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Bertozzi investigated how the kinetic energy of electrons varies with speed - AQA - A-Level Physics - Question 4 - 2022 - Paper 7
Question 4
Bertozzi investigated how the kinetic energy of electrons varies with speed. Which graph shows the variation of kinetic energy with speed? Tick (✓) one box. 
Step 2
Calculate the speed of a particle when its kinetic energy is equal to its rest energy.
Answer
The relationship between kinetic energy (KE) and rest energy (E) is given by the formula:
Setting these equal:
rac{1}{2}mv^2 = mc^2
Dividing both sides by m yields:
rac{1}{2}v^2 = c^2
Thus:
Therefore, the speed is approximately:
Step 3
Discuss the change in the observed mass of a spring when it is stretched.
Answer
The observed mass of the spring changes as it is stretched due to the principle of mass-energy equivalence represented by Einstein’s equation:
When an object is stretched, it gains elastic potential energy, which can be observed as an increase in mass. The difference in observed mass may be negligible, but it is an important concept to understand that mass is related to energy. Thus, when the spring is stretched, the increase in elastic potential energy corresponds to an increase in the observed mass.