Download Newton`s 2nd Law

Newton’s Second
Law of Motion
Force & Acceleration
Larger the net force, greater the
acceleration
implies
Double the Force
Triple the Force
Half the Force
implies
implies
Acceleration “goes as” Force
Force “goes as” Acceleration.
Double the Acceleration
Triple the Acceleration
Half the Acceleration
Think about Cars and Motors
Check Yourself
You push on a crate with 100
Newtons of force. If the
friction force is 100 Newtons,
does the crate accelerate?
No, because net force is zero.
Does that mean that the crate
is not moving?
No, just that its velocity is
constant.
Check Yourself (cont.)
Now push with 150 N; friction is still 100 N.
Does the crate accelerate?
Yes, because net force is now 50 Newtons.
A friend helps push with an additional 150 N.
By how much does acceleration increase?
Net force is now 150+150-100= 200 N. This
is FOUR times larger so acceleration
increases by a factor of four.
Mass & Acceleration
For a given force, greater mass, smaller the acceleration
Double the Mass
Triple the Mass
Half the Mass
implies
implies
implies
Half the Acceleration
Third of the Acceleration
Double the Acceleration
Acceleration goes as
the inverse of mass.
Think about a
shopping cart
Check Yourself
NO
FRICTION
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Newton’s Second Law of
Motion
The acceleration of an object is directly
proportional to the net Force applied to the
object and inversely proportional to the mass
of the object.
Mathematically
written
(Net Force)
(Acceleration) =
A = F/M
(Mass)
F=MxA
M = F/A
Try it…
• Zoo keepers lift a stretcher that holds a
sedated lion. The total mass of the lion and
stretcher is 175 kg, and the upward
acceleration of the lion and stretcher is 0.657
m/s2. What force is needed to produce this
acceleration of the lion and the stretcher?
• Known: m = 175 kg
Equation: F=ma
a = 0.657 m/s2
Work: F = (175) (0.657)
Answer: 115 N
Let’s see what you know…
• What force is needed to accelerate a 1.6 x 103 kg
forward at 2.0 m/s2
• Known: m = 1600 kg
Equation: F=ma
a = 2.0 m/s2
Work: F = (1600) (2.0)
Answer: 3200 N
• The net force on the propeller of a 3.2 kg model
airplane is 7.0 N. What is the acceleration of the
airplane?
• Known: m = 3.2 kg
Equation: a=F/m
F= 7.0 N
Work: a = 7.0/3.2
Answer: 2.2 m/s2
Last chance…
• A baseball accelerates downward at 9.8 m/s2.
If the gravitational force is the only force
acting on the baseball and is 1.4 N, what is
the baseball’s mass?
• Known: F = 1.4 N
Equation: m=F/a
a = 9.8 m/s2
Work: m = 1.4/9.8
Answer: 0.14 kg
Weight on Earth
Acceleration of gravity on Earth is = 9.8 m/s2
From Newton’s Second Law,
(Force) = (Mass) X (Acceleration)
For example, weight (force of gravity) for 1 kg is
( 9.8 Newtons ) = ( 1 kg ) X ( 9.8 m/s2 )
Newton’s Second Law explains why all
objects fall with same acceleration.
Free Fall Acceleration
Ratio of
weight/mass always
the same since
weight depends on
mass.
Falling in a Vacuum
Feather falls slowly due to
air resistance force.
If we remove the air
(create a vacuum) then
feather and coin fall with
same acceleration.
Air Resistance (Drag)
Origin of drag is molecules of gas (or liquid) striking
a moving object. (Friction in the air!)
Drag force depends on:
•Size (area) of the object
•Speed of the object
Larger the size or speed,
larger the drag.
Also depends on shape of object,
density of gas or liquid, etc.
Terminal Velocity
Speed of falling objects increases until drag force
balances weight.
When forces balance, zero acceleration
so constant velocity.
Speed for which air resistance balances
weight called terminal speed.
High terminal speed
(better open the chute!)
Low terminal speed
(large area of chute)
Can a Penny kill you if dropped of
the Empire State Building