Forces and the Laws of Motion
Changes in Motion
A force is an action exerted on an object
which may change the object’s state of
rest or motion.
Forces can cause accelerations.
A force exerted on an object can change the
object’s velocity with respect to time.
Throwing a baseball
Catching a baseball
Hitting a baseball
The SI unit of force is the newton, N.
The newton is defined as the amount of
force that, when acting on a 1 kg mass,
produces an acceleration of 1 m/s2.
1 N = 1 kg x 1 m/s2
1 lb = 4.448 N
1 N = 0.225 lb
Forces can act through contact or at a distance.
Contact forces are the result from the physical contact
between two objects.
Pull on a spring it stretches
Pull a wagon it moves
Push a door it opens
Catch a football it stops
Field forces do not involve the physical contact between
Attraction or repulsion of electrical charges
The effect of a force depends on both magnitude and
direction. Thus, force is a vector quantity.
If you push a toy car it accelerates, if you push harder the
acceleration will be greater. (magnitude)
Diagrams that show force vectors as arrows are called
Book uses blue arrows to show forces
At this point, disregard the size and sape of objects and
assume that all forces act at the center of an object, no
matter where the force is applied.
Force diagrams that show only the forces
acting on a single object are called free-
Force Diagram vs. Free-Body
Force Diagram Free-Body Diagram
In a force diagram, A free-body diagram
vector arrows represent shows only the forces
all the forces acting in a acting on the object of
situation. interest—in this case, the
A person is pulling someone on a sled.
Draw a free-body diagram for the sled.
The magnitude of the forces acting on the
sled are 60 N by the string, 130 N by the
Earth (gravity), and 90 N upward by the
A truck pulls a trailer on a flat stretch of road.
The forces acting on the trailer are 250 000 N
due to gravity, 250 000 N from the road, and 20
000 N to the right from the cable connecting the
trailer to the truck. The forces acting on the
truck are 80 000 N due to gravity, 80 000 N by
the road, 20 000 N to the left exerted by the
cable, and 26 400 N to the right causing the
truck to move forward.
Draw and label a free-body diagram of the trailer and
Newton’s First Law
Newton’s First Law
An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity (that is, constant speed in a straight
line) unless the object experiences a net
In other words, when the net external force on
an object is zero, the object’s acceleration (or
the change in the object’s velocity) is zero.
Inertia is the tendency of an object not to
accelerate, or if it is moving to resist a
change in speed or direction.
Newton's first law is also called the law of
Since in the absence of a net force, a
body will preserve its state of motion.
Newton's first law refers to the net force on
an object. The net force is the vector sum of
all forces acting on an object.
The net force on an object can be found by
using the methods for finding resultant vectors.
Although several forces are
acting on this car, the vector
sum of the forces is zero. Thus,
the net force is zero, and the car
moves at a constant velocity.
The vector Fforward represents the
forward force of the road on the
The vector Fresistance represents
friction and air resistance and is
in the opposite direction.
The vector Fgravity represents the
downward force of gravity.
The vector Fground-on-car represents
the upward force of the road on
Bowling Ball vs. Basketball
Both side by side on the ground.
Supply a net force by pushing each with equal
Inertia is proportional to the object’s mass.
Therefore, mass, which is a measure of the
amount of matter in an object, is also a measure
of the inertia of an object.
Derek leaves his physics book on top of a
drafting table that is inclined at a 35° angle. The
free-body diagram below shows the forces
acting on the book. Find the net force acting on
Select a coordinate system, and apply it to the
Choose the x-axis parallel to and the y-axis perpendicular to the
incline of the table, as shown in (a). This coordinate system is
the most convenient because only one force needs to be resolved
into x and y components.
Tip: To simplify the problem, always
choose the coordinate system in
which as many forces as possible lie
on the x- and y-axes.
Find the x and y components of
Draw a sketch, as shown in (b), to help
find the components of the vector Fg. The
angle q is equal to 180– 90 – 35 = 55.
Fg, x Fg, y
cos q sin q
Fg, x Fg cos q Fg, y Fg sin q
Fg, x (22 N)(cos 55) Fg, x (22 N)(sin 55)
Fg, x 13 N Fg, x 18 N
Add both components to the free-body diagram, as shown in (c).
Find the net force in both the x
and y directions.
Diagram (d) shows another free-body
diagram of the book, now with forces
acting only along the x- and y-axes.
For the x direction: For the y direction:
SFx = Fg,x – Ff SFy = Ft – Fg,y
SFx = 13 N – 11 N SFy = 18 N – 18 N
SFx = 2 N SFy = 0 N
Find the net force.
