Forces in 1 Dimension
Chapter 4
4.1 Force and Motion
• Force is push or pull exerted on object
• Forces change motion
– Makes it important to know the forces on an
object
• Force diagrams (free body diagrams) show
all forces acting on object
– Both direction and magnitude – vector
– Define system
Contact and Field Forces
• Contact – touching
• Field – non-contact
(not touching)
• Every touch
generates force
• Table 4-2, pg 94
Force and Acceleration
• Forces change motion
• Newton’s 2nd Law
– a = Fnet/m
– Net forces and mass will affect the acceleration
of an object
• Applying a force changes the object’s motion –
generates an acceleration
• Changing mass, changes forces
Newton’s 1st 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 that object
is zero”
• Inertia – tendency of an object to resist
change
– Not a force
• Equilibrium – net force on object is zero
4.2 Using Newton’s Laws
• Weight – gravitational force acting on object
– Changes depending on where you are in
Universe
• Scales – How it Works, pg 110
• Apparent weight – weight changes with
motion
– Example: elevator
– Weightlessness – apparent weight is zero
• No contact forces pushing up on object
Drag Force
• Particles of air exert forces on an object
– Often is a huge force
• Drag force – force exerted by a fluid on an
object moving through the fluid
– Direct relationship – increase speed, increase
drag force
– Affected by size and shape of object, and
temperature and viscosity of fluid
Terminal Velocity
• Constant velocity is reached when drag force
equals gravitational force
4.3 Interaction Forces
• Interaction pairs
• Newton’s 3rd Law states that all forces come
in pairs
– One doesn’t cause the other
– Because there is one, there is the other
– Equal in magnitude and opposite in direction
– 2 force diagrams – each one contains one of the
forces in the interaction pair
The Normal Force
• Support force
• Perpendicular
contact force
exerted by a
surface on another
object
Lab
• Force table lab
• Newton’s second lab