It is given that by using track and cart we can record the time and the distance travelled and also the speed of the cart can be recorded. With all this data we can solve questions on the laws of motion.
Like using the first law of motion we can determine the force of gravity acting on the cart that has moved a certain distance and the velocity or the speed of card has already been registered and since time is known putting the values in formula would help us calculate the gravitational pull acting on cart.
The average speed of the football is 15 meters per second. Just divide both of the numbers by 4 :)
Each 
ion contains three extra protons. Hence, the extra charge on each = 
C
Total charge = 0.035 pC
Total charge (Q) = 
C
Let the number of ions be n.
According to question:
n = 72917
Hence, the total number of ions needed to be transferred is 72917
The answer is D
Hope this helps
A wave will "break" because the bottom interferes with its oscillatory motion. Breaking of waves may occur anywhere that the amplitude is sufficient, including in mid-ocean. When waves enter shallow water they break because the motion of water in lower part of the wave nearest the bottom is slowed by friction so that their oscillation is faster than its supporting portion at the bottom. Thus, the wave collapses forward and breaks.
