Introduction to Dynamics

The Study of Forces Where Kinematics is the study of motion, Dynamics is the study of why things move. Sir Isaac Newton formulated the three laws of dynamics when he was 23 years old.

What is a Force? A force is defined as a push, pull, twist or squeeze. Given the symbol F Units are Newtons (N) Forces are vector quantities.

Newton’s First Law of Motion Newton’s first law of motion states: An object will remain at constant velocity (including zero) unless acted upon by an unbalanced force. An unbalanced force is the sum of all forces acting on an object. An unbalanced force is also known as the net force. Symbol for net force = F net or ΣF

Newton’s First Law These forces are balanced ΣF = 0 therefore the object will remain at a constant velocity (which means it will stay at rest or stay moving at the same speed) F 1 = 6.0NF 2 = 6.0N

Newton’s First Law These forces are unbalanced ΣF = 5.0N to the right or +5.0N. Therefore the object will accelerate. F 1 = 6.0NF 2 = 1.0N

Newton’s First Law = Inertia Newton’s First Law is often called the Law of Inertia. Inertia is the tendency for an object to remain at a constant velocity or stationary.

Elastic Force-Hooke’s Law when you stretch or compress a spring by applying a force on it, it will restore itself when the force is removed. The restoring force is called the elastic force (F s ) which is proportional to the distortion or displacement (x) of the material:

Equilibrium position Stretched position Equilibrium position is the position in which the elastic material will return Hooke’s Law: F = - kx “k” is called the spring constant

Every elastic material has a spring constant. With springs, the stiffer the spring the greater the spring constant Usually the equation (F = - kx) is written with a negative sign to indicate that the elastic force is in the opposite direction to the displacement See example problems(p165)

Hooke’s Law Lab Equipment: rubber band, spring scale,meter stick Problem: What is the relationship between the stretch of a rubber band and its elastic force? Procedure: stretch the rubber band to different forces, measure the length & graph F vs x

Data : Calculate the slope of the line and explain what it represents. Force (N) Stretched Length or “x” (m)

(add under graph) Questions: a.if the rubber band was stronger what would happen to “k”? What would the graph look like (sketch on your graph)? b.What would the graph look like if the distances used included the length of the rubber band?