Q1.
Now, conduct a thought experiment. We do a thought experiment when the conditions required for the experiment are difficult to recreate in the real world. Suppose, you find an object and a horizontal floor having such smooth surfaces that the force of friction between them is zero. Imagine, what will happen if you repeat steps 3 and 4 of Activity 6.1 with such an object and a horizontal floor? Will the velocity of the object decrease? Will the object ever come to rest or continue moving forever?
Answer
The object would be set moving by the rubber band, and then it would keep moving for ever with constant velocity. Its velocity would not decrease, and it would never come to rest.
While in contact with the band: net force forwards → velocity rises from 0 to v
After losing contact: friction = 0 N, and no other horizontal force acts
→ net horizontal force = 0 N
→ a = F/m = 0 ÷ m = 0 m s–2
→ velocity stays at v for ever, in a straight line
After losing contact: friction = 0 N, and no other horizontal force acts
→ net horizontal force = 0 N
→ a = F/m = 0 ÷ m = 0 m s–2
→ velocity stays at v for ever, in a straight line
Why it happens: in the real activity it is friction, and only friction, that removes the object's velocity. Take friction away and nothing is left to change the velocity. Newton's first law then applies exactly: an object in motion continues to move with constant velocity unless a net force acts upon it.
Did you know? This is the very argument Galileo made in the 17th century, before Newton stated the law: if a body moves along a horizontal plane and all impediments to its motion are removed, it will continue to move indefinitely. A thought experiment let him reach a correct conclusion about conditions he could never create in his laboratory.