Download Force and Motion

Force and Motion
PSc.1.2 OBJECTIVE: Understand the
relationship between forces and motion.
Objectives
• PSc.1.2.3
• Explain forces using Newton’s
three laws of motion.
4 Forces of the Universe
• Strong: Forces
holding the nucleus
of an atom together
• Weak: Forces as a
result of some atoms
that undergo nuclear
decay
4 Forces of the Universe
• Electromagnetic: A
force based on
electricity, magnetism
and light properties
• Gravitational: The
weakest force caused
by 2 factors, MASS
and DISTANCE
What is a Force?
• A force is an interaction between
TWO objects.
• For example, pushes and pulls are
forces.
What is a Force?
• Forces are vectors (they have both
magnitude and direction).
Adding Forces
• In one dimension, note the direction
using a + or – sign and then add like
scalar quantities (regular numbers
with no direction associated with
them).
Adding Forces
• Examples:
+3 N
+4 N
+
+
+3 N
-4 N
=
+6 N
= 0N
Newton's First Law of
Motion (Inertia)
• An object at rest tends to stay at
rest and an object in motion tends to
stay in motion with the same speed
and in the same direction unless
acted upon by a net outside force.
“Consider a body on which no
net force acts…”
• An important word here is NET. It
means “total” or “sum of all”
(forces).
Newton's First Law of
Motion (Inertia)
• Inertia is the tendency of an object
to resist changes in its velocity,
whether in motion or motionless.
The tendency of an object to
resist changes in its state of
motion (inertia) is dependent
upon its mass.
The more mass an object has,
the more inertia it has – the
greater the tendency it has to
resist changes in its state of
motion.
Newton’s 1st Law and You
Because of inertia, objects (including
you) resist changes in their motion. If a
car going 80 km/h is stopped by a brick
wall, your body keeps moving at
80 km/h.
Stationary and Moving Objects
Balanced and Unbalanced
Forces
• An object is said to be in equilibrium
when there is no unbalanced force
acting upon it.
Balanced and Unbalanced
Forces
• If the book is not at equilibrium, it
subsequently accelerates.
Balanced and Unbalanced Forces
For each
situation,
determine the
net force
(magnitude
and direction)
acting upon
the object.
D)515
N up
A) zero
C) 0
NB)
N
N left
Newton’s Second Law of Motion
• The acceleration of a body is directly
proportional to the net force on it and
inversely proportional to the mass.
 F  ma,
F
a
m
Newton’s Second Law of Motion
F
m a
• Unit of force = N (Newton)
• Unit of mass = kg (kilogram)
• Unit of acceleration = m/s2
Force Example Problem
What net force is required to accelerate
a 1500 kg car at 3.00 m/s2?
GIVEN:
F=?
m = 1500 kg
a = 3.00 m/s2
F
m a
WORK:
F=ma
F = (1500 kg) (3.00 m/s2)
F = 4500 N
Force Example Problem
Joe pushes a 12 kg box with a force of 10. N
pointing east while Jane applies a 15 N force
directed west. What is the rate of
acceleration of this box?
GIVEN:
WORK:
F = 10 N + -15 N a = F  m
m = 12 kg
a
=
(5
N)

