Lecture 8 Physics I Chapter 6 Newton Laws (Friction forces) I am greater than Newton Course website: https://sites.uml.edu/andriy-danylov/teaching/physics-i/ PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Today we are going to discuss: Chapter 6: Newton 1st and 2nd Laws Kinetic/Static Friction: Section 6.4 PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Newton’s laws In 1687 Newton published his three laws in his Principia Mathematica. These intuitive laws are remarkable intellectual achievements and work spectacular for everyday physics PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Newton’s 1st Law (Law of Inertia) In the absence of force, objects continue in their state of rest or of uniform velocity in a straight line i.e. objects want to keep on doing what they are already doing - It helps to find inertial reference frames, where Newton 2nd Law has that famous simple form F=ma PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Inertial reference frame Inertial reference frame – A reference frame at rest – Or one that moves with a constant velocity An inertial reference frame is one in which Newton’s first law is valid. This excludes rotating and accelerating frames (non‐inertial reference frames), where Newton’s first law does not hold. How can we tell if we are in an inertial reference frame? ‐ By checking if Newton’s first law holds! All our problems will be solved using inertial reference frames PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Example Inertial Reference Frame A physics student cruises at a constant velocity in an airplane. A ball placed on the floor stays at rest relative to the airplane. There are no horizontal forces on the ball, so when . Newton’s first law is satisfied, so this airplane is an inertial reference frame. PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Example Non-Inertial Reference Frame A physics student is standing up in an airplane during takeoff. A ball placed on the floor rolls toward the back of the plane. There are no horizontal forces on the ball, and yet the ball accelerates in the plane’s reference frame. Newton’s first law is violated, therefore this airplane is not an inertial reference frame. In general, accelerating reference frames are not inertial reference frames. PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Newton’s Second Law of Motion Newton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass. More force means F ma more acceleration m New unit of force ‐ Newton N kg s2 No! More force means • It takes a force to change either the more fun direction or the speed of an object. • More force means more acceleration; • the same force exerted on a more massive object will yield less acceleration. PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Mass F ma Proportionality defines mass of object. Mass is the measure of inertia of an object, sometimes understood as the quantity of matter in the object. In the SI system, mass is measured in kilograms. Mass is not weight. Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the Moon (gravitational acceleration is about 1/6 g), you will weigh much less. Your mass, however, will be the same. PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Example Force to stop a car. What average net force is required to bring a 1000-kg car to rest from a speed of 20 m/s within a distance of 100 m? Assume acceleration is constant PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics ConcepTest Truck on Frozen Lake A very large truck sits on a A) it is too heavy, so it just sits there frozen lake. Assume there is no B) it moves backward at constant speed friction between the tires and the ice. A fly suddenly smashes C) it accelerates backward against the front window. What D) it moves forward at constant speed will happen to the truck later, E) it accelerates forward after the collision is over? When the fly hit the truck, it exerted a force on the truck (only for a fraction of a second). So, in this time period, the truck accelerated (backward) up to some speed. After the fly was squashed, it no longer exerted a force, and the truck simply continued moving at constant speed. Friction Static Kinetic Friction is always present when two solid surfaces slide along each other. The microscopic details are not yet fully understood. Kinetic/Static Friction: Section 6.4 PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Friction Force Kinetic friction Static friction When an object slides along a surface, Static friction is the contact force that the surface can exert a contact force keeps an object “stuck” on a surface, and which opposes the motion. prevents relative motion. This is called kinetic friction. The kinetic friction force is directed The static friction force is directed tangent to the surface, and opposite to tangent to the surface. the velocity of the object relative to the surface. Static friction points opposite the Kinetic friction tends to slow down the direction in which the object would move sliding motion of an object in contact with a if there were no static friction. surface. PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Friction. Static-vs-Kinetic If a horizontal force is applied on an object , the object does not move. It means a second force, Static Friction, must be opposing the applied force. N N N N F F m A m FA m A m mg mg f mg fk fs s Sliding. no motion Static. Limit. no motion Friction (kinetic) is constant static static Motion is about to start 0 f N F ma k K x x fs S N FA K coefficient of FA fs 0 kinetic friction S coefficient of f F N s A S static friction f fr N S Kinetic friction Static friction fs S N fk K FN An object remains at rest as long as fs < fs max. 45° F The object just begins to slip when fs = fs max. A f F PHYS.1410 Lecture 8 Danylov s A Department of Physics and Applied Physics Coefficients of Friction Note that, in general, μs > μk. End of Class PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Summary PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics Thank you PHYS.1410 Lecture 8 Danylov Department of Physics and Applied Physics.