$A$ bullet of mass $20 \, g$ travelling horizontally with a speed of $500 \, m/s$ passes through a wooden block of mass $10.0 \, kg$ initially at rest on a surface. The bullet emerges with a speed of $100 \, m/s$ and the block slides $20 \, cm$ on the surface before coming to rest. Find the coefficient of friction between the block and the surface. $(g = 10 \, m/s^2)$

$A$ balloon of mass $M$ is descending at a constant acceleration $\alpha$. When a mass $m$ is released from the balloon,it starts rising with the same acceleration $\alpha$. Assuming that its volume does not change,what is the value of $m$?

$A$ helicopter of mass $1000 \;kg$ rises with a vertical acceleration of $15\; m s^{-2}$. The crew and the passengers weigh $300\; kg$. Give the magnitude and direction of the
$(a)$ force on the floor by the crew and passengers,
$(b)$ action of the rotor of the helicopter on the surrounding air,
$(c)$ force on the helicopter due to the surrounding air.

$A$ ball rests upon a flat piece of paper on a table top. The paper is pulled horizontally but quickly towards the right as shown. Relative to its initial position with respect to the table,the ball:
$(A)$ Remains stationary if there is no friction between the paper and the ball.
$(B)$ Moves to the left and starts rolling backwards,$i.e.$,to the left if there is friction between the paper and the ball.
$(C)$ Moves forward,$i.e.$,in the direction in which the paper is pulled.
Which of the following statements is/are correct?

Provide an example from daily life where motion is controlled or initiated by an external force.

From Newton's second law of motion,it can be inferred that