Question

The distance travelled by an object thrown upwards in t seconds is 30t − 4.9t² metres. After how much time would it fall down? At what all times would it be 20 metres above the ground?

Answer 1

Distance travelled by an object thrown upwards in t seconds = (30t − 4.9t²) m

When the thrown object comes back to the ground then the displacement becomes zero.

∴ 30t − 4.9t² = 0

⇒ 4.9t² − 30t = 0

Comparing the above equation with general quadratic equation:

a = 4.9, b = −30 and c = 0

The time taken by the object to fall down = 6.12 seconds

If the distance is 20 m, then 30t − 4.9t² = 20.

⇒ − 4.9t² + 30t − 20 = 0

⇒ 4.9t² − 30t + 20 = 0

Comparing the above equation with general quadratic equation:

a = 4.9, b = −30 and c = 20

Thus, the object would be 20 m above the ground after 0.76 seconds and 5.36 seconds.