Topic 2 Mechanics

Question 1

A student is travelling forwards at constant speed on a skateboard.

The student jumps forward off the skateboard.

Explain how Newton’s laws of motion apply to the student and the skateboard as he jumps forward off the moving skateboard. (6)

Answer 1

-The student applies a force to the skateboard.
-By Newtons 3rd law, the skateboard also applies a force to the student.
-The two forces have an equal magnitude but act in the opposite direction.
-The student accelerates due to Newtons 2nd law.
-The skateboard decelerates due to Newtons 2nd law.
-The skateboard has a higher magnitude of acceleration than the student as it has a smaller mass.

Question 2

Two small identical solid metal spheres, A and B, are suspended by light inextensible threads from a frame.

Sphere A is pulled to one side and released. Sphere A collides with sphere B and stops and sphere B swings upwards.

Using Newton’s laws of motion, explain the motion of the spheres during the collision in terms of the forces acting on them. (6)

Answer 2

-Sphere A applies a force to Sphere B.
-According to Newtons 3rd law, Sphere B will apply an equal and opposite force to sphere A.
-This force opposes the motion of Sphere A.
-Sphere A decelerates according to Newtons 2nd law.
-The resultant force on Sphere B accelerates Sphere B.
-The forces/impulses acting on the spheres are equal so change in speed/momentum are same for each sphere.