What is a gravitational wave and why was it so hard to detect?

gravitational waves are ripples in spece-time caused primarily when objects are accelerated and the energy for the acceleration is transpoted as gravitational radiation.

they are difficult to detect because they require very sensitive technology or you will have to wait unitl black holes collide.

less than one you can't create energy

answer: 2×10^-2kg

explanation: density= mass per volume

vol=mass/density

converting to s.i units we have;

3÷1000kg = 0.003

1.5÷100m = 0.15

0.003/0.15 = 0.02kg