Tutor profile: Shubham K.
Subject: Physics (Newtonian Mechanics)
How do you define centre of mass. Why is it important to consider? How newton's laws are significant for a system of particles?
If you go by the defn of Centre Of Mass, it is the point on a body where we assume that the entire mass is concentrated or the point where its weight acts. Newton's 2nd law of motion is correctly written as Fext=mass * accln of COM. It is the COM which gets the accln when an external force is applied to it. It is also important to understand that COM is the point on a body where sum of moments of every particle on the body is zero i.e. (sigma)mr=0 Thus if we go into COM frame, the complicated motion of particles can be well simplified. Further we take the help of COM to deal with the rotation dynamics. The reason is simple, the energy, momentum analysis becomes much easier and we get a clear picture of trajectory. For a system of particles newton's 2nd law statement becomes very significant which states that rate of change of momentum is directly proportional to force. Now if the net external force on the system is zero, the momentum remains conserved i.e. momentum at each point of time will be same. However different point on the body may have completely different velocities and accelerations but accln of COM is zero throughout.
Subject: Physics (Electricity and Magnetism)
How do you describe the motion of a charged particle placed in a combined electric(E) and magnetic(B) field both being perpendicular to each other.
Path traced by the charged particle will be a helix. Helix structure is very much similar to an elongated spring. If you go by the formula of force experienced by the charged particle placed in a magnetic field i.e. F=q * (v cross B) the component of velocity which is perpendicular to B will be responsible for the circular motion of the charged particle while the electric field will keep the particle accelerated in its direction thus forming a helix.
In which direction does the frictional force act when we walk?
Friction acts in the forward direction when we walk thus friction helps in walking. As we walk on the floor we try to push the ground backward with the help of our toes and we very well know that friction opposes the relative motion between the two surfaces in contact thus friction acts in the forward direction.
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