle of inclination θ of the plane to the horizontal will the box just start to slide down the plane?

Answer:

(b) What is the force acting on the box down the plane, if the angle of inclination of the plane is increased to α

Answer:

- What is the force needed to be applied upwards along the plane to make the box either remain stationary or just move up with uniform speed?
- Answer: is the force needed to be applied upwards along the plane to make the box move up the plane with acceleration a?

Answer:

Q. 42. A helicopter of mass rises with a vertical acceleration of The total mass of the crew and passengers is . Give the magnitude and direction of the

(a) force on the floor of the helicopter by the crew and passengers.

Answer: , where F is the

upward reaction of the floor and is equal to the force downwards on the floor, by Newton՚s 3^{rd} law of motion

(b) action of the rotor of the helicopter on the surrounding air.

Answer: , action of the air on the system, upwards. The action of the rotor on the surrounding air is downwards.

(c) force on the helicopter due to the surrounding air.

Answer: Force on the helicopter due to the air upwards.