If you go from the Earth to live on the Moon you would have the same mass, but your weight would be different. This is because the force of gravity changes depending on which planet you are on. On the Earth the force of gravity is 9.81 Newtons per kg. This means that for every kg of mass you would have a force downwards (your weight) of 10 N. On the Earth:-
Force (weight) = mass x 10 for a 2 kg mass Force = 2 x 10 = 20 N
Qn. 1 Calculate the force on the Earth of an object of mass:-
i) 10 kg; ii) 200 kg; iii) 500 g; iv) 20 g; v) 3.5 kg; vi) 750 g;
NOTE The mass must be in kg.
The Force of Gravity changes on different planets. This is because the planets have different masses. The bigger the mass of the planet the bigger the Force of Gravity. It also depends on the radius of each planet, because the Force of Gravity drops with distance. The bigger the radius of the planet the smaller the Force of Gravity at the surface. To calculate the surface gravity on the other planets:-
The Surface Gravitational Force of Planet
= Mass (relative to the Earth) x (Radius of the Earth)² x 10÷ (Radius of the Planet)²
where the surface gravitational force for the Earth = 10 N/kg.
For Mars, the surface gravitational force for Mars = 0.10 x (6400)²x 10÷(3400)²
Surface gravitational force for Mars = 3.5 N/kg
On the Earth a 1 kg mass would have a weight of 10 N, but on Mars the same mass would have a weight of only 3.5 N.
Qn. 2 Using the above equation complete the table below, filling in the surface gravity values for each planet.
Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto Mass (Relative to the 0.1 0.80 1 0.10 318 95 15 17 0.02 Earth) Radius of Planet km 2450 6000 6500 3400 71500 60500 25509 24750 1150 Surface Gravity N/kg 10
Qn. 3 What is the weight of:-
i) A 20 kg mass on Mercury; ii) A 5 kg mass on Jupiter; iii) A 50 kg mass on Mars;
iv) A 350 g mass on Uranus; v) A 80 kg mass on Neptune; vi) A 3 kg mass on Pluto?
Qn. 4 An object has a weight of 351 N on Neptune. Calculate its mass and then calculate its weight on i) Pluto; ii) Jupiter; iii) Mars and iv) the Moon?
Qn. 5 A person on the Moon throws a rock towards the Earth. What happens to the rock?
Qn. 6 Write an essay saying how gravity would affect life on the Moon and Jupiter?
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