Materials A-level physics

Question

1. A rubber cube of volume 0.8cm3

falls vertically through 6m onto a concrete floor. The cube

experiences a maximum strain of 1.11% and remains in contact with the ground for 0.8s

before rebounding vertically upwards. Given that the cube impacted with the ground such

that one face of the cube was in complete contact with the ground, find the maximum EPE

stored in the cube at maximum strain.

Density of the cube = 0.9kgm-3

A student suggests that the Ek of the cube on impact should be equal to the maximum EPE

stored. Justify whether you agree with the student.

2. A child repeatedly stretches and releases an elastic band. After a short while, the child

realises that the elastic band has warmed up. By drawing a force extension graph, explain

why the child felt the elastic band warming up.

3. Consider the bumper of a car. The material can be made from either Plastic with a stiffness

constant of 1.2, or an alloy with a stiffness constant of 0.9. Explain which material would

have the least impact on the passengers in a car.

Answer 1

1. A part of EK will be spent into deformation of the ground and heating of the rubber; hence, EPE<EK

2. A part of elastic potential energy will be spent into internal energy of the rubber; that is the reason of warming up.

3. The less stiffness constant, the greater impact time, and the smaller the force of impact; of course, for the same change of momentum ∆MV.