Newton’s Laws Forces and Motion

Laws of Motion formulated by Issac Newton in the late 17th century written as a way to relate force and motion Newton used them to describe his observations of planetary motion.

History Aristotle was an ancient Greek philosopher Based on his observations the common belief was that in order for an object to continue moving, a force must be exerted in the direction of the motion This lasted until Issac Newton proposed his “Laws of Motion” based on observations made of bodies free from earth’s atmosphere.

Newton’s 1st Law Inertia An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity unless acted on by an unbalanced force. This statement contradicted Aristotle’s teaching and was considered a radical idea at the time. However, Newton proposed that there was, in fact, an unrecognized force of resistance between objects that was causing them to stop in the absence of an applied force to keep them moving. This new unseen resistance force became known as “friction”.

Newton’s 2nd Law Fnet = ma If an unbalanced force acts on a mass, that mass will accelerate in the direction of the force. Since 8N is greater than 2N, the unbalanced force is to the right so the acceleration is to the right. 2 N 8 N a Newton’s 1st Law says that without an unbalanced force objects will remain at constant velocity (a=0)…so it seems logical to say that if we apply a force we will see an acceleration.

Newton’s 3rd Law Action - Reaction For every action force there is an equal and opposite reaction force. Example: If you punch a wall with your fist in anger, the wall hits your fist with the same force. That’s why it hurts! Action-reaction forces cannot balance each other out because they are acting on different objects.

A Force is… A “push” or “pull” Measured in Newtons (N) in the metric (SI) system and pounds (lbs) in the English system A vector quantity requiring magnitude and direction to describe it Represented by drawing arrows on a diagram

Types of Forces Weight - force of gravity Normal force – surface pushing back Friction - resistance force Applied force - force you exert Tension - applied through a rope or chain Net force – total vector sum of all forces Balanced forces – equal and opposite Unbalanced forces – not equal and opposite

Weight The force of gravity acting on a mass. Weight always acts down! Weight = mass (kg) * acceleration due to gravity Weight is a force…so this is a special case of F=ma and the unit is a Newton.

Mass is… The amount of matter an object is made up of. Measured in kilograms. A universal value, independent of the influences of gravity. NOT a force.

Normal Force (FN) Defined as the force of a surface pushing back on an object. Always directed perpendicular to the surface. This is a contact force. No contact…no normal force. NOT always equal to weight. FN Examples: FN Wa l l Table

Friction A resistance force usually caused by two surfaces moving past each other. Always in a direction that opposes the motion. Measured in Newtons Depends on surface texture and how hard the surfaces are pressed together. Surface texture determines the coefficient of friction (μ) which has no units. Normal force measures how hard the surfaces are pressed together.

Types of friction Static friction is the force an object must overcome to start moving. Kinetic friction is the force an object must overcome to keep moving. Static friction is always greater than kinetic friction!

Calculating the Force of Friction Where f is the force of friction, μ is the coefficient of friction, and FN is the normal force For kinetic friction: For static friction:

May the Net Force be with you Total force acting on an object Vector sum of all the forces The unbalanced force referred to in Newton’s Law of Motion Net force is equal to the mass of an object times the acceleration of that object.

Force Diagrams Force diagrams must include the object and all forces acting on it. The forces must be attached to the object. No other vectors may be attached to the object. Components of forces, axis systems, motion vectors and other objects or surfaces may be included in force diagrams.

Force Diagram Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate. Fapplied = 100 N 10 kg FN f = μFN Weight = Fg = (10 kg)*(9.81m/s2)

To Solve the Problem Now that you have the force diagram! Problem: A 10 kg crate with an applied force of 100 N slides across a warehouse floor where the coefficient of static friction is 0.3 between the crate and the floor. What is the acceleration of the crate. FN f = μFN Fapplied = 100 N a 10 kg Weight = Fg = (10 kg)*(9.81m/s2) Write the Newton’s 2nd Law equation for the x- and y- directions. Now plug in what you know and solve for what you don’t. Algebra!!