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Physical science notes grade 12
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- Physical Sciences
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- 12th Grade
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CURRICULUM FETPHYSICAL SCIENCES
LEARNER BOOKLET
TERM TWO
BACK ONTRACK PROGRAMME
GRADE 12
,WCED Back on Track Tutoring Gr 12 Page 2 of 28
SESSION ONE: 6 MAY 2023
TOPIC: NEWTON’S LAW
Newton's laws and application of Newton's laws
(This section must be read in conjunction with the CAPS, p. 62–66.)
Different kinds of forces: weight, normal force, frictional force, applied force (push, pull),
tension (strings or cables)
• Define normal force, N, as the force which a surface exerts on an object in contact with it,
and which is perpendicular to the surface.
NOTE: The normal force acts perpendicular to the surface irrespective of whether the
plane is horizontal or inclined. For horizontal planes the only forces perpendicular to the
plane should be the weight, w, and the normal force, N. All other forces should be
parallel to the plane. For inclined planes the only forces perpendicular to the plane is the
component of weight, wcosθ, and the normal, N. All other forces should be parallel to the
plane.
• Define frictional force, f, as the force that opposes the motion of an object and which acts
parallel to the surface.
Know that a frictional force:
o Is proportional to the normal force
o Is independent of the area of the surfaces that are in contact with each other
• Define the static frictional force, fs, as the force that opposes the tendency of motion of a
stationary object relative to a surface. The static frictional force can have a range of
values from zero up to a maximum value, μsN. If a force, F, applied to an object parallel
to the surface, does not cause the object to move, F is equal in magnitude to the static
frictional force.
State that the static frictional force is a maximum, fSmax just before the object starts to
move across the surface. The maximum static frictional force, fSmax , is equal to the
magnitude of the maximum horizontal force that can be applied to the object without it
starting to move across the surface.
• Solve problems using fSmax = μs N where fSmax is the maximum static frictional force and μs
is the coefficient of static friction. If the applied force exceeds fSmax, a net force accelerates
the object.
• Define the kinetic frictional force, fk, as the force that opposes the motion of a moving
object relative to a surface. The kinetic frictional force on an object is constant for a given
surface and equals μkN.
• Solve problems using fk = μkN, where fk is the kinetic frictional force and μk the
coefficient of kinetic friction.
Force diagrams, free-body diagrams
• Draw force diagrams. In a force diagram the force is represented by an arrow. The
direction of the arrow indicates the direction of the force and the length of the arrow
indicates the magnitude of the force.
• Draw free-body diagrams. Such a diagram shows the relative magnitudes and directions
of forces acting on an object that has been isolated from its surroundings. The object is
drawn as a dot and all the forces acting on it are drawn as arrows pointing away from the
dot. The length of the arrows is proportional to the magnitude of the respective forces.
• Resolve a two-dimensional force, e.g. the weight of an object on an inclined plane, into
its parallel (F//) and perpendicular ( F⊥ ) components.
• Determine the resultant/net force of two or more forces.
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, WCED Back on Track Tutoring Gr 12 Page 3 of 28
Newton's first, second and third laws of motion
• State Newton's First Law of motion: A body will remain in its state of rest or motion at
constant velocity unless a non-zero resultant/net force acts on it.
• Define inertia as the resistance of an object to any change in its state of motion. The
mass of an object is a quantitative measure of its inertia.
• Discuss why it is important to wear seatbelts using Newton's first law of motion.
• State Newton's Second Law of motion: When a resultant/net force acts on an object, the
Object will accelerate in the direction of the force at an acceleration directly proportional
to the force and inversely proportional to the mass of the object.
In symbols: a α Fnet , constant m and
a α 1/m constant Fnet , and therefore a = Fnet/m and Fnet = ma
• Draw force diagrams and free-body diagrams for objects that are in equilibrium or
accelerating.
• Apply Newton's Second Law of motion, and therefore, to a variety of equilibrium and
non-equilibrium problems including:
o A single object:
- Moving in a horizontal plane with or without friction
- Moving on an inclined plane with or without friction
- Moving in the vertical plane (lifts, rockets, etc.)
o Two-body systems (joined by a light inextensible string):
- Both on a flat horizontal plane with or without friction
- One in a horizontal plane with or without friction, and a second hanging
vertically from a string over a frictionless pulley
- Both on an inclined plane with or without friction
- Both hanging vertically from a string over a frictionless pulley
NOTE: When an object accelerates, the equation Fnet = ma must be applied
separately in the x and y directions. If there is more than one object, a
free-body diagram must be drawn for each object and Newton's second law
must be applied to each object separately.
• Newton’s Third Law of motion When object A exerts a force on object B, object B
SIMULTANEOUSLY exerts an oppositely directed force of equal magnitude on object A.
(The forces are therefore an interaction between two bodies.)
• Identify Newton III force pairs (action-reaction pairs) and list the properties of the force
pairs (action-reaction pairs). When identifying the forces it must be clearly stated which
body exerts a force on which body, and what kind of force it is, e.g. the earth exerts a
downward gravitational force on the object, and the object exerts an upward gravitational
force of equal magnitude on the earth.
Newton's Law of Universal Gravitation
• State Newton's law of universal gravitation: Each particle in the universe attracts every
other particle with a gravitational force that is directly proportional to the product of their
masses and inversely proportional to the square of the distance between their centres.
• Solve problems using F = Gm1m2
r2
• Calculate acceleration due to gravity on the Earth using g = GME
RE2
and on another planet g = GMp Mp is the mass of the planet and
Rp2 rp is the radius of the planet.
Copyright Reserved
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