Download The Force Be With You

Chapter 3
• Definition of FORCE:
– A push or pull on an object
• What forces cause:
– An object to start moving
– An object to stop moving
– Speed up or slow down an object
• Measurement of force:
– Newton (N) which is equal to kgm/s2
– Symbol of Force:
• EXAMPLES OF FORCES:
• Anything that pushes or pulls
–
–
–
–
Gravity
Friction
Air Resistance
Air Pressure
Unbalanced Forces
Forces acting on an object that
are not equal and opposite
Net forces cause an object to
accelerate (speed up, slow
down, or change direction)
Balanced Forces
Opposite and equal forces
acting on an object
If the forces are balanced, the
object will not accelerate
(it will stay still OR move at a
constant velocity)
Which one (unbalanced forces or balanced forces) causes a change in velocity?
Unbalanced!
Using the pink bouncy ball, perform the following tasks and answer the questions about each task.
The "Tasks"
One person pushes the ball
Two people push the ball on
opposite sides at the same
time
One person pulls the ball
towards them
Two people pull the ball
towards them at the same
time
Describe what you see
Net force or
balanced forces?
•
If your velocity changes, are the forces acting on
you balanced or unbalanced?
•
•
unbalanced
If you are moving at a constant speed, are the
forces acting on you balanced or unbalanced?
•
•
balanced
If you are at rest (not moving), are the forces acting
on you balanced or unbalanced?
–
Sit still in your chair.
•
Are the forces balanced or is there a net force?
•
•
Imagine that different forces are acting on a
hockey puck. Draw arrows to show the forces
acting on each hockey puck, and then answer
the questions.
Each N = 1 cm.
Forces
5 N force towards the puck from the left &
5 N force towards the puck from the right
7.5 N force towards the puck on the left &
7.5 N force away from the puck on the right
4 N force towards the puck on the left &
5 N force towards the puck on the right
What is
the Net
force on
the puck?
Will the puck
move? If
so, in what
direction?
Combined
or opposed
forces?
Balanced or
Unbalanced
Forces?
opposed
balanced
0N
won’t move
unbalanced
15 N
to the right
1N
to the left
combined
opposed
unbalanced
Forces
2 N force away from the puck on the left &
4.3 N force towards the puck on the right
5 N force away from the puck on the right &
4 N force towards the puck on the right
3 N force pointing up at the puck &
3 N force pointing down on the puck
Combined
or opposed
forces?
opposed
opposed
opposed
Balanced or
Unbalanced
Forces?
unbalanced
unbalanced
balanced
What is
the Net
force on
the puck?
2.3 N
1N
0N
Will the puck
move? If
so, in what
direction?
to the left
to the right
no movement
Newton’s Second Law:
Force, Mass, and
Acceleration
Acceleration and Force
• Anything that accelerates is acted on by a force.
– Remember: A force is a push or pull on an object.
• Acceleration is caused by applying force.
• Examples:
– Hitting a bag
– Steady pull of gravity
Force Causes Acceleration
• When there is a net force on an object, it
produces acceleration.
• Acceleration also depends on something else.
– Can you think of what that is?
The Effect of Mass
• What happens when you kick a tin can?
– It accelerates (changes motion)
• Now what happens when you kick the same can
full of rocks?
– Ouch!
– And it doesn’t accelerate as much
Inertia
• The full can has more inertia than the empty can
because it has more mass.
• The greater an object’s mass, the greater its
inertia.
• This also means that the greater the mass, the
greater the force needed for acceleration.
What is Mass?
•
•
•
•
Mass is not volume or weight.
Mass is the amount of matter an object has
Volume is a measure space
Weight is the force due to gravity that acts on an
object’s mass
– Your weight is different on the moon that it is here, but
your mass is the same.
Mass and Acceleration
• Recall that the more massive an object is, the
more inertia it has.
• Therefore, more massive objects are harder to
accelerate.
• Acceleration is inversely proportional to mass.
– This means that when one gets bigger, the other one
gets smaller.
Concept Check
• Suppose you apply the same amount of force to
two carts, one cart with a mass of 4 kg, and the
other with a mass of 8 kg.
• Which car will accelerate more?
– The 4 kg car
• How much greater will the acceleration be?
– Acceleration will be twice as much
Newton’s Second Law of
Motion
• Isaac Newton was the first to realize the
connection between force and mass in producing
acceleration.
• His 2nd Law of Motion is one of the most important
rules of nature ever proposed.
Newton’s Second Law of
Motion
• It links force, mass, and acceleration.
• Stated:
The acceleration produced by a net force on an
object is directly proportional to the net force, is
in the same direction as the net force, and is
inversely proportional to the mass of the object.
Equation
• Acceleration = net force
mass
a = _F_
m
• This means acceleration equals the net force
divided by the mass.
Triangle and Units
• Let’s make a triangle for this formula:
• Units
– Acceleration = m/s2
– Mass = kg
– Force = kg ● m/s2 = N
F
m
a
Calculation!!
• Consider a 1000-kg car pulled by a cable with
2000 N of force. What will be the acceleration of
the car?
• a = _F_
m
• a = _2000 N_
1000 kg
• a = 2 m/s2
Friction!!
• What is friction?
• Well, it occurs when two things rub against each
other.
• IMPORTANT – Friction always works against
motion!!
Friction
Motion
What affects Friction?
• Type of surface
– Where is there more friction?
• Between a crate and rough wooded floor
• Between a crate and a polished linoleum floor
• How much they are pressed together
– Where is there more friction?
• Between a crate and an even surface
• Between a crate and downward ramp