Download Newton 2nd law1

Newton’s 2nd Law
Force and Acceleration
Ch. 5.1-5.3
Let’s Review:
Acceleration
• Who remembers what
acceleration is?
• The rate at which velocity is
changing
• Is this an increase or decrease
in speed?
– Both!
•
What else can change
acceleration?
– Change in direction
• Acceleration = CHANGE in
velocity/change in time
Objectives
1. Relationship between acceleration and net
force
2. Relationship between acceleration and
mass
3. Explain Newton’s 2nd Law of Motion
Acceleration
• Force causes acceleration
• Acceleration is proportional
to net force
• aF
• “” doesn NOT mean =
• What does “proportional”
mean?
Acceleration
• Let’s take a look at the
air track again…
Acceleration
• Formula One car
traveling at constant
speed of 185mph in a
straight line. What is the
acceleration?
• Blue-Footed Booby
standing on a rock. What
is the acceleration?
• In both cases a = 0
Zero Acceleration
• Zero acceleration applies both
to the state of rest and
constant velocity.
• No change in state of motion.
Why?
• Forces that are acting on it are
balanced
– There’s exactly enough friction
to counteract the force --> . . .
• Zero net force means zero
acceleration!
• Zero acceleration is the
evidence of zero net force.
Net Force
• Roll ball across floor
– Assume that velocity is constant
• What is the net force here?
• 0 N! If no acceleration, then no net force.
• Suppose I pushed the ball with a force of 5 N.
What would the friction be if the ball maintains a
constant velocity?
• 5 N (in the opposite direction of ball)
• 5 N of force and 5 N of friction cancel each out
and you have constant velocity.
Mass and Acceleration
•
•
•
•
•
Mass resists acceleration
Student/chair demo
Toy car-rubber band demo
More mass --> less acceleration
The same force applied to twice
as much mass results in only 1/2
the acceleration
• For 3 times the mass, only 1/3 the
acceleration results
• Acceleration is inversely
proportional to mass
– Mass increases --> acceleration
decreases
• Acceleration = 1/mass
• Values change in opposite
directions (plug in #’s)
Check Question
• Suppose the pilot announces that the plane is
flying at a constant 900 km/h and the thrust of the
engines is a constant 80,000 N. What is the
acceleration of the airplane?
• Zero, b/c velocity is constant
• What is the combined force of air resistance that
acts on the plane’s outside surface?
• 80,000 N, to produce a zero net force. If
resistance were less, the plane would speed up; if
it were more, the plane would slow down.
The acceleration is constant!
thrust
80,000 N
lift
drag
80,000 N
LET’S DO WORKSHEET
weight 5-1. FIRST SIDE ONLY.
Nonaccelerated Motion
• I’m going to pull a mass across table at a constant
velocity (volunteer to read Newton scale)
• What is the acceleration?
– 0, no change in velocity once the mass is moving steadily
• What is the applied force?
• What is the value of the net force acting on the
mass?
– 0, as evident by the non-changing velocity
Nonaccelerated Motion
continued
• There are people out there who can look at this demo
and tell me how much friction is acting on this object,
as long as there is no acceleration. Are you sitting next
to one of those people?
• This is zero net force on a moving system
– Zero acceleration applies to state of rest and state of
constant velocity
• The forces acting on the book balance to zero
• Zero acceleration is evidence of zero net force
Newton’s Discovery
(This is going to take some brain power…)
• Galileo discovered that
two objects will fall at the
same time
– Drop a book and a pen
– But he didn’t know why…
• Newton put it all together
• One of the most powerful
laws in physics!
• I’m going to try and
explain the relationship
between force, mass, and
acceleration. (2 kids)
• More on this later with
free fall…
Newton’s 2nd Law
• If you place force on an object, it will accelerate in the
direction you push it
– It accelerates in the direction you push it
• Acceleration is directly proportional to force
– If you push it twice as hard, it accelerates twice as much
• Acceleration is inversely proportional to the mass of an
object
– If it gets twice the mass, it accelerates half as much
• acceleration = force/mass,
– a = f/m
• Force = (mass)(acceleration)
– F = ma
Your experience
• Take a minute to think of a situation you
have had that illustrates Newton’s second
law of motion and write it down. Then
share it with your neighbor . . .
Newton’s 2nd Law
• a = f/m
• What do we notice about this
equation?
– Plug in the number 5kg for
mass and 10N for force
– Now plug in 20N for force.
What happens to acceleration?
• Doubling the net force,
doubles the acceleration
(show)
• Doubling the mass, halves
the acceleration
• Pushing an elephant
– cd-rom (Hewitt)
Practice Problems
• For our purposes, let’s always use the
equation, F = ma, and sub numbers into
parts of the equation.
• Remember
– F is always in Newtons
– m is always in grams
– a is always m/s2
• Let's practice