MadSci Network: Other

Re: Will hanging a magnet in front of an iron car make the car go?

Date: Fri Mar 2 14:36:23 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Other
ID: 983551676.Ot


If a magnet, being held by a person in a car, is hanging in front of a car
and braced so that the magnet remains the same distance from the car (so
they will never touch) the car will not move forward.  This can be
explained by using Newton's laws of motion.  According to Newton's 1st law,
an object will remain in motion or remain at rest unless acted on by an
outside force.  Newton's 2nd law states that the net force applied on a
body is proportional to the acceleration.  Finally, Newton's 3rd law states
that for every action there is an equal and opposite reaction.  According
to these principles, in order for the car to move, there needs to be a net
force in (for example) the forward direction (2nd law) otherwise the car
will stay at rest (1st law).  To see if there is a force on the car, we
draw what is called a free body force diagram to illustrate all the forces
present in the system (a further explanation of what a force diagram is can
be found in any introductory level college physics text book).  The force
of gravity is balanced by what is called the normal force so that neither
the car or the magnet will move vertically.  The magnet exerts a horizontal
force on the car.  However, according to Newton's 3rd law, the car also
exerts a force on the magnet.  In other words, for the magnet to attract
the car, the car also has to attract the magnet.  These forces are equal
and opposite.  In this case, the magnet is being held at a set distance
from the car.  The object holding the magnet in place also has forces
acting horizontally outward, essentially pushing against both the car and
magnet with equal and opposite forces.  All these forces cancel one another
out.  Hence this system has no EXTERNAL forces acting on it and the car
will not move.  You could experiment with this at home.  Get a small magnet
and some metal object that the magnet can pick up.  Attach the magnet and
the object together, separated by a short distance so they do not touch, as
per your question (i.e. glue a thin toothpick or something to keep them
apart).  If the magnet is on top, they do not start flying upward, and if
the magnet and object are horizontal, they do not begin to move around the
room.  In order for the magnet to pull something around the room, an
outside force (you) need to be pulling the magnet along.  Without that,
they will just sit there.

Note: If there is no object holding the car and magnet apart, they will
both move towards each other until they are touching, but the center of
mass of the system as a whole will not change.  This means that the
car/magnet system will not move forward.


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