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 F g = 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 (F N ) The normal force and the force of gravity will be equal and opposite in magnitude F g = F N, or F g + F N = 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/s 2 ?

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 (F f ) - 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 F f = μF N 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 F f = μ k F N Kinetic - moving

Newton’s 3rd Law of Motion It is harder to overcome friction when an object is stationary This is called static friction F f = μ s F N 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/s 2 ?

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?