Chapter 3 Force, Mass and Acceleration Newton’s Laws

Sir Isaac Newton An English Mathematician and Physicist that formulated the three laws of motion, law of universal gravitation and invented calculus before the age of 30 An English Mathematician and Physicist that formulated the three laws of motion, law of universal gravitation and invented calculus before the age of 30 Newton’s discoveries helped to answer many questions such as: what causes tides, how do the planets move and why do objects of different masses fall at the same rate? Newton’s discoveries helped to answer many questions such as: what causes tides, how do the planets move and why do objects of different masses fall at the same rate?

What is Force? Force is a push, pull or any action that has the ability to change motion (This doesn’t mean that a force will always make something move– can you think of a situation such as this?) Force is a push, pull or any action that has the ability to change motion (This doesn’t mean that a force will always make something move– can you think of a situation such as this?) There are two units that are commonly used to measure force, Newtons and Pounds. Scientists prefer Newtons(N) There are two units that are commonly used to measure force, Newtons and Pounds. Scientists prefer Newtons(N)

Force vs. Mass The Newton relates to force and motion. A force of 1 Newton causes a 1 kg mass to have an acceleration of 1 m/s 2 The Newton relates to force and motion. A force of 1 Newton causes a 1 kg mass to have an acceleration of 1 m/s 2 We call it a Newton because that is somewhat easier to say than “1 kg times 1 m/s 2 ” We call it a Newton because that is somewhat easier to say than “1 kg times 1 m/s 2 ” There are 4.48 Newtons per pound There are 4.48 Newtons per pound

Newton’s First Law Known as the Law of Inertia Known as the Law of Inertia “An object at rest will stay at rest until acted upon by an unbalanced force, an object in motion will continue with constant speed and direction until acted upon by an unbalanced force” “An object at rest will stay at rest until acted upon by an unbalanced force, an object in motion will continue with constant speed and direction until acted upon by an unbalanced force” Basically – An object at rest remains at rest and an object in motion remains in motion unless acted on by an unbalanced force. Basically – An object at rest remains at rest and an object in motion remains in motion unless acted on by an unbalanced force.

Inertia Objects ability to resist changing its state of motion Objects ability to resist changing its state of motion Inertia is dependant on mass, therefore an object with more mass would have a greater inertia Inertia is dependant on mass, therefore an object with more mass would have a greater inertia Example: Which is harder to move, a bowling ball or a beach ball? The ball with the greater mass is harder to move because it has greater inertia Example: Which is harder to move, a bowling ball or a beach ball? The ball with the greater mass is harder to move because it has greater inertia

Newton’s 2 nd Law Relates the applied force to an object’s mass and its acceleration Relates the applied force to an object’s mass and its acceleration It says that force causes acceleration, mass resists acceleration and the acceleration is a ratio of force over mass It says that force causes acceleration, mass resists acceleration and the acceleration is a ratio of force over mass Common sense: If something has more mass, it would take more force to cause the object to accelerate Common sense: If something has more mass, it would take more force to cause the object to accelerate

Using Newton’s 2 nd Law F = ma F M A An object’s Force equals its mass times its acceleration

2 nd Law A car engine produces a force and you can calculate the acceleration of the car when you know this force and the mass of the car A car engine produces a force and you can calculate the acceleration of the car when you know this force and the mass of the car If you want to increase the acceleration of the car you must decrease the cars mass or increase the force of the engine If you want to increase the acceleration of the car you must decrease the cars mass or increase the force of the engine

Balanced vs. Unbalanced Forces The motion of an object depends on the total of all forces acting on it – Net force The motion of an object depends on the total of all forces acting on it – Net force If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement changing because the net force is zero If the forces are balanced(net force =0), then the object is in equilibrium and there is no chance of movement changing because the net force is zero If there is a net force, then the forces are unbalanced, and movement changes. If there is a net force, then the forces are unbalanced, and movement changes.

