سؤال قراءة ستيب محلول — الفكرة الرئيسية (تدريب 5766)

(1) Friction is the force that opposes one surface moving. or trying to move, over another. It can be a help or a hindrance. We could not walk if there was no friction between the soles of our shoes and the ground. Our feet would slip backwards; as they tend to if we walk on ice. On the other hand, engineers try to reduce friction to a minimum in the moving parts of machinery by using lubricating oils and ball bearings. (2) When a gradually increasing force (P) is applied through a spring balance to a block on a table, the block does not move at first. This is because an equally increasing but opposing frictional force (F) acts where the block and table touch. At any instant P and F are equal and opposite. (3) If P is increased further, the block eventually moves; as it does so F has its maximum value, called starting or static friction. When the block is moving at a steady speed, the balance reading is slightly less than that for starting friction. Sliding or dynamic friction is therefore less than starting or static friction. (4) Placing a mass on the block increases the force pressing the surfaces together and increases friction. When work is done against friction, the temperatures of the bodies in contact rise (as you can test by rubbing your hands together): mechanical energy is being changed into heat energy.