A body constrained to move along the z-axis of a coordinate system is subject to a constant force F given by 
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are unit vectors along the x-, y- and z-axis of the system respectively.
What is the work done by this force in moving the body a distance of 4 m along the z-axis?
The force acting on the body is,
As the body is constrained to move along z-axis, therefore the displacement of the body is,
Now the work done by the force is,
An electron and a proton are detected in a cosmic ray experiment, the first with kinetic
energy 10 keV, and the second with 100 keV. Which is faster, the electron or the
proton ? Obtain the ratio of their speeds. (electron mass = 9.11×10-31 kg, proton mass
= 1.67×10–27 kg, 1 eV = 1.60 ×10–19 J).
Given that,
Kinetic energy, 
K.E of proton, 
K.E is given by,
∴ ratio of their speed is given by,
= 13.54
Thus electron is faster than proton by a magnitude of 13.54.
Since momentum is a vector quantity, therefore, though its magnitude remains constant, if its direction changes, the momentum will change. Because after collision the direction of motion and hence momentum changes, therefore, momentum of molecule will not remain conserved.
Since the speed of molecule remains constant, hence its kinetic energy remains conserved. Therefore collision is elastic.
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