<span>You should stop no closer than 15 feet from a railroad crossing if a train is approaching.
</span>
Answer:
A principle of material science which expect that bunches of particles delivered by development of electrons adjust themselves in bunches (called "spaces") in attractive materials. The heading of attractive field at a point is the heading to which a free north shaft would move in the event that set at that point within the field.
-- The total distance between the towns = 460 km.
-- For the first 3 hours, he travels at an average speed of 60 km/hr. During that time, he covers (60 km/hr) x (3 hrs) = 180 km.
-- At that point, he still has (460km - 180km) = 280 km to go.
-- If he drives that 280km at an average speed of 70 km/hr, it'll take him (280km)/(70km/hr) = 4 hrs to cover it.
-- His total time for the whole trip is (3 hours for the first part) + (4 hours for the last part) = <em>7 hours</em>.
Because alpha particles are positive charged. And they bounce back because of the electromagnetic force between them and the gold foils protons.