|
VISUAL PHYSICS ONLINE 2
DYNAMICS P20 005 A
Newton’s Cradle consists of 7 steel balls of equal mass suspended from
a support.
The ball of the
left is pulled to the side and then released.
The moving ball
strikes the other 6 balls with a speed 3v and
undergoes an elastic collision. The following two diagrams show what may
happen. Use the principles of momentum and energy to decide which diagram is
correct and why the event shown in the other diagram will never occur in
practice.
View solution below only after you have completed the answering the question. |
|
Solution The 7 balls can be considered as the System. It is an
isolated system since zero external forces act on the7 balls. The collisions
between the balls are elastic. Therefore, both momentum Let the mass of each ball be m. Event #1 is the initial collision of the moving at the left-hand side of the Cradle. Event #2 is the motion of the balls or ball on the right after all the collisions. Event #1
Initial momentum of System
Initial
kinetic energy of System Event #2 Diagram (A)
Final momentum of System
Final kinetic energy of System Event #2 Diagram (B)
Final momentum of System
Final kinetic energy of System Momentum is conserved in both Events described by diagrams (A) and (B). However, the kinetic energy is not conserved in Event #2 shown in diagram (B). Diagram (A) is correct. Diagram (B) is incorrect because kinetic energy is not conserved in the collisions. |
