Why do things slow down?
If you stop pedaling your bicycle, air resistance and other friction forces slow you down. Friction forces always act in the opposite direction to movement. A similar thing happens if you stop paddling a canoe-the main friction force on a canoe is water resistance.
Keeping going
When there is friction, you need another force to keep going at constant speed.
When a sailboard is moving, water resistance acts against it. To move at a steady speed, the force of the wind on the sail must balance the friction force.
These examples show that if the resultant force acting on a moving body is zero, the body continues to move at the same speed and in the same direction.
Life without friction
Without friction, a cyclist could freewheel forever in a straight line. She would not need to pedal to keep going at a constant speed.
You can get some idea of what life would be like without friction at an ice rink.
Motion in space
Space is an almost perfect vacuum. There is no air to cause friction. In deep space, where the gravity of stars and planets is negligible, a spacecraft will travel in a straight line at constant speed.
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