Newton's Laws and Friction: Understanding Motion and Forces, Study notes of Physics

Download Newton's Laws and Friction: Understanding Motion and Forces and more Study notes Physics in PDF only on Docsity! Handout #3: Newton’s Laws, Dr. A. Rahman 1 1 Newton’s Laws Newton’s Laws form the foundation of motion in this course. They form a link between the kinematic and dynamic motion of bodies. You will find that a sound understanding of Newton’s laws will serve as the basic insight needed to solve any problem. 1.1 Description We shall provide more detailed elaboration on each law in the following sections. Let’s begin by stating Newton’s three laws: • N1: If no force acts on a body, then the body’s velocity cannot change; that is, the body cannot accelerate. (Restated: If no net force acts on a body ( ~Fnet = 0 ), then the body’s velocity cannot change; that is, the body cannot accelerate.) • N2: The net force (sum of all the forces) on a body is equal to the product of the body’s mass and the acceleration of the body. (Restated ~Fnet = m~a or Σi ~Fi = m~a) • N3: When two bodies interact, the force of body one on body two is equal and opposite to the force of body two on body one. (Restated: ~F12 = −~F21) 1.1.1 N1: Inertia Newton’s first law is sometimes called the law of inertia. The meaning of this physical law is summed up in the common adage ”A body at rest tends to stay at rest until placed in motion and a body in motion tends to stay in motion until a force is acted upon it.” This means that a body will continue its motion along said path as long as no force acts on it. 1.1.2 N2: Σi ~Fi = m~a Newton’s second law relates dynamics (forces) to kinematics (motion). It is said that just about every classical mechanics equation can be derived from it. Please keep this in mind as you work with it. A couple of points regarding the nature of force as presented in this law. First, as you can see from the equation, the force is actually a net force, i.e. the sum of all forces acting on the body. Second, force is a vector, mass is a scalar, and acceleration is a vector. That means solving problems often requires resolving the force vector into its components. These components represent the force’s components in a certain direction. 1.1.3 N3: Action-Reaction Newton’s third law simply states that for every action (force) acting on an object there is an equal an opposite action (force) exerted from the object. This law gives rise to the term action-reaction pairs of forces. An example of this is the normal force of contact that is the reaction force to placing a mass on a surface. They are equal in magnitude but opposite in direction.