Transcript Force

Chapter 5 Forces
(Forces in One Dimension)
Objectives for Section 5.1
Describe how force affects the motion of an object.
Identify different types of forces.
Interpret and construct free-body diagrams.
Explain the relationship between the motion of an
object and the net external force acting on the
Calculate the net force.
State Newton’s three laws of motion and how they
are applied.
Use Newton’s Second Law to calculate the
acceleration of an object.
A. Forces and Motion - study of dynamics
1. Force - a push or a pull it can change the motion
of an object, start or stop movement, and change
shape of object
2. Four basic types
a. gravitational - weakest, attractive
force between objects, acts over
very large distances
b. electromagnetic - results from basic property of
particles. Large compared to gravitational, but over
smaller distances
c. strong nuclear forces - holds nucleus together, limited
in range to diameter of nucleus
d. weak nuclear forces - deals with radiation – alpha,
beta, gamma
3. forces act & cause things to occur
4. Forces can be in contact or act over distances field forces (long-range forces)
a. contact forces – an object from the external
world touches a system and exerts a force on it
b. field forces – an object is pushed or pulled by a
force without actual touching (gravity, magnetic
or electrostatic force)
Force and Motion
What is a force?
How does it affect the motion of the object it
acts on?
A push or pull on an object (system)
It changes it’s motion or shape (starts, stops,
changes motion, it causes acceleration)
System: the object being manipulated
Agent: cause of the force
Types of Forces
Contact force
Example: Your book laying on the
Field force
Example: You drop your textbook
onto the floor.
Free-body Diagrams
Balanced & Unbalanced Forces
With a Balanced force – opposite and equal forces acting on the
same object result in NO motion of the object
Unbalanced forces – two or more forces of unequal strength or
direction acting upon on an object results in the motion of the
A. Force
Balanced Forces (Equilibrium)
forces acting on an object
that are opposite in
direction and equal in size
no change in velocity
A. Force
Net Force
 unbalanced
forces that are not opposite and
 velocity changes (object accelerates)
Newton’s Laws of
“If I have seen far, it is because I have stood on the
shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)
A. Newton’s First Law
Newton’s First Law of Motion
An object at rest will remain at rest and an
object in motion will continue moving at a
constant velocity unless acted upon by a net
Newton’s 3 Laws of Motion
Newton’s 1st Law of Motion:
AKA The Law of Inertia
which states an object at rest will remain at
rest, and an object in motion will remain in
motion at a constant velocity until acted on by
another force.
The greater the mass of
an object the greater the
Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.
Newton’s Second Law
The acceleration of an object is equal to the sum
of the forces acting on the object (the net force)
divided by the mass of the object.
F = ma
Newton's Second Law:
a. F = m a but more easily understood
b. Acceleration is directly proportional to
force and inversely proportional to mass of
an object
c. Second law is a vector equation - direction
of acceleration is the same direction as the
net force
d. Greater the force, the greater is the
acceleration the mass experiences
Unit of Force is the Newton (N)
One newton is the force required to give a
mass of one kilogram an acceleration of
one meter per second squared.
1 N = 1 kg-m/s
(a derived unit)
Net Force
 Sum
of all forces acting on an
 Equilibrium (when the net
forces balance or equal zero)
 Can you have equilibrium when
an object is moving?
Combine all forces acting on object to
determine the net force acting on the object
(1) sum of all forces is the net force.
(2) net force can replace all forces acting on
object and have the same result
(3) forces added using vector math – more later
on the process
(4) net force will have magnitude and direction
– critical to remember
(5) Net force - combination of all forces acting
on an object
Calculating Net Force
Check Your Understanding
1. What acceleration will result when a 12 N net force applied to a
3 kg object?
12 N = 3 kg x 4 m/s/s
2. A net force of 16 N causes a mass to accelerate at a rate of 5
m/s2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
3. How much force is needed to accelerate a 66 kg skier 1
66 kg-m/sec/sec or 66 N
4. What is the force on a 1000 kg elevator that is falling freely at
9.8 m/sec/sec?
 9800 kg-m/sec/sec or 9800 N
Newton’s Third Law
Newton’s Third Law of Motion
When one object exerts a force on a second
object, the second object exerts an equal but
opposite force on the first.
 The magnitudes of the forces are always equal.
The two forces are know as action-reaction
forces or action-reaction pairs.
Newton’s 3 Laws of Motion
Newton’s 3rd Law of Motion:
For every action there is an equal & opposite reaction.
If an object is not in motion, then all forces acting on it are balanced and the
net force is zero!
Friction – the force that one surface exerts on another when the two rub
against each other.
Sliding friction
Fluid friction
Rolling friction
5.1 Concept Review
You exert a force on a black box and measure its
acceleration and then exert the same force on a brown
box and find its acceleration is three times greater.
