1. Forces can be either internal or external. Internal forces act within an object between its parts, while external forces act on the object as a whole from outside. 2. The main types of internal forces are tension, compression, torsion, and shear. Only internal forces can change the form of energy within a system without changing the total energy. 3. External forces can change the total energy of a system by doing work on it. Common external forces include gravity, applied forces, wind forces, and contact forces between objects. The interaction of internal and external forces determines an object's motion or stability.

2. • A push or pull
•Force has a size & direction
•All motion is created by a force
•All forces are exerted by one
object on another
•It has both magnitude & direction,
therefore it is a vector quantity.

3. •Measured in Newtons (N)
•The more Newtons,the greater the
force
•One tool to measured the the force
of gravity is the spring scale.

4. Forces are also described as a push
or pull on an object. They can be due to
phenomena such as gravity, magnetism,
or anything that might cause a mass to
accelerate.
Common Symbol = F
SI unit = newton
Derivations from = F=ma
other quantities

5. 1.External Forces
2.Internal Forces

6. EXTERNAL FORCES
• External forces include
the applied force, normal
force, tension force,
friction force, and air
resistance force
• Fapp
• Ffrict
• Fair
INTERNAL FORCES
• Internal forces include
the gravity forces,
magnetic force, electrical
force, and spring force
• Fgrav
• Fspring

7. Examples of External & Internal
Forces
1. A baseball. The forces holding the particles of baseball together are
internal forces; the forces of a bat hitting the ball is an external
force on the ball ; so is gravity.
2. A car . The parts of the car exert forces on each other, the pistons
push the rods, the axle pushes the wheels , etc. All these forces are
internal to the car as a whole. But the friction that the ground
exertion the tires is an external force. So I the weight of the car,
and the force the ground exerts to hold up the car.

8. INTERNAL FORCES
• When the only type of force doing net work upon an object is an
internal force (for example, gravitational and spring forces), the
total mechanical energy (KE + PE) of that object remains constant.
• For example, as an object is "forced" from a high elevation to a
lower elevation by gravity, some of the potential energy of that
object is transformed into kinetic energy. Yet, the sum of the
kinetic and potential energies remains constant.
• When the only forces doing work are internal forces, energy
changes forms - from kinetic to potential (or vice versa); yet the
total amount of mechanical is conserved. Because internal forces
are capable of changing the form of energy without changing the
total amount of mechanical energy, they are sometimes referred to
as conservative forces.

9. • Internal forces act between
diff erent parts of the same
structure. There are four types
of internal forces: tension,
compression, torsion, and
shear.

10. Figure 4 The main cables in a
suspension bridge, have tension
forces acting on them
Tension
When you pull on an elastic band,
the force of your fi nger pulling on
the elastic band is an external force.
This force creates an internal force
called tension, which causes all of
the particles of the elastic band to
pull apart. Tension can act on a
variety of objects, for example, a
stretched skipping rope, a
trampoline, an electrical power
line, guitar strings, and the cables of
a suspension bridge.

11. Compression
• An object that is pressed or squeezed experiences
compression.
• Compression is an internal force that presses the particles of
an object together. The springs inside a mattress undergo
compression when you lie down on the mattress.
Compression also occurs when you kick a soccer ball, step on
the sole of your shoe, or lay your head on a foam cushion.
Compressed objects usually return to their original shape aft
er the external force is removed.

12. Figure :These mattress springs have
compression forces acting on them.

13. Torsion
It acts in an object when the object is
twisted .
Torsion is evident when a skater twists
in a jump, a washcloth is
wrung out, and a doorknob is turned.
Torsion can be created when
both ends of a structure are twisted.
Torsion can also be created when
only one end of a structure is twisted
while the other end remains
stationary .
Figure : This figure skater is
experiencing torsion force as she
twists her body in a spin .

14. Shear
Shear forces occur when forces push or
pull in opposite directions within an
object. Shear forces usually result in an
object being bent, torn apart, or cut. A
strong wind that is blowing horizontally
against a tree anchored to the ground
causes shear forces inside the tree.
These forces can cause it to bend or
break . Scissors use shear
force to cut paper in half. Th e blades of
the scissors move in opposite directions
and create two pushing forces against
the paper, which results in the paper
being cut.
Figure : Shear forces
inside the trees may cause
them to snap in a strong wind.

15. EXTERNAL FORCES
• When net work is done upon an object by an external force,the
total mechanical energy(KE+PE) of that object is changed.
• If the wok is positive work, then the object will gain the energy.
• If the work is negative work, then the object will lose the energy.
• The gain or loss in energy can be in the form of potential energy, kinetic
energy, or both.
• Under such circumstances, the work that is done will be equal to the
change in mechanical energy of the object
• Because external forces are capable of changing the total mechanical
energy of an object, they are sometimes referred to as nonconservative
forces

16. The most obvious external force acting on
structures is gravity. On Earth, gravity
always acts downward. Gravity is a non-
contact force. Non-contact forces are those
applied to an object by another object not
in contact with it. Applied forces, or
contact forces, also act on an object from
the outside. You apply external forces
when you push a swing, pull an elastic, or
throw a ball. External forces on buildings
include wind, earthquakes, the weight of
people on the floors of the building, and
the weight of the building itself. A structure
is designed so that external forces will not
cause it to break or fall over.
In Figure: The
magnitude and direction
of the applied force are
the same.

17. Another important external force is the force in the
direction opposite to gravity. Think of the forces on
you when you are sitting on a stool. You know that
the force of gravity on you (your weight) is an
external force that pulls you downard.