(12
kg)
a=?
F
m a
a = - 0.42 m/s2
Force Example Problem
If a 134 N force accelerates mass by
6.5 m/s2, what is the object’s mass?
GIVEN:
F = 134 N
m=?
a = 6.5 m/s2
F
m a
WORK:
m=F a
m = (134 N)  (6.5 m/s2)
m = 21 kg
Force Problem #1
• A freight train slows down as it
approaches a train yard. If a
force of 3,800,000 N is
required to provide a
deceleration of 0.33 m/s2,
what is the train’s mass?
1.2 x 107 kg
Force Problem #2
• A 69,000,000 kg freight train
accelerates from rest to 11.0 m/s
in 57.6 s. What is the size of the
unbalanced force?
1.32 x 107 N
Force Problem #3
• Suppose an empty grocery cart
rolls downhill in a parking lot.
The cart has a maximum speed
of 1.3 m/s when it hits the side
of the store and comes to rest
0.30 s later. If an unbalanced
force of 65 N stops the cart, what
is the mass of the grocery cart?
15 kg
Force Problem #4
• A 4.0 kg shotput is thrown with
30 N of force. What is its
acceleration?
7.5 m/s2
Newton’s 2nd Law proves that different
masses accelerate to the earth at the
same rate, but with different forces.
Newton’s Third Law of Motion
• For every action, there is an equal
and opposite reaction OR for every
force, there is an equal and opposite
force.
• The action and reaction forces must
act on different objects.
Newton’s Third Law of Motion
• Problem:
 How can a horse
pull a cart if the cart
is pulling back on
the horse with an equal but
opposite force?
 Aren’t these “balanced
forces” resulting in no
acceleration?
NO!!!
Newton’s Third Law of Motion

Explanation:
• forces are equal and opposite
but act on different objects
• they are not “balanced forces”
• the movement of the horse
depends on the forces acting
on the horse
Newton’s Third Law of Motion
When you sit in
your chair, your
body exerts a
downward force
on the chair and
the chair exerts
an upward force
on your body.
Newton’s Third Law of Motion
Flying gracefully
through the air,
ravens depend on
Newton’s third law of
motion. As the raven
pushes down on the
air with its wings, the
air pushes its wings
up and gives the
raven lift.
Newton’s Third Law of Motion

The hammer
exerts a force
on the nail to
the right.

The nail exerts
an equal but
opposite force
on the hammer
to the left.
Newton’s Third Law of Motion
The baseball forces the bat to the
left. Identify the reaction.
The bat forces the baseball to the
right.
Newton’s Third Law of Motion
The bowling ball forces the pin to
the left. Identify the reaction.
The pin forces the bowling ball to
the right.
Newton’s Third Law of Motion
 The
rocket exerts a
downward force on
the exhaust gases.
The gases exert an
equal but opposite
upward force on the
rocket.
Newton’s Third Law of Motion
The enclosed air particles push the
balloon wall outwards. Identify the
reaction.
The balloon wall forces the air
particles inward.
Newton’s Third Law of Motion
• Action-Reaction Pairs

Both objects accelerate.

