These interactions impart forces to the bodies that are interacting. For example:

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1 Name: Class: Date: MECHANICS Dynamics & Statics Handout 1 1. Learning Objectives At the end of this lesson, you should be able to: i) State Newton s 3 rd law of motion. ii) Identify forces acting on an object and draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most two dimensions). 2. Fundamental Interactions (not in syllabus) When two bodies come together, not necessarily in contact, they may mutually influence and cause a physical change to each other. We say that there exists an interaction between these two bodies. Physicists suggest that there may only be 4 kinds of fundamental interactions, from which all other phenomena arise. These are: (1) Gravity (2) (3) Nuclear Strong Interaction (4) Nuclear Weak Interaction These interactions impart forces to the bodies that are interacting. For example: The magnetic interaction (or more precisely, the electromagnetic interaction) exists between two magnets. This results in a pair of magnetic forces, being exerted on the two magnets separately. 1

2 3. Newton s Third Law Newton s Third Law, also known as the Law of Reciprocal Actions, states that: Whenever a body A applies a force (known as the action ) on a body B, body B will exert another force (known as the reaction ) that is equal in magnitude, but opposite in direction to the force applied by A. The two forces hence act on two different bodies that are interacting with one another. Characteristics of Action-Reaction Pairs: i. They are equal in magnitude ii. They are opposite in direction iii. They act on bodies. iv. They act along the. v. They arise from the same type of interaction. Action-reaction pairs always satisfy the above characteristics. 4. Types of Forces The table below lists some examples of forces and the interaction from which they arise. FORCE Weight (Gravitational Force) Electrostatic Force Magnetic Force Friction Air Resistance/ Water Resistance Normal Contact Force Tension INTERACTION Gravity a. Friction Arises when two bodies in contact are in opposing motion relative to one another. The force of friction of one body opposes the motion of the same body. b. Normal Contact Force Arises perpendicular to the surface of contact between two bodies that are being pushed together by an external force. It tends to prevent one body from sinking into the other. 2

3 5. Drawing Free Body Diagrams When drawing a free body diagram, we will consider the forces acting on one body that is of interest to us, and ignore the other body with which the body of interest is interacting with. For instance, there are altogether 3 bodies interacting with one another in the diagram below. However, we are only interested in the magnet on the top right corner. As such we will ignore the Earth and the other magnet. (Note that the picture below is greatly simplified. In fact there exists gravitational influence between the two magnets because they both possess mass, and another magnetic influence between the Earth and the magnet of interest because the Earth too has a magnetic field) Let s look at more examples: Example1: The Book on the Table Can you draw a free-body diagram of the book in the space provided on the next page? What about the table? 3

4 Free-body Diagram of the Book Free-Body Diagram of the Table Can you identify the action-reaction pairs of forces for the book and the table? 4

5 Example 2: Box on Slope What are the forces acting on the box? Draw a free-body diagram for the box. Example 3: Tarzan on a vine What are the forces acting on the Tarzan? Draw a free-body diagram for Tarzan. ~end~ 5