What can you conclude?
Mass of the brown box is 1/3 that of the black box
What is a Newton?
 Force require to accelerate 1 kg by 1m/s2
 1 N = 1 kg-m/s
How can you feel the inertia of a pencil or your
text book?
By changing it’s motion (accelerating it)
Objectives for Section 5.2
Describe the relationship between the mass and
weight of an object using Newton’s 2nd Law
Demonstrate an understanding of frictional
forces and the use of coefficients of friction
Be able to calculate acceleration based on net
Define free fall and terminal velocity due to air
Weight Force
Using Newton’s Second Law you can derive the
formula for weight force.
Mass and Weight
a. weight - due to gravitational force
(1) w = m g (F = m a)
direction is downward
b. mass - amount of matter in an object.
c. gravitational mass
(a) measured using a balance to
compare weights of two objects
(b) unknown mass on one side, known
mass on the other
d. Inertial mass: measured by the force is required
to accelerate the mass: m = F/a
e. weight is a vector, mass is a scalar
Friction is the name given to the force that acts
between materials that touch as they move past
each other.
• Friction is caused by the irregularities in
the surfaces of objects that are touching.
• Even very smooth surfaces have
microscopic irregularities that obstruct
• If friction were absent, a moving object
would need no force whatever to remain in
More on Friction……………
1. Force that opposes motion between
two surfaces in contact.
2. Amount depends on:
a. Kinds of surfaces in contact – this
determines the coefficient of friction ()
b. Amount of force pressing surfaces
together – the normal force
(More weight more friction)
3. Expressed as Ff = FN
3. Friction is caused by microwelds
4. Types of friction:
a. Static (usually the greatest)
b. Sliding
c. Rolling (usually the least)
d. Fluid Friction (air or water resistance)
Drag Force
The friction force exerted by a fluid on the
object moving through the fluid.
Example: Air resistance
Air resistance (drag force)
1. Force that opposes
motion of objects through air
2. Pushes up on falling
3. Affected by object’s
speed, size, shape
4. Without drag force, all objects
fall at the same rate
5. Terminal velocity is the max
speed at which an object can fall
Objects with similar air resistance fall at the
same rate.
Everything falls at the same rate of speed in a
 That rate is the gravitational constant.
On earth (-9.8 m/sec²)
Video! Falling Objects, Gravity, Air Resistance,
on the moon with Apollo.
Terminal Velocity
The constant velocity that is reached when the
drag force equals the force of gravity.
What reaches a terminal velocity faster, a heavy,
compact object or a light object with larger
surface area?
Objectives for Section 5.3 Net Forces
Identify forces acting on an object and calculate
net forces
Determine acceleration based on the net force
Find the direction and magnitude of normal
B. Using Newton’s Second Law
1. Free body diagrams - critical for solving
a. Sketch object under consideration
b. Draw and label all external forces acting on object
c. Assume a direction for each force. If your
selection ends up negative(-) means it goes the
other way
Combine all forces acting on object to
determine the net force acting on the object
(1) sum of all forces is the net force.
 (2)
net force can replace all forces acting on object and
have the same result
 (3)
forces added using vector math
 (4)
net force will have magnitude and direction – critical
to remember
 (5)
Net force - combination of all forces acting on an
Tension Force
Practice Problem
A 50.0 kg bucket is being lifted by a rope. The
rope will not break if the tension is 525 N or
less. The bucket started at rest, and after being
lifted 3.0 m, it is moving at 3.0 m/s. If the
acceleration is constant, is the rope in danger of
breaking? Remember Fg = mg = 50kg(9.8m/s2 )= 490N
Fnet = Ftension – Fg or Ftension = Fnet + Fg
Fnet = ma; use Vf2 = Vi2+2ad or a = Vf2-Vi2/2d
a = (3m/s)2-(0m/s)2/2(3m) = 1.5m/s2
Fnet = 50kg(1.5m/s2) = 75N
Ftension = Fnet + Fg = 75N + 490N = 565 N, yes it is in danger!
The Normal Force
Weight & Normal Force
In which figure is the box’s weight equal to the
normal force in magnitude?
In which figure is the magnitude of the normal
force greater than the weight of the box?
The magnitude of the normal force is greater
than the weight of the box in Figure b.
Are mass and gravity the only factors that
contribute to the normal force of an object?
The weight of the box and the magnitude of the
normal force are equal in Figure a.
External forces other than gravity and the mass
of the object may change the normal force that
an object exerts.
In which figure (or figures) does the box have
an apparent weight different from that caused
by its mass and the effect of gravity alone?
The box’s apparent weight is different from the
weight caused by its mass and gravity in Figures
b and c.
Practice Problem
Poloma hands a 13 kg box to a 61 kg Stephanie,
who stands on a platform. What is the normal
force exerted by the platform on Stephanie?
