Download Introduction to Dynamics

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 = Fnet or ΣF
Newton’s First Law
F1= 6.0N
F2= 6.0N
• 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)
Newton’s First Law
F1= 6.0N
F2= 1.0N
• These forces are unbalanced
• ΣF = 5.0N to the right or +5.0N.
• Therefore the object will accelerate.
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 (Fs) 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:
Force
(N)
0.0
1.0
2.0
3.0
4.0
5.0
Stretched
Length or
“x” (m)
• Calculate the slope of the line and explain
what it represents.
(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?