While riding a bicycle,if we stop pedalling,why does the bicycle begin to slow down?

$A$ car of mass $1000 \, kg$ moving with a velocity of $15 \, m/s$ collides with a tree and comes to rest in $5 \, s$. What will be the force exerted by the car on the tree?

$A$ truck starts from rest and rolls down a hill with constant acceleration. It travels a distance of $400 \, m$ in $20 \, s$. Find its acceleration. Find the force acting on it, if its mass is $7$ metric tons.

Calculate the ratio of momentum,when
$(i)$ velocity of an object is doubled.
$(ii)$ mass of an object is halved.
$(iii)$ both mass and velocity are increased by three times.

How would you arrive at a mathematical formula to measure force using the second law of motion?

State the reason for the following:
$(i)$ Some leaves may get detached from a tree if we vigorously shake its branch.
$(ii)$ It is advised to tie any luggage kept on the roof of a bus with a rope.