Two students taking a Physics class witness a small girl pul

Two students taking a Physics class witness a small girl pulling a sled behind her in the snow. She stops suddenly to tie her shoe. The sled keeps moving forward and hits her leg making the little girl cry. Student 1: When the girl stops to tie her shoe she stops pulling on the sled. But the frictional force still acts on the sled. This is the reaction force to the girl s pull force and this force makes the sled move forward and hit the girl. This is due to Newton s Third Law. Student 2: No. There is no force acting on the sled. So the net force on the sled is zero and the sled moves at the same velocity that it was moving at before the girl stopped. This is why the sled hit the girl s leg. This is due to Newton s Second Law. Which student do you agree with and why? Discuss each part of all statements made by both students and confirm its validity. It is all right to disagree with both students.

Solution

For Student 1: It is correct that the friction force is the reaction force to girl\'s action and it is correct that the friction still acting on the sledge but it will not cause it to move forward.Friction always opposes the motion.So the students claim that the sled moved forward due to newton\'s 3rd law is wrong. So student 1 claim is invalid.

For Student 2: It is wrong to say that there is no force acting on the sled - frictionl force is action on the sled.Frictional force always opposes the motion so the sled cannot travel with constant velocity untill an external force is applied.So the students claim that the sled hit the girls leg due to Newtons second law is invalid.

Thus we disagree with both the students.

Right explanation: As the girl left the sled the sled kept moving due to inertia of motion and hit her .This is Newtons 1st law of motion.