To describe how a force affects the
motion of an object.
 To interpret and construct free body
diagrams.
 To recognize Newton's laws of motion in
real life situations.

Intuitively, Force is any kind of push or pull on an object.
When you push a grocery cart, a
motor lifts an elevator, or a hammer
hits a nail, a force is being exerted.
If an object is at rest, to start it moving
requires force, that is a force is
needed to accelerate an object from
zero velocity to moving.
A force exerted in different directions
has a different effect. So force has a
direction as well as magnitude, and is
indeed a vector.
Every object continues in its state of rest, or of uniform velocity in a
straight line, as long as no net force acts on it.
The tendency of an object to maintain its
state of rest or uniform motion in a
straight line is called inertia.
http://www.youtube.com/watch?v=Q0Wz5P
0JdeU&feature=related
Watch the following video and then discuss it in groups.
Share your comments with the rest of the group
http://www.stevespanglerscience.com/experiment/egg-drop-inertia-trick
You may try this at home, if and
only if, you clean any mess you
make.
The acceleration of an object is directly proportional to the net force
acting on it, and is inversely proportional to its mass. The direction of
the acceleration is in the direction of the net force acting on the object.
𝐹
=𝑚
𝑎
http://www.youtube.com/watc
h?v=WzvhuQ5RWJE&featur
e=related
From Newton’s second law we can make a more precise definition of
force as an action capable of accelerating an object.
In SI units, the unit of force is called newton (N)
One Newton is the force required to impart an acceleration of 1 m/𝑠 2 to a
mass of 1kg.
Thus 1N=1kg* m/𝑠 2
Other units of force are the dyne (g*cm/𝑠 2 ) and pound (lb)
For next class please bring the following
materials per team:
• Tablecloth
• A full bottle of water (Closed)
• Unbreakable dinner plates, saucers (The
heavier the better)
• Spoons (metallic preferably)
Perform the magician’s tablecloth trick.
Explain your results according to Newton’s
first law of movement.
What average net force is required to bring a 1500 kg car to rest from a speed
of 100 km/h within a distance of 55m.
Hint: first use the kinematic equation: 𝑣 2 = 𝑣 2 𝑖 + 2𝑎(𝑥 − 𝑥𝑖 )
Solution: -1.1 x 104 N
Whenever an object exerts a force on a second object, the second
exerts an equal force in the opposite direction of the first.
This law is sometimes paraphrased
as “to every action there is an equal
and opposite reaction” This is
perfectly valid. But to avoid confusion
it is very important to remember that
the “action” force and the “reaction”
force are acting on different objects.
http://www.youtube.com/watch?v=cP0Bb3
WXJ_k&feature=related
In groups discuss:
• What exerts the force on a car?
• What makes the car go forward?
Share your results with the rest of the group.
http://www.youtube.com/watch?v=r8E5dUnLmh4&feature=related
The magnitude of the force of gravity on
an object is commonly called the
object’s weight.
An object’s weight will vary according to
the force of gravity effects on different
planets or in space.
For example de acceleration due to
gravity on the moon is about one sixth
the one on Earth.
The net force on an object at rest is zero according to Newton’s Laws. Therefore
the downward force of gravity on an object must be balances by an upward
force. When a contact force acts perpendicular to the common surface of
contact, it is referred to as the Normal Force.
When solving problems involving Newton’s
laws and force, it is important to draw a
diagram showing all the forces on each
object involved.
Such diagram is called a free-body diagram.
Choose one object, and draw an arrow to
represent each force acting on it. If the
problem involves more than one object, a
separate free-body diagram is needed for
each object.
Pull a 10.0 kg box with a cord as shown on the figure. The magnitude of the
force is 40.0 N and is exerted at a 30̊ angle. Calculate the acceleration of the
box, and the magnitude of the upward force 𝐹𝑁 exerted by the table on the
box.
Hint: Draw a free body
diagram like this:
Solve for components of the force pulling the box.
Now apply Newton’s Second Law in x to find the acceleration.
The acceleration of the box is 3.46 m/𝑠 2 to the right.
Next we solve for the vertical components to find 𝐹𝑁 .
Note that the normal force is lower than the weight because part of the pull of
the cord is in the upward direction.
Two boxes connected by a light cord (weightless) are resting on a frictionless
table. The boxes have a mass of 12 kg and 10 kg. A horizontal force of 40.0 N
is applied to the 10 kg box as shown. Find the acceleration of each box and
the tension in the cord connecting the boxes.
Acceleration = 1.82 m/𝑠 2 Tension = 21.8 N
Friction exists between two solid surfaces because of its roughness. Even the
smoothest looking surface is quite rough on a microscopic scale.
Sliding friction is usually called kinetic friction (kinetic is from the Greek for
moving).
When an object slides along a rough surface, the force of kinetic friction acts
opposite to the direction of the object’s velocity.
Experiments show that the friction force is approximately proportional to the
normal force between the two surfaces. In many cases depends very little on
the surface of contact so we can write the proportionality as an equation
inserting a constant of proportionality.
So Kinetic Friction:
𝑭𝒇𝒓 = 𝝁𝒌 𝑭𝑵
The term 𝜇𝑘 is called the coefficient of kinetic friction, and its value depends
on the nature of the two surfaces.
There is also static friction, which refers to a force parallel to the two surfaces
that can arise even when they are not sliding.
Suppose you are pushing a desk and it does not move. You are exerting a
force on the desk, but it isn’t moving so there must be another force on the
desk keeping it from moving. (The net force is zero on an object that doesn’t
move). This is the force of static friction exerted by the floor on the desk.
If you push hard enough, the desk will eventually move, and kinetic friction
takes over.
So Static Friction:
𝑭𝒇𝒓 ≤ 𝝁𝒔 𝑭𝑵
Pull a 10.0 kg box with a cord as shown on the figure. The magnitude of the
force is 40.0 N and is exerted at a 30̊ angle. Calculate the acceleration of the
box. This time assume a coefficient of kinetic friction of 0.30.
Solution: 1.1 m/𝑠 2
The inclined plane is one of the original six simple machines. It is a flat
surface whose endpoints are at different heights. By moving the object up the
plane rather than vertically the amount of force needed is reduced at the
expense of increasing the distance the object is moved.
www.youtube.com/watch?v=EgXOVGjIeyI
Solving problems is usually easier if we chose the xy coordinate system so
the x axis points along the incline and the y axis is perpendicular to the
incline
as
shown
in
the
figure.
Remember that the normal force is not vertical but perpendicular to the
sloping surface of the plane, yet weight is indeed vertical and pointing
down.
The skier in the figure has just begun descending the 30̊ slope. Assuming
the coefficient of kinetic friction is 0.10, calculate (a) her acceleration and
(b) the speed she will reach after 4.0 s.
Solution: a= 4 m/𝑠 2 V=16 m/s
Watch the following video about static friction:
www.youtube.com/watch?v=jSu0Tvlm6LY&feature=related
www.youtube.com/watch?v=QMW_uYWwHWQ&feature=related
Then answer the questions.
1. What are the wooden clamps for?
2. In the first tug-o-war Adam and Jaime find that the main problem
is traction, explain why?
3. What is the scale measuring?
4. What was the problem with using cars to pull the interleaved
books?
5. What was the equivalent force needed to separate the books
equivalent to?
6. Can you think of another real life situation where friction plays an
important role?
• Giancoli , Douglas C. Physics Sixth Edition. USA Pearson 2005
• Serway, Raymond A. Essentials of College Physics. USA Thomson 2007