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PHYSICS LECTURE F – 1: INTRO TO FORCES
FORCES & FREE-BODY DIAGRAMS
FORCE
 A vector quantity
 An action exerted on an object which may change the object’s state of rest or motion
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NEWTON
 SI unit of force
 The amount of force that , when acting on a 1 kg mass, produces an acceleration of 1 m/s2
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FORCE CLASSIFICATION
 Contact forces: the force exerted when two physical objects come in direct contact with each
other.
 Field forces: forces that do not involve physical contact between two objects.
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FORCE DIAGRAMS AND FREE-BODY DIAGRAMS
 Force is a vector
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In a force diagram, vector arrows represent all the forces acting in a situation
A free-body diagram shows only the forces acting on the object of interest
o Do not include forces that the object exerts on other objects
For now, assume all forces act at the center of an object – all forces are drawn as if they act at
the point, no matter where the force is applied
PHYSICS LECTURE F – 1: INTRO TO FORCES
SAMPLE PROBLEM A
A truck pulls a trailer on a flat stretch of road. The forces acting on the trailer are the force due
to gravity (250, 000 N downward), the force exerted by the road (250,000 N upward), and the
force exerted by the cable connecting the trailer to the truck (20,000 N to the right). The forces
acting on the truck are the force due to gravity (80,000 N. downward), the force exerted by the
road (80,000 N upward), the force exerted by the cable (20,000 N left), and the force causing the
truck to move forward (26,400 N. to the right).
a. Draw and label a free-body diagram of the trailer.
b. Draw and label a free-body diagram of the truck.
c. Draw and label a free-body diagram of the truck & trailer system.
PHYSICS LECTURE F – 1: INTRO TO FORCES
EQUILIBRIUM
 Equilibrium: the state in which the forces on an object are balanced.
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Equilibrium: The state in which the net force on an object is zero.
o The net force acting on a body in equilibrium must be equal to zero.
o NET FORCE: The vector sum of all forces acting on an object. (or) A single
force whose external effects on a rigid body are the same as the effects of several
actual forces acting on the body.
o Determining Net Force
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PHYSICS LECTURE F – 1: INTRO TO FORCES
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A body in equilibrium cannot/will not accelerate.
FORCES BALANCED?
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO
YES
NO