Gravity is the curvature of the universe caused by objects with mass.
The more mass an object has, the more it curves space-time. This is why planets like earth can attract things towards them.
Isaac Newton - figured out how gravity works. Objects "fall down" towards the earth at 9.8 m/s².

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​Gravity

Gravity is not the same all over the earth. There are variances in the effect of gravity on the earth's surface. Also, as an object moves away from the earth, it is affected less by gravity.

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Earth's Gravity

It is the resistance between two surfaces that occur when they contact each other.
Kinds of Friction
Static - the strongest kind of friction. Generated by two motionless objects.
Sliding - is generated by two objects in motion while in contact with each other.
Rolling - is generated by a rolling object and the surface it's rolling on.

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Friction

Friction prevents slipping.
Friction slows down objects.
Friction opposes the motion of objects.
Friction and heat are the enemies of machines. To reduce friction:
Apply lubricants
Install rollers or bearings
Streamline the design of objects to reduce air resistance

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Effects of Friction