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Newtons Second Law of MotionForce and Acceleration

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Title: Newtons Second Law of MotionForce and Acceleration

1
Chapter 5

Newtons Second Law of Motion--Force and
Acceleration

2
Force Causes Acceleration

In order to make an object at rest move, it must
accelerate
suppose you hit a hockey puck
as it is struck it experiences acceleration, but
as it travels off at constant velocity (assuming
no friction) the puck is not accelerating
if the puck is struck again, then it accelerates
again the force the puck is hit with causes the
acceleration
acceleration depends on net force
to increase accelerationincrease net force
double accelerationdouble the force
Force Acceleration
directly proportional

3
Mass Resists Acceleration

Acceleration depends on mass
to decrease accelerationincrease mass
to increase accelerationdecrease mass
double the mass ½ the acceleration
Acceleration 1/mass
inversely proportional

4
Newtons Second Law

Newton was the first to realize that acceleration
produced when something is moved is determined by
two things
how hard or fast the object is pushed
the mass of the object
Newtons 2nd Law
The acceleration of an object is directly
proportional to the net force acting on the
object and is inversely proportional to the
objects mass

5
Newtons Second Law

a F/m or F ma
Robert and Laura are studying across from each
other at a wide table. Laura slides a 2.2 kg
book toward Robert. If the net force acting on
the book is 1.6 N to the right, what is the
books acceleration?
A 30.0 g arrow is shot by William Tell through an
8.00 cm thick apple sitting on the top of his
sons head. If the arrow enters the apple at
30.0 m/s and emerges at 25.0 m/s in the same
direction, with what force has the apple resisted
the arrow?

6
Newtons Second Law

3. An applied force of 50 N is used to
accelerate an object to the right across a
frictional surface. The object encounters 10 N of
friction. Use the diagram to determine the normal
force, the net force, the mass, and the
acceleration of the object. (Neglect air
resistance.)
4. Rose is sledding down an ice-covered hill
inclined at an angle of 15.0 with the
horizontal. If Rose and the sled have a combined
mass of 54.0 kg, what is the force pulling them
down the hill?

7
Newtons 2nd Law Kinematics

A 4.60 kg sled is pulled across a smooth ice
surface. The force acting on the sled is of
magnitude 6.20 N and points in a direction 35.0
above the horizontal. If the sled starts at
rest, how fast is it going after being pulled for
1.15 s?
The fire alarm goes off, and a 97 kg fireman
slides 3.0 m down a pole to the ground floor.
Suppose the fireman starts from rest, slides with
a constant acceleration, and reaches the ground
floor in 1.2 s. What was the force exerted by
the pole on the fireman?

8
Friction

Acts on materials that are in contact with each
other
Always acts in a direction to oppose motion
Depends on type of surfaces
Rubber against concrete produces more friction
than steel against steel
Occurs in liquids and gases
Air resistance, running in water
When friction is present an object can still move
at a constant velocity
The friction force must balance outside forcenet
force would be zero (no acceleration)

9
Friction

Does friction act on an object at rest?
No, there must be movement
Suppose a biker cruises with a constant velocity
but the thrust from his pedaling is 10 N. What
is the acceleration of the bike?
0 m/s2 the bike is moving at a constant speed
What is the force of air resistance (friction)
acting on the bike?
10 N in the direction opposite the motion of the
bike these forces must balance or the bike would
be accelerating

10
Friction

Types of Friction
Static
The resistance force that must be overcome to
start an object in motion
Kinetic/Sliding
The resistance force between two surfaces already
in motion
Rolling
The resistance force between a surface and a
rolling object
Fluid
The resistance force between and object and a
fluid/gas (air resistance)

11
Sliding Friction

Ffriction µFnormal
µ the coefficient of sliding friction (has no
units)
1. Brian is walking through the school cafeteria
but does not realize that the person in front of
him has just spilled his glass of chocolate milk.
As Brian, who weighs 420 N, steps in the milk,
the coefficient of sliding friction between Brian
and the floor is suddenly reduced to 0.040. What
is the sliding force of friction between Brian
and the slippery floor?

