Question

what is the weight of a 70 kg body on the surface of the planet whose mass is 1/7th of that Earth and the radius is half of the earth

Answer 1

We know g = G  M /r^2

M= mass of planet, r is radius

on Earth it's about 10

on new planet g = 1/7 times ( due to mass is less) and 4 times (due to radius, remember square)

so basically 4/7 times g of Earth

g = 4/7 g

Weight = mg (m = mass of object, g is gravitational acceleration of that particular planet)

Weight on new planet = 70 * 4/7 g

=40 g

about 400 N (if taking g=10)

= 392 N (if taking g=9.8)

Answer 2

Using the formula of gravity derived from the force formula.