Part 1: Inertia and Forces NEWTON’S LAWS OF MOTION What is a force? A push or a pull May be unbalanced or balanced What does a force do? Gives energy to an object Can set an object in motion, make it stop, or change its motion Balanced Forces Forces on an object that are equal in size but opposite in direction are called balanced forces. Balanced Forces When the forces on an object are balanced, the object will either be at rest or at constant velocity. Unbalanced Forces Whenever you have one force that is greater than the force that is opposite to it, you have an unbalanced force. Unbalanced Forces An unbalanced force always results in an acceleration! Starting motion Stopping motion Changing speed Changing direction INERTIA Inertia The tendency of an object to stay at rest or in motion. Objects with greater mass have greater inertia Inertia keeps astronauts in motion Seat belts protect against inertia GRAVITY Gravity The attractive force between all objects in the universe F = M1 M2 d2 More mass means greater gravity More distance means less gravity Massive objects have lots of gravity My gravitational attraction is small no matter what the distance! Weight vs Mass Planet Mass of each planet (kg) Acceleration due to Gravitational force (m/s2) Mercury 3.3 x 1023 3.7 Venus 4.87 x 1024 8.87 Earth 5.98 x 1024 9.8 Mars 6.42 x 1023 3.693 Jupiter 1.90 x 1027 20.77 Saturn 5.69 x 1026 10.4 Uranus 8.69 x 1025 8.43 Neptune 1.02 x 1026 10.71 Weight vs Mass What is the mass of my 10 kg cat on Earth? What is his weight? What is his mass on the moon? (acceleration due to gravity = 1.6 m/s2) What is his weight on the moon? The earth exerts a force on the student and the student exerts a force on the earth because of GRAVITY! Inertia keeps satellites in orbit Newton’s First Law of Motion The law of inertia: An object at rest stays at rest an object in motion stays in motion in the same direction unless acted upon by an outside force. Unbalanced forces change motion Unbalanced Forces When the forces are unbalanced, motion will be in same direction as the greatest force. Unbalanced forces change motion Newton’s Second Law of Motion Force = Mass x acceleration A force is needed for acceleration Acceleration will occur in the direction of the unbalanced force. Which cars are accelerating? If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed. Newton’s 2nd Law Force = mass x acceleration F_____ M A Force is measured in newtons. A newton is a kg m s2 Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree? Answer: M F_____ A 1000kg * 50m/s/s = 50,000 kgm or 50,000N s2 F = ma A car is traveling at constant speed. Is a force acting on it? Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero. More 2nd Law Problems: 1. What is the acceleration on a mass of 50kg if a force of 10N is applied? 2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass? Answers: _F_ MA 1. ___10N___ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg 2. F______ 15kg 10m/s2 150 kgm/s2 or 150N

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