One fine summer day a group of students was jumping from a railroad bridge into the river
below. they stepped off the bridge with an acceleration (due to gravity) of 9.8 and they hit the
water 4.0 s later. how high (in meters) was the bridge?

78.4 m

Explanation:

This problem is a good example of Vertical motion, where the main equation for this situation is:  

(1)  

Where:  

is the height of the jumpers

is the initial height of the jumpers

is the initial velocity of the jumpers  

is the time at which the jumpers reach the water

is the acceleration due to gravity  

Having this clear, let's find from (1):

(2)  

(3)  

Finally:

 This is the height of the bridge

a.metals can become magnetic if they have many spinning electrons that create magnetic fields.d.only some metals are attracted by magnets; these metals are called ferromagnetic.

d.wrapping an insulated wire around a metal with ferromagnetic properties and applying an electric current.