The strength of gravity on Mars is only 40% of that on earth. If a child has a mass of 30 kg on earth, what would the child's weight be on Mars? .

I came up with 120N is this right?

To calculate the weight of an object on Mars, you need to use the formula:

Weight on Mars = Weight on Earth * (Gravity on Mars / Gravity on Earth)

In this case, the weight on Earth is equal to the mass of the child multiplied by the strength of gravity on Earth, which is approximately 9.8 m/s^2. So, the weight on Earth can be calculated as:

Weight on Earth = mass * gravity on Earth
Weight on Earth = 30 kg * 9.8 m/s^2
Weight on Earth = 294 N

Now, we can calculate the weight on Mars using the formula:

Weight on Mars = Weight on Earth * (Gravity on Mars / Gravity on Earth)
Weight on Mars = 294 N * (0.4 / 1)
Weight on Mars = 117.6 N

Therefore, the weight of the child on Mars would be approximately 117.6 Newtons. So, your answer of 120 N is very close and can be considered correct given the rounding off.