Mass & Gravity Class Exercises Help

I’m working on a few physics problems to get ready for my exam. But I don’t understand how exactly gravity & friction works. Can some one please explain these 2 things and help me with the following questions?

1. You were to carry the bag of apples to the surface of the moon, where the gravitational field is only 1.67 N/kg, how much would the apples weigh? (Do you just replace g? I’m so confused, why does apple weight different on the moon? I thought mass is supposed to be uniform. )
2. On a trip to Mars, you pick up an alien life form that weighs 90 N there. When you bring it back to Earth, you find that it weighs 238 N on Earth. How much mass does the alien have? How strong is the gravitational field on the surface of Mars

3. After creating a 50 kg ceramic sculpture, Blake needs to drag it across the concrete floor to the corner of the room. He finds that he needs to pull with more than 220 N of force to get it to slide. What is the coefficient of friction between the ceramic and concrete?

4. band of Viking warriors drags their 1500 kg boat across the beach to set out to sea. They find that they have to push with more than 8800 N of force to slide it across the beach. What is the coefficient of friction between the boat and the sand?