Newton's Laws of Motion

• First law: If the net force on a body is zero, it does not accelerate (is its velocity zero?).
• Second law: The acceleration of an object is related to the total force on it by F_net = ma.
• Third law: If an object A exerts a force F on an object B, then B exerts a force –F on A.

Types of Forces

• Weight: Near the Earth's surface the gravitational force is always F_G = mg, downwards.
• Normal force: A force that acts on an object in contact with a solid surface. Perpendicular to the surface; its magnitude F_N depends on the situation, and can be found using the fact that the surface prevents the body from sinking into it (up to the surface's breaking point).
• String tension: A force that acts on an object attached to a string (cable, rope). It is directed along the string away from the body, and its magnitude F_T depends on the situation – up to the string's breaking point. The tension is the same at all points along a string, provided that no tangential forces (including the string's own weight) are applied to the string.
• Friction: A force that acts on an object in contact with a solid surface (we do not treat friction with air and other fluids). It is along to the surface, and there are two types. Static friction: Its value depends on the situation, but cannot exceed a maximum value, F_{s s} F_N. Sliding friction: (a.k.a. kinetic friction) F_k = _k F_N.
• Elastic force: Exerted by a spring in the direction opposite to the displacement, F = –kx.

Solving Problems

• Setup: Sketch of situation, choice of axes (may depend on forces present), drawing of free-body diagram, resolving all forces present along the two axes if relevant.
• Using Newton's laws: Writing down the equations, using the information available (from the free-body diagram and the statement of the problem).
• Kinematical aspects: Some problems require the use of equations that relate the acceleration to other information on the motion.

Static Equilibrium Situations

• Concepts: What is equilibrium? Torque,  = r_perp F.
• Conditions for equilibrium: No net force and no net torque on the object,

1st:   F_net = 0 ;   2nd:  _net = 0 .

• Skills: Be able to solve static equilibrium problems in two dimensions, using both conditions for equilibrium and (if necessary) resolving forces into components.