Or FORCE-BODY-DIAGRAMS FREE-BODY-DIAGRAMS Or FORCE-BODY-DIAGRAMS & NEWTON’S LAWS

BIG IDEA 3: The interactions of an object with other objects can be described by forces. 3.A.2.1: I can represent forces in diagrams or mathematically using appropriately labeled vectors with magnitude, direction, and units during the analysis of a situation. [SP 1.1] 3.A.3.1: I can analyze a scenario and make claims (develop arguments, justify assertions) about the forces exerted on an object by other objects for different types of forces or components of forces. [SP 6.4, 7.2] 3.A.3.3: I can describe a force as an interaction between two objects and identify both objects for any force. [SP 1.4] 3.A.4.1: I can construct explanations of physical situations involving the interaction of bodies using Newton’s third law and the representation of action-reaction pairs of forces. [SP 1.4, 6.2]

3.A.4.2: I can use Newton’s third law to make claims and predictions about the action-reaction pairs of forces when two objects interact. [SP 6.4, 7.2] 3.A.4.3: I can analyze situations involving interactions among several objects by using free-body diagrams that include the application of Newton’s third law to identify forces. [SP 1.4] 3.B.1.2: I can design a plan to collect and analyze data for motion (static, constant, or accelerating) from force measurements and carry out an analysis to determine the relationship between the net force and the vector sum of the individual forces. [SP 4.2, 5.1] 3.B.1.3: I can re-express a free-body diagram representation into a mathematical representation and solve the mathematical representation for the acceleration of the object. [SP 1.5, 2.2] 3.B.2.1: I can create and use free-body diagrams to analyze physical situations to solve problems with motion qualitatively and quantitatively. [SP 1.1, 1.4, 2.2]

The analysis of dynamics problems involves the selection and analysis of the relevant forces acting on the object under consideration. The object of interest is referred to as the system. The world around the object that exerts forces on it is called the environment. system

Each force has a specific identifiable, immediate cause called agent. The agent can be animate such as a person, or inanimate such as a desk, floor or a magnet. The agent for the force of gravity is Earth's mass. The ‘agent’ initiates or causes the force to happen. agent

For the purpose of developing force diagrams for problem analysis, all forces will be categorized as either contact or field forces. Contact forces are all forces that result from the contact between the system and its surroundings at the systems boundaries. Examples of contact forces are static and kinetic friction, tension forces and normal force.

Contact and Field Forces

Field forces or long-range forces result from the systems interaction with force fields such as magnetic, electric, or gravitational fields. A force has both magnitude and direction. Force is a vector quantity and therefore it can be represented with a free-body-diagram.

Circle the system and identify every place where the system touches the environment. It is at these places that contact forces are exerted. Identify the contact forces: FN f Fpush Identify any field forces: FG or W

Next replace the object by a dot. Each force is represented as an arrow that points in the correct direction. The tail of the force vector is always on the particle.

FN FG

FN FG

FN FF FG

FT FG

FT1 FT2 FG

FN FF FG

FN FT FG

FN1 FN2 FG

FN FA θ FF FG

FT FG

FG

FD FG FG

System of Interest Look for forces ACTING on the System… 3rd Law f - All forces opposing the motion System 1: Acceleration of the professor and the cart System 2: Force the professor exerts on the cart Look for forces ACTING on the System… Not outside of the system ?!?

If Hand force is 150N Find the acceleration of the system With what force does the 5kg block push on the 10 kg block?

Tom pushes two identical blocks on a horizontal frictionless table from the left. The force that block 1 exerts on block 2 is F12. The force that block 2 exerts on block 1 is F21. Compare the magnitude of F12 and F21. 1 2 FLEFT F12 < F21 F12 = F21 F12 > F21 Cannot be determined