Add the net forces in the x and y directions together as
vectors to find the total net force. In this case, Fnet = 2
N in the +x direction, as shown in (e). Thus, the book
accelerates down the incline.
Your Turn II
An agricultural student is designing a support to keep a
tree upright. Two wires have been attached to the tree
and placed at right angles to each other. One wire
exerts a force of 30.0 N on the tree and the other 40.0
N. Determine where to place a third wire and how much
force it should exert so that the net force acting on the
tree is zero.
A gust of wind blows an apple from a tree. As the apple
falls, the gravitational force on the apple is 2.25 N
downward, and the force of the wind on the apple is
1.05 N to the right. Find the magnitude and direction of
the net force of the apple.
Inertia and the Operation of a Seat
While inertia causes passengers
in a car to continue moving
forward as the car slows down,
inertia also causes seat belts to
lock into place.
The illustration shows how one
type of shoulder harness
When the car suddenly slows
down, inertia causes the large
mass under the seat to continue
moving, which activates the lock
on the safety belt.
Equilibrium is the state in which the net force on an
object is zero.
Objects that are either at rest or moving with constant
velocity are said to be in equilibrium.
Newton’s first law describes objects in equilibrium.
Tip: To determine whether a body is in equilibrium, find the net
force. If the net force is zero, the body is in equilibrium. If there is a
net force, a second force equal and opposite to this net force will
put the body in equilibrium.
Newton’s Second and Third
Newton’s Second Law
Pushing a car on a flat surface.
By yourself acceleration will be small.
With friends acceleration will be much
Acceleration is directly proportional to the
force acting on the object.
Mass of the object also affects
Newton’s Second Law
The acceleration of an object is
directly proportional to the net force
acting on the object and inversely
proportional to the object’s mass.
SF = ma
net force = mass acceleration
SF represents the vector sum of all external forces
acting on the object, or the net force.
Roberto and Laura are studying across
from each other at a wide table. Laura
slides a 2.2 kg book toward Roberto. If
the net force acting on the book is 1.6 N
to the right, what is the book’s
SF = ma
a=1.6 N/2.2 kg = 0.73 m/s2 to the right
Your Turn III
The net force on the propeller of a 3.2 kg model
airplane is 7.0 N forward. What is the
acceleration of the airplane?
The net force on a golf cart is 390 N north. If
the cart has a total mass of 270 kg, what are
the magnitude and direction of the cart’s
A soccer ball kicked with a force of 13.5 N
accelerates at 6.5 m/s2 to the right. What is the
mass of the ball.
Newton’s Third Law
If two objects interact, the magnitude of the force
exerted on object 1 by object 2 is equal to the
magnitude of the force simultaneously exerted on object
2 by object 1, and these two forces are opposite in
In other words, for every action, there is an equal
and opposite reaction.
Because the forces coexist, either force can be called the
action or the reaction.
Action and Reaction Forces
Action-reaction pairs do not imply that the net force on
either object is zero.
The action-reaction forces are equal and opposite, but
either object may still have a net force on it.
Consider driving a nail into wood with
a hammer. The force that the nail
exerts on the hammer is equal and
opposite to the force that the hammer
exerts on the nail. But there is a net
force acting on the nail, which drives
the nail into the wood.
The gravitational force (Fg) exerted on an
object by Earth is a vector quantity, directed
toward the center of Earth.
The magnitude of this force (Fg) is a scalar
quantity called weight.
Weight changes with the location of an object in
Calculating weight at any location:
Fg = mag
ag = free-fall acceleration at that location
Calculating weight on Earth's
ag = g = 9.81 m/s2
Fg = mg = m(9.81 m/s2)
Television resting on a table.
Gravity is acting on the TV.
Why doesn't the TV continue to fall to the
center of the Earth?
There must be an equal and opposite
force acting on the TV to oppose gravity.
This is called normal force.
The normal force acts on a surface in a direction
perpendicular to the surface.
The normal force is not always opposite in direction to
the force due to gravity.
In the absence of other forces,
the normal force is equal and
opposite to the component of
gravitational force that is
perpendicular to the contact
In this example, Fn = mg cos q.
Picture a gallon of milk at rest on a table.
Net force equals zero.
Newton’s second law tells us if we apply a force
horizontally to the milk it will accelerate and
remain in motion unless acted on by another
Experience tells us if we push the mike it will
move, but stop almost immediately.
The other force is FRICTION!
Static friction is a force that resists the
initiation of sliding motion between two surfaces
that are in contact and at rest.
Kinetic friction is the force that opposes the
movement of two surfaces that are in contact
and are sliding over each other.
