Physics 1 – Oct 27, 2016 P3 Challenge – Two masses are connected by a cord and pulled to the right by another cord. (Assume frictionless surface.) Draw free body diagrams for a) green mass, b) red mass, c) the two blocks together Today’s Objective: Second Law Problem Solving T F Get out Forces homework for check

Agenda, Assignment IB 2.2 Forces Agenda Assignment: Forces, Newton’s Laws, etc… Agenda Common forces Newton’s Laws Free body diagrams Dynamics w/ kinematics Assignment: Forces Worksheet

Forces Motion is caused by a net force. Kinematics (both 1D and 2D) are used to describe and predict motion Dynamics answers the question WHY does motion occur (or not occur) Motion is caused by a net force. A force is a vector quantity and is measured in Newtons, N 1 N = 1 kg∙m/s2 A net force is the vector sum of all forces ON a body of interest

Common Forces Weight, W = mg mass times the acceleration due to gravity Distinguish between mass and weight Present anytime you are on or near the surface of the earth Always directed downward, toward center of earth Tension, T in a string connected to an object Pulling force applied through a string Always directed along the string , away from the body of interest

Common Forces Applied force, FA or just F Generic force Friction, f May be a push or a pull resulting from direct contact Could be directed anywhere Friction, f Present anytime two surfaces move/slide (or might move) relative to one another Always directed opposite the direction of motion Often set to zero (frictionless) to simplify a problem

Common forces Normal reaction force, R or N (avoid confusion with unit N) Present anytime there is a surface that something is resting on. Always directed perpendicular to the surface Spring force, Ts = kx a variable force that depends on x k is called spring constant; represents the “stiffness” of a spring Proportional to the amount of displacement from equilibrium, x May be a compression or extension of the spring Present anytime there is a spring Always directed to restore the spring to its equilibrium position.

Common forces Drag force, Fdrag Present anytime an object is moving through a fluid (liquid or air) Is often negligible (especially in air with compact objects) Always directed opposite the direction of motion Upthrust, Fup Also known as buoyant force (in water) or lift force (in air) Present anytime an object is floating in a fluid Always directed upward, opposite gravity

Newton’s Laws – the First Newton’s First Law: A body at rest stays at rest and a body in motion stays in motion with a constant velocity unless acted upon by a force. Known as the Law of Inertia IB Language: When the net force on a body is zero, the body will move with constant velocity (which may be zero). Converse is also true. If a body is not moving with constant velocity, then there must be a net force.

Examples of the First Release from a rotation: a mass rotating on the end of a horizontal string on a table top is released. The mass will continue moving in the last direction it was going and trace out a tangent to the circular motion. Need for a safety belt: If you don’t have a safety belt on and the car stops, inertia will send you through the windshield. Swaying on the subway: When the subway starts you fall back while hands and feet try to exert enough forward forces to create the acceleration. When the subway stops, you fall forward while hands and feet exert a stopping force.

Newton’s Laws – the Second The acceleration of an object produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force. The proportionality constant is the mass of the object. In math: Fnet = ma Most problem solving will be applications of the second law: w/ kinematics 1D Forces 2D without friction Multiple bodies 2D w/ Friction Inclines More on this later…

Newton’s Laws – the Third Newton’s Third Law: For every action there is an equal and opposite reaction Warning: Not all equal and opposite forces form an action reaction pair!! For an action-reaction pair, the two forces involved must act on different bodies of interest. IB Language: When a body A exerts a force on body B, body B will exert and equal but opposite force on body A.

Free body diagram A free body diagram (FBD) is a key part of solving dynamics problems Includes any and all forces acting on your body of interest To create an FBD: Sketch the system Decide on your body of interest. Turn into a point body. Are you on earth? Add a Weight, W force pointing down. Is the body of interest sitting on a surface? Add a normal force, R perpendicular to the surface. Are there sliding surfaces? Add a friction f opposite the direction of motion Are there other things in the environment? Add a T or a FA or …. for each

Kinematics and Dynamics Variable in common to both systems is acceleration. Ex: What average force is needed to accelerate a 7.00 g bullet from rest to 175 m/s over a distance of 0.700 m along the barrel of a rifle? Ex: A 10. kg bucket is lowered by a rope in which there is 63 N of tension. What is the acceleration of the bucket? Is it up or down? How long before the bucket hits the water 13 meters below?

Exit Slip - Assignment List three types of forces and the symbol most commonly used to represent that type of force. What’s Due on Thurs Oct 27? (Pending assignments to complete.) Forces Worksheet What’s Next? (How to prepare for the next day) Read p57-75