Transcript Chapter 3: Newton`s Second Law of Motion

Chapter 3: Newton’s Second
Law of Motion
Force and Acceleration
1
Galileo Developed the
Concept of Acceleration
• Galileo used inclined planes (ramps)
• He found that balls rolling down inclines
rolled faster and faster
• Acceleration = change in velocity/ time
interval
2
Acceleration can be negative
• Example: a car is moving in a certain
direction. The driver steps on the
accelerator. The car speeds up. The
driver lets up on the accelerator and the
car starts to slow down. The car
decelerated (negative acceleration)
• Deceleration is indicated by an arrow in
the opposite direction of motion
3
Question: If a car makes a turn, but is
going the same speed, is it accelerating?
• Speed doesn’t change, but direction does
• Remember that acceleration is change in
velocity/time
• Velocity is speed with direction
• So, in the scientific sense the car is
accelerating because the car’s direction is
changing as it is going around the turn.
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Explanation of Acceleration
• Look at Figure 3.4 on page 36
• What would be the reading of the
speedometer at:
• Time - 3 sec = ______ m/sec2
• Time - 4 sec = ______ m/sec2
• Time - 5 sec = ______ m/sec2
5
Force causes Acceleration
• Acceleration depends on net force. Larger
force, larger acceleration and vice versa.
Note: with mass constant.
• Example: Suppose you pull a wagon with a
net force of 20N. Friction and gravity are
working to slow down the wagon - Lets say
10N of force
• The net force on the wagon is 10N and the
wagon moves in the direction of the larger
force which is the pull that you generated
6
Acceleration ~ net force
• ~ stands for directly proportional
• As one increases, the other also
increases
• As net force increases so does
Acceleration
• There is a general rule: if the net force
is X2 then accel. Is doubled
• If net force is X3, acel. Is tripled
7
Mass of an object corresponds to
or relates to inertia
• The greater an object’s mass the
greater it’s inertia
• Mass is also a measure of how much
material (# and kinds of atoms) an
object contains
8
Don’t Confuse Mass and
Volume
• Volume is the amount of space an
object takes up
• Example: 2 bags of equal size (1)
contains cotton the other contains rocks
• Equal volumes but not equal masses
9
Mass resists Acceleration
• The more massive an object is the more
difficult to accelerate it
• Acceleration ~ 1/mass - When Force is
constant
• Inverse relationship = mass and
acceleration change in opposite ways
• As one goes up the other goes down
and vice versa
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Mass resists Acceleration
• The more massive an object the more
difficult to accelerate
• Twice as much mass has 1/2 the
acceleration
• X3 as much mass results in 1/3 the
acceleration
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Newton’s Second Law
• Force = mass x acceleration
• Units
• Force = (N)
• Mass = kilograms
• Acceleration = meters/sec2
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Newton’s 2nd Law
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Friction always acts in a
direction to oppose motion
• Friction occurs when one object rubs
against something else
• Friction occurs for solids, liquids, and
gases
• The amount of friction between two
surfaces depends on the kinds of
material and how much they are
pressed together
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Friction
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Mass vs. Weight
• Mass is the amount of matter in an
object
• Weight is the force due to gravity that
acts on an objects’ mass
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Mass vs. Weight
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Objects in Free Fall have
equal Acceleration
• Gravity - 9.8 m/sec2
• Neglecting air drag or air resistance
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Acceleration is less when air
drag acts
• For Free Fall the downward force is
weight
• The upward force is air drag (air
resistance)
• Net force = weight (N) - air drag (N)
• When air drag = weight we have ∑F = 0
19
Does that mean the object in
free fall comes to a stop?
• No, the object no longer picks up speed
• Example: Parachutist
• The diver reached terminal speed
(velocity)
20
Forces on a Parachutist
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