Force: a push or pull acting on a body that causes or tends to cause a change in the linear motion of the body Characteristics of a force magnitude direction point of application. line of action Net Force: resultant force (overall effect of multiple forces acting on a body) Example: push from side and front = at angle Contraction of muscles Do not accelerate the body Classifying Forces Units of Force Friction (external force) Characteristics of a force Dynamic friction

1. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Affiliated G.G.S.IP.U, Delhi
YUGSHAKTI
ASSISTANT PROFESSOR(IT)
PHYSICS
Paper Id: 20109
Paper code: BCA 109
Topic: CLASSIFICATION OF FORCES

2. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Force
– Force: a push or pull acting on a body that causes
or tends to cause a change in the linear motion of
the body
• Characteristics of a force
– magnitude
– direction
– point of application.
– line of action
– Net Force: resultant force (overall effect of
multiple forces acting on a body)
• Example: push from side and front = at angle

3. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Classifying Forces
• Internal Force: acts within the object or system
whose motion is being investigated
– action / reaction forces both act on different parts
of the system
• tensile-internal pulling forces when the structure is under tension
• compressive- internal pushing (squeezing) forces act on the ends
of an internal structure
– do not accelerate the body
• Orientate segments, maintain structural integrity

4. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Internal Forces
• Examples
– Contraction of muscles
– Do not accelerate the body

5. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Classifying Forces
 External Force: acts on object as a result of
interaction with the environment surrounding it
non-contact - occur even if objects are not
touching each other
 gravity, magnetic
contact - occur between objects in contact
 fluid (air & water resistance)
 reaction forces with another body (ground, implement)
 vertical (normal) reaction force
 acts perpendicular to bodies in contact
 shear reaction force
 acts parallel to surfaces in contact (friction)

6. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
F = ma
• Force may also be defined as the product of a
body's mass and the acceleration of that body
resulting from the application of the force.
• Units of force are units of mass multiplied by
units of acceleration.

7. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Units of Force
• Metric system (systeme internationale -SI)
– Newton (N)
• the amount of force necessary to accelerate a mass
of 1 kg at 1 m/s2
• English system
– pound (lb)
• the amount of force necessary to accelerate a mass
of 1 slug at 1 ft/s2
• equal to 4.45 N

8. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Weight (external force)
• Weight - the amount of gravitational force
exerted on a body. wt=mag.
• Since weight is a force, units of weight are
units of force - either N or lb.
• As the mass of a body increases, its weight
increases proportionally.

9. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Weight
• The factor of proportionality is the
acceleration of gravity, which is
-9.81m/s2 or- 32 ft/s2.
• The negative sign indicates that the
acceleration of gravity is directed downward
or toward the center of the earth.

10. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Weight
• On the moon or another planet with a
different gravitational acceleration, a body's
weight would be different, although its mass
would remain the same.
– Space station example

11. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Weight
• Because weight is a force, it is also
characterized by magnitude, direction, and
point of application.
• The direction in which weight acts is always
toward the center of the earth.

12. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Friction (external force)
• Component of a contact force that acts
parallel to the surface in contact
– acts opposite to motion or motion tendency
– reflects interaction between molecules in
contact
– reflects force “squeezing” surfaces together
– acts at the area of contact between two
surfaces

13. • Static friction: surfaces not moving relative to each
other
– Maximum static friction: maximum amount of friction
that can be generated between two static surfaces
• Dynamic friction: surfaces move relative to each
other
– constant magnitude friction during motion
• always less than maximum static friction
Friction

14. F =  N
Friction depends on
Nature of materials in contact ()
Force squeezing bodies together (N)
Known as the normal contact
(reaction) force
Friction is FUN

15. –Nature of materials in contact
•Coefficient of friction ()
–value serves as an index of the
interaction between two surfaces in
contact.
N
F

Coefficient of Friction ()

16. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Factors Affecting Friction
• The greater the coefficient of friction, the
greater the friction
• The greater the normal contact force, the
greater the friction

17. • Normal reaction force (NRF)
– AKA - normal contact force
– force acting perpendicular to two surfaces in contact.
– magnitude intentionally altered to increase or
decrease the amount of friction present in a
particular situation
• football coach on sled
• push (pull) upward to slide object
• pivot turn on ball of foot
Reaction Force

18. TRINITY INSTITUTE OF PROFESSIONAL STUDIES
Sector – 9, Dwarka Institutional Area, New Delhi-75
Coefficient of Friction
• Hot, soft, rough surfaces have higher
coefficients of friction
– Tires, concrete, etc
• Cold, hard, smooth surfaces have lower
coefficients of friction
– Ice, marble, etc