Newton's Laws

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Previously Asked Questions

Q:     If two forces act on a moving body, could the body possibly move (a) at a constant speed or (b) at a constant velocity?

A:     If b can be true then a is also true by the same method.  If two forces are equal and opposite on an object they will cancel each other out so that there is no net effect on the moving body.  Therefore, if an object was traveling with a constant velocity the and two equal and opposite forces were applied to it, the forces would cancel and the object would continue moving with a constant velocity

Q:    A vertical force F is applied to a block of mass m that lies on a floor.  What happens to the magnitude of the normal force N on the block from the floor as magnitude F increases from zero if force F is (a) downward and (b) upward?

A:    If the force F was downward then the magnitude N would increase from its initial value of mg.  If the force F was upward then the magnitude N would decrease from mg to zero (when the box would move up off the floor.


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As the force pressing down on an object increases the normal to the force also increases. (Note: the object's mass and weight remain the same)

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As the force pulling up on an object increases the normal to the force will decrease. (Note: the object's mass and weight remain the same)

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Newton's Laws

Newton's First Law

   "If no net external force acts on an object, then if the object is a rest it will remain at rest and if it is in motion of any kind it will remain in motion in straight line with constant speed."


Newton's Second Law

    The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass: a = Fnet / m.

The most often used formulation of Newton's Second Law is: sigma.gif (836 bytes) F = ma.

Newton's Third Law

When two objects interact, the force exerted on object A by object B is equal in magnitude to the force exerted on object B by object A and opposite to it in direction.

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Newton's Second Law sigma.gif (836 bytes) F = ma.  
Newton's Second Law (component version) sigma.gif (836 bytes) Fx = max
sigma.gif (836 bytes) Fy   = may
sigma.gif (836 bytes) Fz  = maz
SI Unit for Force (Newton) 5-3.gif (234 bytes)
Weight W = mg
Newton's Third Law FAB = - FBA

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Mechanics List of Topics

Measurements Newton's Laws Potential Energy and Conservation of Energy Rotation of
Rigid Bodies
Vectors Forces and Fields Linear Momentum Angular Momentum Mechanical
Motion of Point-Mass Objects in One Dimension The Gravitational Field Collisions Torque Mechanical Waves
Motion of Point-Mass Objects in Two and Three Dimensions Kinetic Energy
and Work
Circular Motion of Point-Mass Objects Equilibrium Sound