Weight vs. Mass Mass is the amount of matter in an object, usually measured in kilograms Mass is the amount of matter in an object, usually measured in kilograms Weight is the force of gravity acting on a certain mass, measured in Newtons or pounds Weight is the force of gravity acting on a certain mass, measured in Newtons or pounds A certain mass of something has a “weight”, which is the force acting on that mass. It varies depending on the gravity. A certain mass of something has a “weight”, which is the force acting on that mass. It varies depending on the gravity.

Gravity, Mass and Weight Imagine that you travel to the moon. Which of these will be the same as on earth: Gravity, your mass or your weight? Imagine that you travel to the moon. Which of these will be the same as on earth: Gravity, your mass or your weight? An object will have the same mass on the moon as on earth – it still has the same amount of matter An object will have the same mass on the moon as on earth – it still has the same amount of matter An object weighs less on the moon but has the same mass. It weighs less because there is less gravity. An object weighs less on the moon but has the same mass. It weighs less because there is less gravity. You would weigh more on Jupiter than Earth because Jupiter has more gravity than Earth You would weigh more on Jupiter than Earth because Jupiter has more gravity than Earth

Gravity A force that pulls every mass toward another mass A force that pulls every mass toward another mass Earth is the biggest mass for us so other masses are pulled toward earth’s center Earth is the biggest mass for us so other masses are pulled toward earth’s center On earth, every kilogram of mass weighs 9.8 Newtons On earth, every kilogram of mass weighs 9.8 Newtons

You can figure out the weight by multiplying the mass times the acceleration of gravity You can figure out the weight by multiplying the mass times the acceleration of gravity F= m X a F= m X a Weight is a force (pulled to earth’s center) Weight is a force (pulled to earth’s center) The acceleration due to gravity on Earth is 9.8 m/s 2 The acceleration due to gravity on Earth is 9.8 m/s 2

Gravity, Cont. On Earth, every object will fall at the same rate (not counting air friction) On Earth, every object will fall at the same rate (not counting air friction) The Acceleration of gravity is 9.8 m/s 2 meaning that every second, a falling object accelerates 9.8 m/s The Acceleration of gravity is 9.8 m/s 2 meaning that every second, a falling object accelerates 9.8 m/s In other words, every second something is falling it is moving 9.8 m/s faster In other words, every second something is falling it is moving 9.8 m/s faster If you drop a bowling ball and a match box car at the same time, they will hit the ground at the same time If you drop a bowling ball and a match box car at the same time, they will hit the ground at the same time

Newton’s Third Law “For every action, there is an equal and opposite reaction” “For every action, there is an equal and opposite reaction” There must be a PAIR of objects There must be a PAIR of objects The action and reaction act on separate objects The action and reaction act on separate objects If you press down on the table, you are exerting a force on it. Is the table exerting a force on your hand? If you press down on the table, you are exerting a force on it. Is the table exerting a force on your hand? Your force on the table is action force, the table’s force on you is reaction force Your force on the table is action force, the table’s force on you is reaction force

3 rd Law Skateboard – foot on ground is action – earth against skateboard is reaction Skateboard – foot on ground is action – earth against skateboard is reaction Throw ball on a skateboard Throw ball on a skateboard Recoil – backward acceleration from reaction force Recoil – backward acceleration from reaction force

Law of Universal Gravitation The force of attraction (gravity) between 2 objects depends on the mass of the objects and the distance between them The force of attraction (gravity) between 2 objects depends on the mass of the objects and the distance between them More noticable between large objects (planets and stars) More noticable between large objects (planets and stars) Force = G x mass 1 x mass 2 Force = G x mass 1 x mass 2 d 2 d 2 G = 6.67 x 10-11

Friction Forces that work against the motion of the object Forces that work against the motion of the object Force of friction is always in the opposite direction of the motion Force of friction is always in the opposite direction of the motion Must consider when finding net force Must consider when finding net force Will cause motion to stop eventually unless force is continually supplied Will cause motion to stop eventually unless force is continually supplied

Types of Friction Air Friction – caused by air moving around objects Air Friction – caused by air moving around objects Sliding Friction – two objects sliding against one another Sliding Friction – two objects sliding against one another Rolling Friction – one object rolling over another object Rolling Friction – one object rolling over another object Viscous Friction – object moving in water or fluid Viscous Friction – object moving in water or fluid