12
Friction

While redecorating her apartment, Kelly slowly
pushes an 82 kg china cabinet across the wooden
dining room floor, which resists motion with a
force of 320 N. What is the coefficient of
sliding friction between the china cabinet and
the floor?
3. A rightward force is applied to a 10-kg
object to move it across a rough surface at
constant velocity. The coefficient of friction,
µ, between the object and the surface is 0.2. Use
the diagram to determine the gravitational force,
normal force, applied force, frictional force,
and net force.
(Neglect air resistance.)

13
Applying ForcePressure

Pressure the amount of force per unit area
P F/A
Units are Pascals (Pa) N/m2
Suppose you are standing on the ground. Do you
exert more pressure when you stand on both feet
or stand on one foot?
The force, your weight, is the same in both
cases. Two feet have more area than one foot,
therefore, there will be more pressure exerted if
you are standing on one foot.

14
Pressure

Brooke comes home from school and puts her books
down on the kitchen table. The books have a
combined weight of 25 N and the area in contact
is 0.19 m by 0.24 m. What pressure do the books
apply on the table?
2. A full coffee mug has a mass of 0.60 kg and
an empty mug has a mass of 0.30 kg. a.) Which
mug, the full or empty one, applies a greater
pressure on the table? b.) If the full mug
applies a pressure of 1200 N/m2, what is the area
of the circular ring of coffee left on the table
by the bottom of the mug?

15
Free Fall Explained

Do heavy objects always fall faster than light
objects?
Only in the presence of air resistance
Newtons Second Law (F ma)
Mass is only a factor in the force not in the
acceleration
A heavier object will strike the ground with a
much greater force than a lighter object, but
they will have the same acceleration and drop time

16
Falling Air Resistance

Air resistance obviously affects the amount of
time it takes an object to drop.
Think of an elephant and a feather if we dropped
both off the school, which would land first?
Without air resistance, they land at the same
time
With air resistance, the elephant lands first

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ff.gif
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ar.gif
17
Falling Air Resistance

For the elephant
net force is only slightly decreased by the air
resistance because the elephant has a large
weight (downward force)
For the feather
the net force is greatly decreased by the air
resistance because the feather has a small weight
(downward force)

18
Falling Air Resistance

Does the elephant or the feather experience a
greater force due to air resistance?
Air resistance of a falling body depends on
How big the body is
How fast the body is falling
Air resistance is the result of an object plowing
through a layer of air and colliding with air
molecules.
The more air molecules which an object collides
with, the greater the air resistance force.
Subsequently, the amount of air resistance is
dependent upon the speed of the falling object
and the surface area of the falling object.
Based on surface area alone, it is safe to assume
that (for the same speed) the ELEPHANT would
encounter more air resistance than the feather.

19
Falling Air Resistance

Why does the elephant fall faster if it
experiences more air resistance than the feather?
Objects accelerate when forces are unbalanced
The feather has a smaller force of gravity,
therefore its air resistance (even though it is
smaller than the elephants) equals its force of
gravity much faster than for the elephant.
When these forces are equal, the feather has
stopped accelerating and therefore reached its
terminal speed.

20
Falling Air Resistance

Terminal Speed the speed reached when the
acceleration stops
Terminal Velocity the terminal speed with its
direction
Ping pong ball 9 m/s baseball 20 m/s
When the force of air resistance of a falling
object is equal to the objects weight, what will
the net force be?
What will the acceleration be?
Does this mean that the object stops falling?

21
Falling Air Resistance

With air resistance, Newtons Second Law becomes
a Fnet/m (weight air resistance)/m
a (mg R)/m g R/m
R resistance force

22
Falling Air Resistance

A skydiver jumps. As she falls faster through
the air, does air resistance increase, decrease,
or remain the same?
Does net force increase or decrease?
As she falls faster and faster does her
acceleration increase, decrease, or remain the
same?
How can a skydiver control his/her velocity?

23
Falling Air Resistance