Laws of Motion

Q: Rocket can move in air free space , but Jet planes can not . Why ?

Ans: As you know that rocket is self contained , as both the fuel and the oxygen needed to burn the fuel are available inside the rocket . Therefore , a rocket can move in air free space . A jet plane has fuel only . It needs to take oxygen from the atmosphere to burn the fuel . That is why a jet plane cannot operate in air free space .

Q: A bullet fired from a gun is more dangerous than an air molecule hitting a person , though both of them have almost same speed . Why ?

Ans: As we know that mass of bullet is much greater than the mass of an air molecule . Due to greater mass of bullet and almost same speed of air molecule , the linear momentum of bullet is much larger than the linear momentum of an air molecule . Hence force required to stop a bullet will be much larger than the force required to stop an air molecule . That is why a bullet fired from a gun is more dangerous than an air molecule .

Q: A man is raising himself and the crate on which he stands with an acceleration of 5 ms^-2 by a massless rope-and-pulley arrangement. Mass of the man is 100 kg and that of the crate is 50 kg. If g = 10 ms^-2, then the tension in the rope is

(a) 2250 N

(b) 1125 N

(c) 750 N

(d) 375 N

Q: A massive rod of length 1 m placed straight on a smooth horizontal surface is pulled longitudinally by a force F of 10 N as shown in the figure. The tension in rod varies as T = 10√x. The linear mass density of rod is

(a) λ ∝ x

(b) $\lambda \propto \frac{1}{\sqrt{x}}$

(c)  λ ∝ √x

(d) λ ∝ x²

Q: A uniform rope of length 20 m and mass 20 kg is shown in the hanging part on each side is 6 m. C is the middle point of ABC part of rape. Find the tension at point C. Assume that the rope is in equilibrium .

(a) 20 N

(b) 20√5 N

(c) 60 N

(d) 40 N