If something weighs 100 newtons on earth and it travels to a planet with 4 times the gravity of earth how much would it weigh?
By Newton's second law of motion, mass, force and acceleration are related as:
F = m x a
where, F is force acting on a body, m is its mass and a is its acceleration. When we consider the gravitational force on a body, the acceleration will be the acceleration due to gravity (g, on earth) and the force can be considered to be its weight. If on a different planet the gravity is different from that of Earth, we will find that the weight of the object is different.
Here, the weight of the body is 100 N (on Earth).
The other planet has 4 times the gravity of Earth, that is, the acceleration due to gravity on that planet is 4 times that on Earth, or g' = 4g
Thus, the planet will exert 4 times the gravitational pull on the body as compared to our planet.
Hence, F' = m x a = m x g' = m x 4g = 4 (mg) = 4 x 100 N = 400 N.
Thus, the body would weigh 400 N on the other planet (remember mass of the body will stay constant).
Hope this helps.