FORCE
FORCE OF PULL EXERTED BY AN OBJECT ON ANOTHER
CONTACT FORCE
•FORCE BETWEEN TWO BODIES THAT ARE IN DIRECT
CONTACT WITH EACH OTHER
•EX. FRICTION
NON-CONTACT FORCE
• FORCE THAT ACTS EVEN IF THE
INTERACTING BODIES ARE SEPARATED
BY A DISTANCE
• EX. GRAVITATIONAL FORCE,
ELECTROSTATIC FORCE, MAGNETIC
FORCE
Sir Isaac Newton (1643-1727)
an English scientist and mathematician
famous for his discovery of the law of
gravity also discovered the
three laws of motion.
Today these laws are known as
Newton’s Laws of Motion and describe
the motion of all objects on the scale we
experience in our everyday lives.
LAW OF INERTIA
•NEWTON'S FIRST LAW STATES THAT A BODY AT REST WILL
REMAIN AT REST, AND A BODY IN MOTION WILL REMAIN IN
MOTION AT A CONSTANT VELOCITY IN A STRAIGHT LINE
UNLESS ACTED UPON BY AN EXTERNAL FORCE.
WHAT IS MEANT BY
UNBALANCED FORCE?
INERTIA: THE TENDENCY OF AN OBJECT TO RESIST
CHANGES IN ITS STATE OF MOTION
THE FIRST LAW STATES THAT ALL OBJECTS HAVE
INERTIA. THE MORE MASS AN OBJECT HAS, THE
MORE INERTIA IT HAS (AND THE HARDER IT IS TO
CHANGE ITS MOTION).
LAW OF ACCELERATION
•NEWTON’S SECOND LAW STATES THAT IF A NET EXTERNAL FORCE ACTS
ON A BODY, THE BODY ACCELERATES.
• THE DIRECTION OF ACCELERATION IS THE SAME AS THE DIRECTION OF
THE NET FORCE. THE MASS OF THE BODY TIMES THE ACCELERATION OF
THE BODY EQUALS THE NET FORCE VECTOR.
Force equals mass times acceleration.
F = ma
Force is measured in
Newtons (N) = mass (kg) x acceleration (m/s2)
Or
kg m/s2
An unbalanced force causes something to accelerate.
A force can cause motion only if it is met with an unbalanced force.
Depends on the net force acting on the object
Net force (Fnet): The sum total and direction of all forces acting on the
object.
Net forces: Always cause acceleration.
Balanced forces cause no
acceleration.
Unbalanced forces
cause acceleration.
WHAT DOES F = MA MEAN?
Force is directly proportional to mass and acceleration.
Small Force
= Small Acceleration
F
a
F
Small acceleration
Large Mass
a
Large acceleration
F
a
Acceleration is INVERSELY related to the mass of the object.
Solving Newton Second Law Problems
1. Draw a free body diagram
2.Break vectors into components if needed
3.Find the NET force by adding and subtracting forces that are on
the same axis as the acceleration.
4.Set net force equal to “ma” this is called writing an EQUATION OF
MOTION.
NOTE: To avoid negative numbers, always subtract the smaller
forces from the larger one. Be sure to remember which direction
is larger.
• A 50 N APPLIED FORCE DRAGS AN 8.16 KG LOG TO THE RIGHT ACROSS A
HORIZONTAL SURFACE. WHAT IS THE ACCELERATION OF THE LOG IF THE FORCE OF
FRICTION IS 40.0 N?
• An elevator with a mass of 2000 kg rises with an acceleration
of 1.0 m/s/s. What is the tension in the supporting cable?
QUESTIONS
• SUPPOSE THAT THE ACCELERATION OF AN OBJECT IS ZERO. DOES THIS MEAN
THAT THERE ARE NO FORCES ACTING ON IT?
• WHEN A BASKETBALL PLAYER DRIBBLES A BALL, IT FALLS TO THE FLOOR AND
BOUNCES UP.
• IS A FORCE REQUIRED TO MAKE IT BOUNCE? WHY?
• IF A FORCE IS NEEDED, WHAT IS THE AGENT.
LAW OF INTERACTION
•NEWTON’S THIRD LAW STATES THAT WHEN TWO BODIES INTERACTS,
THEY EXERT FORCES ON EACH OTHER THAT AT EACH INSTANT ARE
EQUAL IN MAGNITUDE AND OPPOSITE IN DIRECTION.
•THESE FORCES ARE CALLED ACTION-REACTION FORCES.
•EACH OF THESE TWO FORCES ACTS ON ONLY ONE OF THE TWO BODIES;
THEY NEVER ACT ON THE SAME BODY.