The amount of acceleration
depends on the mass of the object.
F
a 
m

Small mass  more acceleration

Large mass  less acceleration
Objectives
• PSc.1.2.1
• Explain how gravitational force affects
the weight of an object and the
velocity of an object in freefall.
Force of Gravity
• The force of gravity on an object
is called weight.
• At the Earth’s surface, gravity
exerts a force of 9.8 N on every
kilogram of mass.
Weight vs. mass
• Weight and mass are not the same.
• Mass is a fundamental property of
matter measured in kilograms (kg).
• Weight is a force measured in
Newtons (N).
• Weight depends on mass and
gravity.
Weight vs. mass
A 10-kilogram rock has the same mass
no matter where it is in the universe.
On Earth, the10 kg. rock weighs 98 N.
On the moon, the same rock only
weighs 16 N.
Force of Gravity
• The force of gravity, by Newton's
Second Law, is
Fg = m g
where g is the acceleration due to
gravity (9.8 m/s2).
Fg
m g
Force of Gravity
• When you measure weight in pounds
on a postal scale, you are measuring
the force of gravity acting on an
object.
Gravitational Force
Determine the gravitational force of 50.0 kg
of physical science textbooks.
GIVEN:
Fg = ?
m = 50.0 kg
g = 9.8 m/s2
Fg
m g
WORK:
Fg = m g
Fg = (50.0 kg) (9.8 m/s2)
Fg = 490 N
Gravitational Force
Determine the mass of an object that
weighs 1200 N.
GIVEN:
Fg = 1200 N
m=?
g = 9.8 m/s2
Fg
m g
WORK:
m = Fg ÷ g
m = (1200) ÷ (9.8 m/s2)
m = 122 kg
Gravitational Force
Determine the acceleration due to gravity
on Jupiter in which a 12 kg object exerts a
gravitational force of 300 N.
WORK:
GIVEN:
Fg = 300 N
m = 12
g=?
g = Fg ÷ m
g = (300) ÷ (12 kg)
g = 25 m/s2
Fg
m g
Gravitational Force Problem #1
• Determine the gravitational force
on a 1.25 kg object at rest on the
surface of the Earth.
12.3 N
Gravitational Force Problem #2
• A 20.0 kg block is pulled by along
a surface by a horizontal force of
at a constant speed. Determine
the weight of the block.
196 N
Gravitational Force Problem #3
• If the weight of a book is 14.0 N,
what is the mass of the book?
1.43 kg
Objectives
• PSc.1.2.2
• Classify frictional forces into one of
four types: static, sliding, rolling and
fluid.
Friction
• Friction is a force that opposes
motion, and so its vector always
points away from the direction of
movement.
Friction
• Friction is a
force that
resists the
motion of
objects or
surfaces.
• Many kinds of
friction exist.
Objects on Earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
Friction Opposes Motion
Force on box
by person
Force on floor
by box
Force on person
by box
Force on box
by floor
It’s the sum of all the forces that
determines the type of motion. 57
Friction
• Friction is greater...
• between rough
surfaces
• when there’s a
greater force between
the surfaces
(e.g. more weight)
Friction
Different Types of Friction
• As its name suggests, static friction
occurs when the body is not moving
(i.e. "static"). It is the force which
makes it difficult to start something
moving.
Different Types of Friction
• On the other hand, sliding (kinetic)
friction occurs when the body is in
motion. This is the force which causes
objects to slow down and eventually
stop.
Friction
• Static friction is always greater
than kinetic friction.
Friction
• Rolling friction is the resistance
that occurs when a round object
such as a ball, tire, or wheel rolls
on a surface.
Different Types of Friction
• Fluid friction is the friction that
occurs in fluids such as water, air,
and other liquids.
Different Types of Friction
• Air resistance is fluid friction
that occurs in air.
Terminal Velocity
• As an object
falls, the
downward
force of
gravity causes
the object to
accelerate.
Terminal Velocity
• However, as an object falls
faster, the upward force of air
resistance increases.
• This causes the net force on a
sky diver to decrease as the sky
diver falls.
Terminal Velocity
• Finally, the upward air
resistance force becomes large
enough to balance the
downward force of gravity.
Terminal Velocity
• This means the net force on the
object is zero.
• Then the acceleration of the object is
also zero, and the object falls with a
constant speed called the terminal
velocity.
Applied Force: Push and Pull
• A force which may act on an object
could be any physical push or pull, an
applied force.
• This could be caused by a person
pushing a crate on the floor, a child
pulling on a wagon or the wind
pushing on the ship.
Friction Problem #1
• The figure shows a block that
is being pulled along the floor.
According to the figure, what is
the acceleration of the block?
+2 m/s2
Friction Problem #2
• A 20.0 kg mass is pulled by along
a surface by a horizontal force of
at a constant speed. Friction is
20.0 N. What is the force applied
to the mass?
20.0 N
Friction Problem #3
• A 20.0 kg mass is pulled by along
a surface by a horizontal force of
100 N. Friction is 20.0 N. What
is the acceleration of the mass?
4.00 m/s2
Friction Problem #4
• A 49.0-N block is pulled by a
horizontal force of 50.0 N along a
rough horizontal surface at a
constant acceleration of
6.00 m/s2. What is the force of
friction?
20.0 N
Sample Friction Problem #5
A 10.0 kg box rests on a
horizontal floor. Static
friction is 39.2 N, and kinetic
friction is 29.4 N.
Determine the force of
friction acting on the box if a
horizontal external applied
force is exerted on it of
magnitude
(a) 0,
(Answer: (a) 0)
(b) 10. N,
(Answer: (b) 10. N)
(c) 20. N,
(Answer: (c) 20. N)
(d) 38 N, and
(Answer: (d) 38 N)
(e) 40. N
(Answer: (e) 29.4 N)