NCERT Physics Class 11 Exemplar Ch 5 Laws Of Motion Part 8

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

  1. Show that the trajectory is an ellipse.

  2. Show that


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

(a) giving it only horizontal velocity, and


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




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?


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


  1. 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?

  2. Answer: is the force needed to be applied upwards along the plane to make the box move up the plane with acceleration a?


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 3rd 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.

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