$A$ sledge of mass $80 \, kg$ moves over the frozen surface of a lake with a velocity of $30 \, m s^{-1}$ and comes to rest in $5 \, s$. Find its negative acceleration and the frictional force between the lake and the sledge.

Why is it advised to tie luggage kept on the roof of a bus?

$A$ passenger in a moving train tosses a coin which falls behind him. It means that the motion of the train is

During a game of table tennis,if the ball hits a player,it does not hurt him. On the other hand,when a fast-moving cricket ball hits a spectator,it may hurt him. State the reason.

Give reasons and state the laws related to these statements:
$(a)$ It is easier to push an empty box than to push a box full of books.
$(b)$ It is difficult for a fireman to hold a hose which ejects a large amount of water with high velocity.

Explain the following:
$(i)$ Why does a boatman push the river bank backward with a long bamboo pole when launching his boat into the water?
$(ii)$ Why does a boat tend to leave the shore when passengers are alighting from it? How does the boatman overcome this difficulty?
$(iii)$ Why does a gun recoil when fired?
$(iv)$ How do rockets operate in space?