# Newtons Laws of Motion Content Standards SPS 8

- Slides: 36

Newton’s Laws of Motion Content Standards SPS 8. Students will determine relationships among force, mass, and motion. b. Apply Newton’s three laws to everyday situations by explaining the following: -Inertia -Relationship between force, mass and acceleration -Equal and opposite forces

Newton’s Laws of Motion Objectives: • Differentiate between balanced and unbalanced forces • Draw free-body diagrams for objects at rest and in motion • State and apply Newton’s first law of motion • Write Newton’s second law using appropriate units for mass, force, and acceleration. • Demonstrate your understanding of the distinction between mass and weight. • Apply Newton’s second law to problems involving one or more bodies in constant acceleration • State and illustrate examples of Newton’s third law of motion.

Newton’s Laws of Motion ESSENTIAL QUESTIONS 1. Why do different objects respond differently to equal forces? 2. What is the difference between balanced and unbalanced forces

Force • A push or a pull • Measured in Newtons (N) • Net force is the total of all forces acting on an object • When net force = 0 the forces are balanced • When net force ≠ 0 the forces are unbalanced

Force cont. • Unbalanced forces CAN change motion • Balanced forces CANNOT change motion • Inertia - the tendency of an object to resist any change to it’s motion

Force • BALANCED FORCESMINI DEMONSTRATION

Force-balanced BALANCED FORCES

Force-unbalanced forces • UNBALANCED FORCESMINI DEMONSTRATION

Drawing Free-Body Diagrams Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation • • F-NORMAL F-FRICTION F-APPLIED F-GRAVITY

Among other accomplishments, Sir Isaac Newton (1642 -1727) invented calculus, developed the laws of motion, and developed the law of gravitational attraction.

Newton's First Law of Motion (The Law of Inertia) • objects tend to remain either at rest or in uniform straight line motion (i. e. , motion with constant velocity) until acted upon by an unbalanced force • inertia: concept introduced by Galileo

Newton’s Laws of Motion • Inertia can be described as the tendency of an object to keep doing whatever’s it’s doing.

Newton’s Laws of Motion INERTIA Inertia of Rest: The tendency of the body to remain at rest until and unless an unbalanced force acts on it Inertia of Motion: The tendency of the body to remain in constant motion until and unless an unbalanced force acts on it Note: Inertia does not have any unit

Newton's First Law of Motion (The Law of Inertia) – Relationship between mass and inertia – mass of an object: a measure of the amount of inertia the object has • an object with a larger mass has more inertia (i. e. , more resistance to a change in its motion) • an object with a small mass has less inertia

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck. Slide from www. science-class. net

Top view of a person standing in the aisle of a bus. (A) The bus is at rest, and then starts to move forward. Inertia causes the person to remain in the original position, appearing to fall backward.

(B) The bus turns to the right, but inertia causes the person to retain the original straight line motion until forced in a new direction by the side of the bus.

Newton's Second Law of Motion • A net force acting on an object produces an acceleration (a change in the motion of the object) F = ma m = mass of the object a = acceleration F = net force acting on the object

Newton's Second Law of Motion –the acceleration is: • directly proportional to the net force acting on the object • inversely proportional to the mass of the object

Acceleration and Mass - With Friction Zero F F a a/2 Pushing two carts with same force F produces one-half the acceleration. The acceleration varies inversely with the amount of material (the mass).

If the force of tire friction (F 1) and the force of air resistance (F 2) have a vector sum that equals the applied force (Fa), the net force is zero. Therefore, the acceleration is zero (i. e. , velocity is constant)

More mass results in less acceleration when the same force is applied. With the same force applied, the riders and the bike with twice as much mass will have half the acceleration (with all other factors constant). Note that the second rider is not pedaling.

More about: F = ma • the unit of force in the metric system is: Newton (N) 1 N = 1 kg m/s 2 • the unit of force in the English system is: pound (lb) 1 lb = 1 slug x 1 ft/s 2 (slug is the unit of mass in the English system)

More about: F = ma • the unit of force in the metric system is: Newton (N) 1 N = 1 kg m/s 2 m=F/a a=F/m m-kg a- m/s 2

More about: F = ma • What resultant force will give a 3 kg mass an acceleration of 4 m/s 2? • Remember F = m a • Given Value: m= 3 kg a=4 m/s 2 • Unknown Value F= ? m= 3 kg

More about: F = ma • Example 2: What resultant force F is required to give a 6 kg block an acceleration of 2 m/s 2? m= • Remember F = m a 6 kg • Given Value: m= 6 kg a=2 m/s 2 • Unknown Value F= ?

More about: F = ma • The Weight of an object: –the downward pulling force of the Earth on that object (the force of gravity on the object) –is equal to the mass of an object (m) times the acceleration due to gravity (g) W = mg m=W/g g=W/m

Weight and Mass • Weight is the force due to gravity. It is directed downward and it varies from location to location. • Mass is a universal constant which is a measure of the inertia of a body. • W = mg m=W/g g=W/m • W-newton m-kilogram • G-acc. due to gravity - m/s 2

Weight and Mass Problems • What is the weight of a 10 -kg block? 10 kg g=9. 8 m/s 2 m W • W = mg = (10 kg)(9. 8 • W = mg m=W/g g=W/m • W-newton m-kilogram • G-acc. due to gravity - 9. 8 m/s 2 2 m/s )

Weight and Mass Problems • What is the mass of a block that weighs 60 N? m= ? kg g=9. 8 m/s 2 m W • W = mg therefore m=W/g • m= 60 N/9. 8 m/s 2 • W-newton m-kilogram 2 • G-acc. due to gravity - 9. 8 m/s

A parallel between the mass and the weight of an object mass • The amount of substance (matter) contained in an object • a scalar quantity (no direction) • metric unit: kg weight • The force of gravity on an object • a vector ( direction: vertically down) • metric unit: N (English unit: lb)

A parallel between the mass and the weight of an object weight mass • calculated as: W = mg • the same everywhere (g = gravitational acceleration) in the universe • changes with location – ex. : mass of an (with change in g) object on the Moon – on the Moon: g. Moon = is the same as on 1. 6 m/s 2 the Earth – weight of an object on the Moon is about six times less than on the Earth

Newton's Third Law of Motion Whenever two objects interact, the force exerted by the first object on second is equal in size and opposite in direction to the force exerted by the second object on the first. F 1 = F 2 F 1

Newton’s Third Law: For every action force, there must be an equal and opposite reaction force. Forces occur in pairs. Action Reaction

Acting and Reacting Forces Use the words by and on to study action/reaction forces below as they relate to the hand the bar Action Reaction

Acting and Reacting Forces The action force is exerted by the _hands on the ___bar__. The reaction force is exerted by the bar on the Action hand. Reaction

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