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Question

How much work is required to stop an electron (m= 9.11 x 10^-31 kg) which is moving with a speed of 1.10 x 10⁶ m/s?

Answer 1

Kinetic Energy and the Work-Energy Principle statement is net work = change of kinetic energy, Wnet=∆KE. Then, to stop the electron ( that is it kinetic energy will be zero m(v_fin)²/2 =0) you will need to spend work=its kinetic energy, that is, W=m(v_fin)²/2 -m(v_in)²/2=-(9.11 x 10^-31 kg)(1.10 x 10⁶ m/s)²/2=5.51*10^-19 J

Negative sign shows that to stop an object we need to spend some energy.