How many days have passed if 19.2 mg of calcium-47 decayed to 1.2 mg of calcium-47?

This is a problem involving half-lives. We can solve this problem by using the equation as shown: A=Aoe^-(ln2/t)T where A is the final amount (1.2milligram), Ao is the initial amount (19.2mg), t is the half-life (4.5 days) and lastly T time elapsed (required). Plug in the values in the equation and you should get  18 days. Eighteen days have passed.

i believe it is-

d.) gravity pulled particles of dust and gas together to form planets.

(newtons first law)

conservation of momentum law states :

momentum before collision = momentum after collision

momentum p (kg-m/s)= mass * velocity

say moving car=1 000 kg, velocity = 10 m/s then p = 1 000*10= 10 000 kg-m/s

say still car = 2 000 kg, velocity = 0 m/s then p = 2 000* 0 = 0 kg-m/s

total momentum prior to collision = 10 000 + 0 = 10 000 kg-m/s

momentum after collision = 10 000 kg-m/s

mass now = 1 000 kg+2 000 kg = 3 000 kg

10 000 = 3 000 kg* velocity m/s

10 000/3 000 = velocity

3.33 m/s = velocity after collision

an impulse is a force applied over time , momentum is exclusive of external forces and a perfect collision is implied