Newton's Laws of Motion and Forces: An Introduction - Prof. A. Hawthorne-Allen, Study notes of Physics

Download Newton's Laws of Motion and Forces: An Introduction - Prof. A. Hawthorne-Allen and more Study notes Physics in PDF only on Docsity! Chapter 4 – Motion and Force: Dynamics Vocabulary Force – any kind of push or a pull on an object. A more precise definition of force is an action capable of accelerating an object. Newton’s First Law of Motion – Every body continues in its state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. Inertia – the tendency of a body to maintain its state of rest or of uniform motion in a straight line. Law of Inertia – Newton’s first law is often referred to as this. Inertial Reference Frames – reference frames in which Newton’s first law does hold. Noninertial Reference Frames – reference frames where the law of inertia does not hold. Mass – a measure of the inertia of a body. Newton’s Second Law of Motion – The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object. a = ΣF/F/m Net Force (ΣF) –ΣF) –F) – the vector sum of all forces acting on the body. Newton (ΣF) –N) – in SI units, the unit of force. One Newton is the force required to impart an acceleration of 1m/s² to a mass of 1 kg. Thus 1N = 1kg*m/s². Newton’s Third Law of Motion – Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. This is sometimes paraphrased as “to ever action there is an equal and opposite reaction.” Weight – the force of gravity on an object, FG, whose magnitude is mg. Contact Force – A force exerted by two objects that are in contact. Normal Force FN – when a contact force acts perpendicular (normal) to the common surface of contact. Free-body Diagram (ΣF) –Force Diagram) – an arrow representing each force acting on a given body is drawn. Make sure to include every force acting on that body. Tension (ΣF) –FT) – when a flexible cord pulls on an object, the cord is said to be under tension. Rolling Friction – when a body rolls across a surface. Kinetic Friction – when a body slides across a surface. Coefficient of Kinetic Friction (ΣF) –μk) – depends on the nature of the two surfaces Static Friction – a force parallel to the two surfaces that can arise even when they are not sliding. μs is the coefficient of static friction. Notes 4-1 Force Forces do not always give rise to motion. A force has direction as well as magnitude, and is indeed a vector. 4-2 Newton’s First Law of Motion If no force is applied to a moving object, it will continue to move with constant speed in a straight line. An object slows down only if a force is exerted on it. The object moves with constant speed when no net force is exerted on it. Newton’s first law does not hold in every reference frame. For most purposes, we can usually assume that reference frames fixed on the Earth are inertial frames. 4-3 Mass The more mass a body has, the harder it is to change it state of motion. It is harder to start it moving from rest, or to stop it when it is moving, or to change its motion sideways out of a straight-line path. Mass is a property of a body itself (it is a measure of a body’s inertia, or its “quantity of matter”). Weight is a force, the force of gravity acting on a body. 4-4 Newton’s Second Law of Motion Since a change in speed or velocity is an acceleration, we can say that a net force gives rise to acceleration. The acceleration of a body is inversely proportional to its mass. 4-5 Newton’s Third Law of Motion Remember that the “action” force and the “reaction” force are acting on different objects. A rocket does not accelerate from the gasses rushing out the back of the engine pushing on the ground, but instead the rocket exerts a strong force on the gases, expelling them; and the gases exert an equal and opposite force on the rocket. Every material, no matter how hard, is elastic, at least to some degree. 4-6 Weight – the Force of Gravity; and the Normal Force On the moon, on other planets, or in space, the weight of a given mass will be different. When an object is at reason on the Earth, the gravitational force on it does not disappear. Two forces acting on one object that remains at rest must be equal. The sum of these two forces must be zero, therefore the must be of equal magnitude and in opposite directions. 4-7 Solving Problems with Newton’s Laws: Vector Forces and Free-Body Diagrams Newton’s second law tells us that the acceleration of an object is proportional to the net force acting on the object. When solving problems involving Newton’s laws and force, it is very important to draw a diagram showing all the forces acting on each object involved. When concerned only about translational motion, we can draw all the forces on a given body as acting at the center of the object, thus treating the object as a point. If the cord has negligible mass, the force exerted at one end is transmitted undiminished to each adjacent piece of cord along the entire length to the other end. Flexible ropes and cords can only pull. They can’t push because they bend. 4-8 Applications Involving Friction, Inclines When a body is in motion along a rough surface, the force of kinetic friction acts opposite to the direction of the body’s velocity. The magnitude of the force of kinetic friction depends on the nature of the two sliding surfaces. The force of friction between hard surfaces depends very little on the total surface area of contact.