Kinetic friction is always less than the
maximum static friction.
Friction Forces in Free-Body
In free-body diagrams, the force of friction is
always parallel to the surface of contact.
The force of kinetic friction is always opposite
the direction of motion.
To determine the direction of the force of static
friction, use the principle of equilibrium. For an
object in equilibrium, the frictional force must
point in the direction that results in a net force
The Coefficient of Friction
The quantity that expresses the dependence of
frictional forces on the particular surfaces in
contact is called the coefficient of friction, m.
Coefficient of kinetic friction: mk
Coefficient of static friction: Fs,max
If the value of m and the normal force are
known, then the magnitude of the force of
friction can be calculated directly.
Ff = m Fn
Coefficients of Friction
A 24 kg crate initially at rest on a
horizontal floor requires a 75 N horizontal
force to set it in motion. Find the
coefficient of static friction between the
crate and the floor.
Fs,max = 75 N
Fn = mg (24 kg x 9.81 m/s2)
Plug and chug
ms = 75 N / (24 kg x 9.81 m/s2)
ms = 0.32
Your Turn IV
Once the crate in the previous problem is in motion, a
horizontal force of 53 N keeps the crate in moving with a
constant velocity. Find μk between the crate and the
A museum curator moves artifacts into place on various
different display surfaces. Use the values in Table 2 on
page 138 to find the Fs, max and Fk for the following
Moving a 145 kg aluminum sculpture across a horizontal steel
Pulling a 15 kg steel sword across a horizontal steel shield.
Pushing a 250 kg wood bed on a horizontal wood floor.
Sliding a 0.55 kg glass amulet on a horizontal glass display case.
A student attaches a rope to a 20.0 kg box of
books. He pulls with a force of 90.0 N at an
angle of 30.0° with the horizontal. The
coefficient of kinetic friction between the box
and the sidewalk is 0.500. Find the acceleration
of the box.
Draw a diagram
Choose a convenient
coordinate system, and find
the x and y components of all
Fapplied,y = (90.0 N)(sin 30.0º) = 45.0 N (upward)
Fapplied,x = (90.0 N)(cos 30.0º) = 77.9 N (to the right)
Choose an equation or situation:
A. Find the normal force, Fn, by applying the
condition of equilibrium in the vertical direction:
SFy = 0
B. Calculate the force of kinetic friction on the box:
Fk = mkFn
C. Apply Newton’s second law along the horizontal
direction to find the acceleration of the box:
SFx = max
A. To apply the condition of equilibrium in the vertical
direction, you need to account for all of the forces in
the y direction:
Fg, Fn, and Fapplied,y. You know Fapplied,y and can use
the box’s mass to find Fg.
Fapplied,y = 45.0 N
Fg = (20.0 kg)(9.81 m/s2) = 196 N
Tip: Remember to
Next, apply the equilibrium condition, pay attention to the
SFy = 0, and solve for Fn. direction of forces.
SFy = Fn + Fapplied,y – Fg = 0 In this step, Fg is
subtracted from Fn
Fn + 45.0 N – 196 N = 0 and Fapplied,y
Fn = –45.0 N + 196 N = 151 N because Fg is
B. Use the normal force to find the force of kinetic
Fk = mkFn = (0.500)(151 N) = 75.5 N
C. Use Newton’s second law to determine the
SFx Fapplied Fk max
Fapplied, x Fk
77.9 N 75.5 N 2.4 N 2.4 kg m/s 2
m 20.0 kg 20.0 kg 20.0 kg
a = 0.12 m/s2 to the right
Your Turn V
A student pulls on a rope attached to a box of books and
moves the box down the hall. The student pulls with a
force of 185 N at an angle of 25.0° above the horizontal.
The box has a mass of 35.0 kg, and μk between the box
and the floor is 0.27. Find the acceleration of the box.
A 75 kg box slides down a 25.0° ramp with an
acceleration of 3.60 m/s2.
Find the μk between the box and the ramp.
What acceleration would a 175 kg box have on this ramp?
A box of books weighing 325 N moves at a constant
velocity across the floor when the box is pushed with a
force of 425 N exerted downward at an angle of 35.2°
below the horizontal. Find the μk between the box and
Air resistance is a form of friction. Whenever
an object moves through a fluid medium, such
as air or water, the fluid provides a resistance to
the object’s motion.
For a falling object, when the upward force of
air resistance balances the downward
gravitational force, the net force on the object is
zero. The object continues to move downward
with a constant maximum speed, called the
There are four fundamental forces:
Strong nuclear force
Weak nuclear force
The four fundamental forces are all field