Journey Into Space update

1. The force of gravity acting on an object is a measure of the gravitational pull acting on that object. It is a force. 
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3. Mass is a measure of how difficult it is to make the object go faster. The force of gravity acting on an object can change with where it is; the mass always stays the same.  source
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5. Think about a stationary, immobile car on a steep hill.  If it's in neutral - not in gear - what happens when you release the handbrake? It rolls down the hill.
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7. No force or effort?  Does that mean the car has no mass?
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9. A car most definitely has mass. About 1 tonne of it (or more). And there is a force - that due to gravity (also known as the weight force). 
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11. On a slope a part of the gravitational (weight) force due to the Earth acts down the slope. 
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13. So gravity provides the “effort”. 
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15. A better description is “the car would roll down the hill due to gravity but the handbrake stops it (using friction between brake pads and wheel).

What happens when he steps forward?

  Useful site

16  Additional note

Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do on Earth. source

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 Be careful on your bike.