1. Copy and complete the sentences.
(a) When a bicycle moves through the _______________, a force of friction acts in the opposite _____________.
This force ______________ the bicycle down.
When a canoe moves through the water, a force of friction acts in the _______ direction.
This ___________ the canoe down.
(b) An ice rink is almost _________________ free. If there was no friction at all, a curling stone would carry on moving at ____________ speed in a ______________ line. It would _____________ stop.
2. What is the resultant force on the sailboard when it is traveling at constant speed?
3. What force balances the air resistance on the moving jet?
4. What is the resultant force on the jet?
5. When was the space probe Voyager 1 launched?
6. How fast is Voyager traveling away from the Sun?
7. Voyager has no engine, so why does it keep moving?
8. Explain why Voyager is now moving in an almost straight line.
Tuesday, October 9, 2007
Sunday, October 7, 2007
6-Moving at constant speed
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.
Friday, October 5, 2007
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