FN = Fg(Steph) + Fg(box)
FN = (m(Steph)+ m(box))g
FN = (13kg+ 61kg)9.8m/s2
FN = 725 kgm/s2 or 725 N
2. Scales (Elevator Problems)
a. weight on scale is from the normal force
of the scale pushing back up on the object
which is pushing down due to gravity
b. with elevator at rest, a = 0 and Fnet = 0
Fnet = 0 = FN - W
FN = W
Scales reading is
normal force true weight of
c. elevator moving up so “a” is positive and the
Fnet =m a
Fnet = m a = FN - W
FN = m a + W
Scales reads an
apparent weight – not
true weight but net force
normal force
acting on object.
d. elevator moving down, “a” is negative and the
Fnet = -ma
normal force
Fnet = -ma = FN - W
FN = - m a + W
Practice Problem
Your mass is 75.0 kg, and you are standing on a
bathroom scale in an elevator. Starting from rest, the
elevator accelerates upward at 2.00 m/s2 for 2.00 s and
then continues at a constant speed. Is the scale reading
during acceleration greater than, equal to, or less than the
scale reading when the elevator is at rest?
W = mg = 75kg(9.8m/s2) = 735N,
at rest Fnet = 0 so FN= W or FN = Fscale = 735N
But during upward acceleration………
Fnet = ma = FN - W or FN = ma+W= 75kg(2m/s2)+(735N)
So FN = Fscale= 885N (It reads greater during upward acceleration!)
Newton’s Third Law
All forces come in pairs and the forces in a pair
act on different objects and are equal in strength
and opposite in direction
Interaction Forces
Action - Reaction
“For every action there’s an
equal but opposite reaction.”
If you hit a tennis ball with a racquet, the
force on the ball due to the racquet is the
same as the force on the racquet due to the
ball, except in the opposite direction.
If you drop an apple, the Earth pulls on the
apple just as hard as the apple pulls on the
If you fire a rifle, the bullet pushes the rifle
backwards just as hard as the rifle pushes the
bullet forwards.
Practice Problem
When a softball with a mass of 0.18 kg is
dropped, its acceleration toward Earth is equal
to g, the acceleration due to gravity. What is the
force on Earth due to the ball, and what is
Earth’s resulting acceleration? Earth’s mass is
6.0 x 1024 kg.
Fball = ma = .18kg(9.8m/s2) = 1.76N
Fball = FEarth (Action-Reaction Pair)
FEarth = 1.76N = ma;
a = 1.76N/mEarth
a = 1.76N/6.0 x 1024 kg = 2.94 x 10-25 m/s2
Earth / Apple
How could the forces on the tennis ball, apple, and
bullet, be the same as on the racquet, Earth, and rifle?
The 3rd Law says they must be, the effects are different
because of the 2nd Law!
0.40 kg
3.92 N
3.92 N
5.98  1024 kg
A 0.40 kg apple weighs 3.92 N
(W = mg). The apple’s weight
is Earth’s force on it. The
apple pulls back just as hard.
So, the same force acts on
both bodies. Since their
masses are different, so are
their accelerations (2nd Law).
The Earth’s mass is so big, it’s
acceleration is negligible.
Earth / Apple
The products are the same, since the forces are the same.
little mass
Apple’s big
big mass
Earth’s little
Lost in Space
Suppose an International Space Station
astronaut is on a spacewalk when her tether
snaps. Drifting away from the safety of the
station, what might she do to make it back?
Fhand on bowling ball is the force that the
hand exerts upward on the bowling ball.
Fbowling ball on hand is the force that Earth
exerts downward on the bowling ball.
Fbowling ball on Earth is the force that the
bowling ball exerts upward on Earth.
Fhand on bowling ball and Fbowling ball on hand;
FEarth on bowling ball and Fbowling ball on Earth.
are interaction pairs because they are of
equal magnitude and opposite direction
and act on different objects.
Fbowling ball on hand acts only on the hand,
Fbowling ball on Earth acts only on Earth, and
Fhand on bowling ball and FEarth on bowling ball act
only on the bowling ball.
The movement of the ball is due to
unbalanced forces on it, not the
balanced force of interaction pairs that
act on each object.
Due to the 3rd Law, when you swim you push the water
(blue), and it pushes you back just as hard (red) in the
forward direction. The water around your body also
produces a drag force (green) on you, pushing you in the
backward direction. If the green and red cancel out, you
don’t accelerate (2nd Law) and maintain a constant velocity.
Note: The blue vector is a force on the water, not the on
swimmer! Only the green and red vectors act on the swimmer.
Demolition Derby
When two cars of
different size collide,
the forces on each are
the SAME (but in
opposite directions).
However, the same
force on a smaller car
means a bigger