Newton’s Laws of Motion Chapters 2,3,6,7 Forces Newton’s Laws of Motion Chapters 2,3,6,7

Forces A push or pull that could cause a change in motion Measured in Newtons (N) kg∙m/s2 Many Types Tension Weight Friction Normal

Concurrent Forces Two concurrent forces acting on an object….. Two forces acting concurrently…… Means the forces are acting on the same object at the same time

Newton’s 1st Law (N1L) An object at rest will stay at rest unless acted upon by an external force An object in motion will remain in motion unless acted upon by an external force.

Law of Inertia (N1L) Inertia Inertia Mass Property of objects to resist changes in motion Mass is the measure of inertia Inertia Mass

Law of Inertia (N1L) More mass, more inertia, less likely to change motion Less mass, less inertia, more likely to change motion

Law of Equilibrium (N1L) The sum of all the forces acting on an object equals zero. Object may be at rest or moving at constant velocity

Example Box at rest on a table

Example Force of table pushing up equals weight of box pushing down Force of table pushing up – weight of box = 0

Normal Force, FN Force exerted by a surface whenever an object is in contact with a surface Normal force is always perpendicular to the surface

Force Diagram Also called Free Body Diagram Shows all forces acting on an object Arrows originate on object and point in the direction that the force is acting Weight Table Pushing Up

Example A 50N chandelier hangs from ceiling by a single cord. Draw a free body diagram and determine the tension in the cord. ΣF = 0 Tension, T T - w =0 T = w Weight, w T = 50N

Another Example ΣF = 0 T – w – F =0 T = w + F T = 50N + 20N T = 70N A 50N object hangs from ceiling by a single cord. A student pulls down on the object by 20N. Draw a free body diagram and determine the tension in the cord. ΣF = 0 T T – w – F =0 T = w + F w = 50N F = 20N T = 50N + 20N T = 70N

Newton’s 2nd Law (N2L) The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the mass of the object

N2L

N2L Example A 5kg box is being pulled by a 10N force. What is the acceleration of the box? 5kg F = 10N

N2L Example A 5kg box is being pulled by 2 forces as shown. What is the acceleration of the box? F1 = 10N F2 = 5N 5kg

Weight, w Force due to gravity, Fg

Friction, Ff Force that opposes motion. Always in the direction opposite to motion Friction v

Types of Friction Static Friction Kinetic Friction Friction when an object is stationary, not moving Must be overcome for an object to start moving Kinetic Friction Friction when an object is moving Always less than static friction

Static Friction A student exerts a force of 100N against a crate resting on the floor. If the crate does not move, what is the force of friction? Ff F=100N

Kinetic Friction A student exerts a force of 90N to keep a crate sliding across a level floor at a constant speed. What is the force of friction? v Ff F=90N

Another Example A 2000kg car is accelerating at a rate of 3m/s2. If the engine is exerting a force of 8000N, what is the force of friction? What type of friction is this? Ff F=8000N a

Another Example Kinetic Friction Ff F=8000N a

One More Example A 75kg skydiver is falling at his terminal velocity. What is the force of the air resistance? What type of friction is this? Ff w = mg

One More Example Ff Kinetic Friction w = mg

Friction What factors affect friction? Type of material Weight Ff F

Friction Why does surface area not matter? FN Ff F Fg

Normal Force, FN Force exerted by a surface whenever an object is in contact with a surface Normal force is always perpendicular to the surface FN Fg

Friction The frictional force is due to a surface acting on an object The frictional force is dependent on the force exerted by the surface The frictional force is proportional to the Normal Force

Friction Ff=μFN Coefficient of Friction, μ Factors in the type of material/surface Two Types μs, coefficient of static friction μk, coefficient of kinetic friction

Coefficient of Friction,μ

Example If it takes a force of 50N to start a 100N crate moving across a level floor, what is the coefficient of friction? FN Ff F=50N Fg=100N

Example If it takes a force of 25N to keep a 100N crate moving across a level floor at constant speed, what is the coefficient of friction? FN Ff F=25N Fg=100N

One More Example A block of copper sliding on steel requires a force of 25N to keep it moving across a level floor at constant speed, what is the weight of the block? FN Ff F=25N Fg

One More Example FN Ff F=25N Fg

Newton’s 3rd Law (N3L) For every action there is an equal and opposite reaction Forces occur in Action/Reaction pairs

N3L When an object exerts a force on a second object, the second object exerts a force on the first object that is equal in magnitude and opposite in direction Must be two objects acting on each other

N3L Example A Chandelier hanging from a ceiling Wire pulls up on chandelier Chandelier pulls down on wire

N3L Example

N3L Example

N3L Example Action: Man pushes stroller Reaction: Stroller pushes man

Forces Review A student pushes a crate across a floor at a constant speed. What forces are acting on the crate? FN v Ff F Fg