killer tips for physics.....
Hi guys and gals these are some of the concepts throughout physics which i was able to pick up throughout my jee preparation by some deep thought....
hope u find them useful!! some of u might already know many of these but still.....here goes...
1) F=ma is not the correct definition of force...it is valid only when m is constant..
F=d(mv)/dt = vdm/dt+mdv/dt is correct...
2) Frictional force does not depend upon the area of contact...
3) All forces at the molecular level are conservative...at macroscopic level forces such as normal force,tension.friction etc are all
due to the net resultant of these forces acting at molecular level(electrostatic forces)... but if we were to study the force of friction at molecular level for example...
we would have to consider the motion of every molecule and that would be impossible....hence we approximate by using a single
force which can represent the sum of all these forces... but this becomes non conservative....this is because....when the force of friction acts...the macroscopic effects are observable
and they can be represented by this non conservative approximation...but the molecular effects cannot...hence the molecular effects are that the particles vibrate and this is what we term as internal energy...
but if we were to consider at the molecular level there is no such thing as internal energy etc and the motion can be FULLY described by electrostatics...
4) frictional force need not always do negative work on a body it can do positive work also
5) all the four fundamental forces of nature act at the speed of light... hence if apply a force at one end of a body....the particles at the other end feel the effects only after finite amount of time...
hence usually in problems like "consider an atwood machine with 2 equal masses in which the string above one of the blocks is cut...find the tension in the string JUST AFTER it is cut." the value of tension JUST AFTER will
be mg...because the tension has to take time to adjust itself...just like all forces...
6) work done is the dot product of force applied and displacement of point of application of the force....not displacement of center of mass....
but for deformable bodies it becomes more complicated....only for rigid bodies we can say in terms of CM as all points remain at a fixed distance from CM....its not physically correct though...
7) work-KE theorem is valid only for particle like objects and rigid bodies... .THIS THEOREM IS INVALID FOR DEFORMABLE BODIES....
8) angular momentum and angular velocity need not be in the same direction always....
EXPLANATION:
consider a collision of a point mass with a rigid body of finite dimensions...initially let the angular momentum of the point mass about CM be L... then the final angular momentum about the CM is Ixwxi+Iywyj+Izwzk and final angular velocity is wxi+wyj+wzk u can easily see that that.. angular momentum is not a scalar multiple of angular velocity....and it will be a scalar multiple only if the object has 3 fold symmetry about the CM such as a sphere a cube etc....
9) a body can roll on a surface even if it is frictionless...
10) velocity is different for different wavelengths of light ...so when we say refractive index of a wavelength we say nw=cw/vw where vw is the velocity of a PARTICULAR wavelength of light and cw is the speed of light of THAT wavelegth in air.......but when it comes to a mixture of different wavelengths....their net velocity is given by something known as phase velocity...so u wud use nm=cm/vm where nm is the refractive index for the mixture and cm and vm are the phase velocities in the respectively...
PS: all the diagrams shown here are those drawn by me unless specified...
and watch out for more "KILLER TIPS!!" ;)
Back to Community Shelf
Awaiting Review for Nickels![[Post New]](/templates/default/images/icon_minipost_new.gif){kind=icon}
![[Thumb - spring.JPG]](/upload/2008/2/8/70c7551452dbf94dc825b02c5406cbb4_17497.JPG_thumb)
91
