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Note 11 Working with Forces The definition of force is best expressed not as what it is but as how it affects motion. The way it affects motion is described in Newton’s second law of motion. Remember, this is applied to one object. The net force on that object is equal to the mass of that object times the acceleration of that object, and both the net force and the acceleration point in the same direction. ! ! ΣF = ma A single force will always produce an acceleration. If an object has an acceleration, then the net force must be non-zero. If the net force were zero, then the acceleration is zero. This means the velocity is constant. The object can be at rest or has a constant non-zero velocity. This is the essence of Newton’s first law of motion. The unit of force is given the name the newton, N. According to Newton’s second law, the newton is made up of the following SI units. ⎡ kg ⋅ m ⎤ [N ] = [kg][m/s 2 ] = ⎢ 2 ⎥ ⎢⎣ s ⎥⎦ 1 N = 0.225 lbf (pound force) So how is it that the scale shows mass and force together? link Categories of Force There are two categories of forces: non-contact and contact forces. Non-contact forces are those that do not required physical contact. We will deal with only gravity. Contact forces do required physical contact. For us, these are all other forces that are not gravity. page 1 Force Vectors The way to deal with force vectors is to place all of the forces on a single object onto its own coordinate system centered on the object. This is called a free-body diagram. The label of each force vector contains a type, a source, and a target. Here are two free-body diagrams of a ball hung by a string before and after release to free fall. Ftension from string on ball ball Fgravity from Earth on ball ball Fgravity from Earth on ball Some forces have shortcuts. The force from gravity is also referred to as the weight, W, and the tension force is T. Tstring on ball ball WEarth on ball ball WEarth on ball Lastly, unless there are many forces, the source name can be omitted and the target name is clear as long as you label the object of the free-body diagram. T ball W ball W page 2