Transcript Chapter 4: Forces in One Dimension

Chapter 4: Forces in One
Dimension
• Launch Lab on pg. 87
• Which string broke in step 2?
• Which string broke in step 3?
Force and Motion
• Force: a push or a pull on an object
• Forces can cause an object to:
– Speed up, slow down, change directions
• Direction in which the force is exerted also
plays a role
Force and motion
• F is a symbol that for a vector and represents
the size and direction of a force
• F just represents magnitude
Considering how force affects motion
• Consider the object “called system” and
everything around the object that exerts
forces on it is called the external world
Contact force vs. field force
• Contact force examples: touch something to
apply a force
• Field force examples: gravitational force from the
earth acts on objects without touching them, also
magnets
• There needs to be an agent causing a force on a
system (example: your hand “agent” causes a
force on the book “system”
Free-Body Diagrams
• The object is represented with a “dot”
• Represent each force with an arrow that points in the
direction that the force is applied
• The length of the arrow should proportional to the size
of the force
• Always draw the force arrows pointing away from the
particle even when the force is a push
• Label each force
• Choose a direction that represent (+)  this is usually
the direction with the greatest force because it makes
the problem easier to solve and reduces negative
values
Practice problems
Force and Acceleration
• Would an object continue to accelerate when
there was no longer a force applied?
• Think about a spaceship in outer space…does
it have velocity? Is there still a force acting on
it? Could it accelerate?
Force and Acceleration
• The relationship between force and acceleration
is a linear relationship where the greater the
force is, the greater the resulting acceleration.
• Talk about the cart pulled by a stretched rubber
band example, if you pull more carts with the
same rubber band…acceleration decreases
therefore… the slope in an acceleration vs. force
graph is dependant on mass
F = ma
• a = F/m … tells you that a force applied to an
object causes that object to experience a
change in motion, the force causes the object
to accelerate.
• It also tells you that for the same object if you
double the force, the acceleration doubles
What is the unit for force?
• If F=ma (Newton’s 2nd law)… then one unit
could be: kg·m/s2
• 1 kg·m/s2 = 1 Newton (N)
Think about different objects falling
• Acceleration as we learned is the same due to
gravity
• Therefore different objects have different
masses, and therefore require different
amounts of forces.
Combining forces
• Net force: the vector sum of all the forces on
an object
Equal and opposite direction forces
• Net force = __________N
• Draw a picture below
Equal or unequal forces same direction
• Net force =
• Draw a picture below
Unequal forces opposite directions
• Net force =
• Draw a picture below
Equal or Unequal forces perpendicular
• Net force =
• Draw a picture below
Newton’s First Law
• “an object that is at rest will remain at
rest, and an object that is moving will
continue to move in a straight line with
constant speed, if and only if the net
force acting on the object is zero”
Newton’s First Law
• Inertia- the tendency of an object to resist
change
• Equilibrium- if the net force on an object is
zero, then the object is in equilibrium (if it is at
rest or if it is moving at a constant velocity)
– As long as the object is not experiencing a change
in speed or direction is at equilibrium
Table 4-2
Force
Symbol
Definition
Direction
Table 4-2
Force
Symbol
Definition
Direction
Section Review pg. 95
• 9-12, 14.
Section Review pg. 95
• 9-12, 14.