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lecture notes-Chapter-4-posted

# lecture notes-Chapter-4-posted - Chapter 4 Force and...

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1 Chapter 4: Force and Newton’s Laws of Motion Force Newton’s Three Laws of Motion The Gravitational Force Contact Forces (normal, friction, tension) Application of Newton’s Second Law Apparent Weight Air Resistance Fundamental Forces

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What is force? 2
Question 7 A vector F with magnitude 10 makes an arbitrary angle with respect to the x-axis. Which of the following is not a possible value for F x ? A. F x = 0 B. F x = 5 C. F x = 10 D. F x = 15

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Question 8 Two forces are acting on an object. F 1 = 8 N, to the right ; F 2 = 6 N, up . What is the magnitude of the net force? A. 2 N B. 10 N C. 14 N
5

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What happens to an object there is no net force acting on the object? 6
7 Galileo: What is going to happen if the second incline is horizontal?

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8 §4.2 Inertia and Equilibrium: Newton’s First Law of Motion Newton’s 1 st Law (The Law of Inertia): If the net force acting on an object is zero, then its velocity (speed and direction of motion) does not change. Inertia is a measure of an object’s resistance to changes in its motion (velocity).
9 If the object is at rest, it remains at rest (speed = 0). If the object is in motion, it continues to move in a straight line with the same speed. No force is required to keep a body in straight line motion when effects such as friction are negligible.

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10 An object is in translational equilibrium if the net force on it is zero.
11 If the object is at rest, it remains at rest (speed = 0). If the object is in motion, it continues to move in a straight line with the same speed. No force is required to keep a body in straight line motion when effects such as friction are negligible.

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Question 4 An ice skater glides across a frozen pond at a constant velocity . What is the net force acting on the skater? A. More than her weight. B. Equal to her weight. C. Less than her weight but more than zero. D. Depends on her speed. E. Zero.
Question 6 If no force is acting on an object suddenly becomes zero, the object is A. at rest. B. slowing down. C. moving with a constant speed. D. at rest or moving with a constant speed.

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What happens if the net force acting an object is not zero? 14
15 §4.3 Net Force, Mass, and Acceleration: Newton’s Second Law of Motion Newton’s 2nd Law: The acceleration of a body is directly proportional to the net force acting on the body and inversely proportional to the body’s mass. Mathematically: a F F a m m net = = net or

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16 An object’s mass is a measure of its inertia. The more mass, the more force is required to obtain a given acceleration. The net force is just the vector sum of all of the forces acting on the body, often written as Σ F .
17 If a = 0, then Σ F = 0. This body can have: Speed = 0 which is called static equilibrium, or speed 0, but constant, which is called dynamic equilibrium.

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18 An astronaut playing shuffleboard (a) on Earth and (c) on the Moon.
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