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?