4 notes H phy students to print cutnell Honors Physics (1).docx

4 notes H phy students to print cutnell Honors Physics (1).docx

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Chapter 4 Forces 4.1 Introduction of Forces A. Terminology 1. Kinematics is the study of how things move. (velocity, acceleration, etc.) 2. Dynamics is the study of why things move. 3. Force is a push or pull a. Forces cause an object to change velocity by acceleration, deceleration, overcoming opposing force or changing directions. b.Contact force - an object that touches another object that causes it to change velocity. c.Non-contact or action at a distance forces - when a force can be exerted without touching. For example a magnet can pull on metal or gravity will pull down a skydiver. 4. Free - body diagram - a representation of an object and the forces acting on it. B. Forces acting on an object are vectors drawn as free body diagrams 1. One dimensional vector addition The weight of the book is 10N in the negative direction. The force of the table pushing up is 10N in the positive direction (called normal force). Σ F y ¿ is the sum of the forces in the y direction. This is also called net force or F net ) Σ F y = Fy F y + ¿ ) Σ F y = 10 N 10 N + ¿ ) Σ F y =0N The book doesn’t accelerate anywhere. 2. Two guys push a car. One with a force of 275 N and another with 395 N. The opposing force of the car is 560 N. The forward force is 275 N + 395 N in the positive x direction The opposing force is 560 N in the negative x direction
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Σ F x = F x F x + ¿ ) Σ F x = 560 N 275 N + 395 N + ¿ ) Σ F x = 110 N You can also draw velocity and acceleration vectors Free body diagrams 3. A rope lifts a bucket with constant speed . (Ignore air resistance) . Σ F y = Fy F y + ¿ ) Σ F y =0N 4. A rope lifts a bucket with constant increasing speed . (Ignore air resistance) Σ F y = Fy F y + ¿ ) Σ F y =+N 5. A soccer ball is kicked uphill. Draw velocity vectors and deceleration vectors velocity vectors acceleration vectors velocity vectors velocity is constant acceleration = 0 Gravit y accel eratio n down ward U p w ar d fo rc e velocity vectors velocity is not constant In this case acceleration is constant
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Σ F y = Fy F y + ¿ ) Σ F y =-N 6. A soccer ball is kicked downhill. Draw velocity vectors and acceleration vectors. 4.2 Newton’s 1st Law of Motion In 1665 Sir Isaac Newton started working on his 3 laws of motion. When he published them in 1687, they addressed the relationship between acceleration and force. A. Newton’s first law of motion: An object with no net force acting on it remains at rest or moves with constant velocity in a straight line. (Law of inertia) B. Mass - measure of inertia. The SI unit for mass is Kg. 4.3 Newton’s 2nd Law of Motion A. Newton’s second law of motion: The acceleration of a body is directly proportional to the net force and inversely proportional to the mass. In other words F= m a force (newtons) = mass (kg) x acceleration ( m/s 2 ) Unit of force F= (mass)kg X(acceleration) m/s 2 F= kg m/s 2 = N (newton’s) Force is measured in newtons If mass stays the same but force doubles so does acceleration.
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