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## 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 or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.
• Define frictional force, f, as the force that opposes the motion of an object and which acts parallel to the surface.
Define static frictional force, fs, as the force that opposes the tendency of motion of a stationary object relative to a surface.
Define kinetic frictional force, fk, as the force that opposes the motion of a moving object relative to a surface.

Know that a frictional force:
• Is proportional to the normal force
• Is independent of the area of contact
• Is independent of the velocity of motion

• Solve problems using:

is the maximum static frictional force and:
us
is the coefficient of static friction.

• NOTE:
• If a force, F, applied to a body parallel to the surface does not cause the object to move, F is equal in magnitude to the static frictional force.
• The static frictional force is a maximum

just before the object starts to move across the surface.
• If the applied force exceeds
,
a resultant/net force accelerates the object.

• Solve problems using:

where fk is the kinetic frictional force and uk the coefficient of kinetic friction.

### Force diagrams, free-body diagrams

• Draw force diagrams
• Draw free-body diagrams. (This is a diagram that shows the relative magnitudes and directions of forces acting on a body/particle that has been isolated from its surroundings)
• Resolve a two-dimensional force (such as the weight of an object on an inclined plane) into its parallel (x) and perpendicular (y) components.
• Determine the resultant / net force of two or more forces.

### 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.
• 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.
• Draw force diagrams and free-body diagrams for objects that are in equilibrium or accelerating.
• Apply Newton's laws of motion to a variety of equilibrium and non-equilibrium problems including:
• A single object:
- Moving on a horizontal plane with or without friction
- Moving on an inclined plane with or without friction
- Moving in the vertical plane (lifts, rockets, etc.)
• Two-body systems (joined by a light inextensible string):
- Both on a flat horizontal plane with or without friction
- One on 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
• State Newton's third law of motion: When one body exerts a force on a second body, the second body exerts a force of equal magnitude in the opposite direction on the first body.
• Identify action-reaction pairs.
• List the properties of action-reaction pairs.

### Newton's Law of Universal Gravitation

• State Newton's Law of Universal Gravitation: Each body in the universe attracts every other body with a 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:
• Calculate acceleration due to gravity on a planet using:
• Describe weight as the gravitational force the Earth exerts on any object on or near its surface.
• Calculate weight using the expression:
w = mg.
• Calculate the weight of an object on other planets with different values of gravitational acceleration.
• Distinguish between mass and weight.
• Explain weightlessness.