How do you calculate how dense the Earth is? I keep trying but I get 3.51*10¹⁰kg/m³. I don't know how or where I'm going wrong.

a roller coaster car filled with passengers has a mass of 4.80 kg. what speed must it have at the top of the 7.5 m tall circular loop so the car doesn't fall off?

How do you express a major axis with a length of 2.992x10¹¹m in AU?

You are asked to design a rollercoaster. It will have ten attached cars, each of which will have a mass of 500 kg and carry six people with average mass of 70.0 kg. You want to design a vertical circular loop with a radius of 55.0 m. The centre of the circle would have a height of 71.0 m above the ground.

A cyclist exerts a force of 125 N to accelerate from rest to a speed of 10.0 m/s. What is the cyclist’s power output? Consider the difference between average velocity and instantaneous velocity. 

a rabbit running at 12m/s runs past a dog who immediately gives chase. if the dog accelerates at 2.40m/s^2 and both run in a straight line, how long til the dog catches the rabbit?