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Newton’s 3rd Law of Motion
By: Heather Britton
Newton’s 3rd Law of Motion
• Newton’s 3rd Law of Motion states
• Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first
Newton’s 3rd Law of Motion
• This law is also known as action/reaction
• It is important to remember that the forces act on different objects
Newton’s 3rd Law of Motion
• Gravity causes objects to accelerate and is therefore a force
• The force caused by gravity is called weight
• Fg = mg
• Weight is equal to the mass multiplied by the acceleration of gravity
Newton’s 3rd Law of Motion
• Gravity does not go away when an object is at rest (sitting on a table)
• When two objects are in contact vertically there is a contact force
• One object pushes down on the other object
Newton’s 3rd Law of Motion
• The other object pushes up on the first object
• This is called the normal force (FN)
• The normal force and the force of gravity will be equal and opposite in magnitude
• Fg = FN, or Fg + FN = 0 (no acceleration)
Newton’s 3rd Law of Motion
• Example 1
• What is the weight of a 90 kg person on Earth?
Newton’s 3rd Law of Motion
• What is the weight of a 50 kg person on Mars where the acceleration of gravity is 3.7 m/s2?
Newton’s 3rd Law of Motion
• Forces can be applied at angles like velocity
• Trig functions can be used to break forces at an angle into components along axes
• The pythagorean theorem may also be used
Newton’s 3rd Law of Motion
• Force of friction (Ff) - a force that opposes motion
• A value of zero is frictionless
• A value greater than the applied force means no acceleration
Newton’s 3rd Law of Motion
• An object sliding past another object has a value of friction based on a percent of the friction force and the force needed to move the object
• This value is called the coefficient of friction (μ)
• μ - the Greek letter mu
Newton’s 3rd Law of Motion
• The force of friction is the product of the normal force and the coefficient of friction
• Ff = μFN
• Both forces are measured in Newton’s therefore μ has no units
Newton’s 3rd Law of Motion
• It is easier to overcome friction when an object is moving
• This is called kinetic friction
• Ff = μkFN
• Kinetic - moving
Newton’s 3rd Law of Motion
• It is harder to overcome friction when an object is stationary
• This is called static friction
• Ff = μsFN
• Static - not moving
Newton’s 3rd Law of Motion
• Example 3
• A smooth wood block is placed on a wood table top. You must exert a force of 14 N to keep the 40 N block moving at constant velocity. What is the coefficient of kinetic friction? If a 20 N brick is placed on the block what force will be required to keep it moving at constant velocity
Newton’s 3rd Law of Motion
• Example 4
• A spring scale hangs from the ceiling of the elevator. It supports a package that weight 25 N. What upward force does the scale exert when the elevator is not moving?
Newton’s 3rd Law of Motion
• Example 4 (cont.)
• What force must the scale exert when the object and elevator accelerate upward at 1.5 m/s2?
Newton’s 3rd Law of Motion
• Example 5
• The handle of a lawn mower makes a 45° angle with the lawn. If a person applies 50 N of force to the handle how much of that force actually propels the lawn mower?
Newton’s 3rd Law of Motion
• Example 5 (cont.)
• If the lawn mower has a mass of 25 kg, what is the normal force exerted by the lawn?
• What is the normal force if the person pulls the mower?
Newton’s 3rd Law of Motion
• Example 6
• A sign that weighs 168 N is supported by two ropes, a and b, that each make a 22.5° with the horizontal. The sign is not moving. What force does each rope exert on the sign?