Forces & Newton 1. What Is a Force? A Force is an interaction between two bodies.

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1 Forces & Newton 1 What Is a Force? A Force is an interaction between two bodies. Convention: F a,b means the force acting on a due to b. A Force is a push or a pull. A Force has magnitude & direction (vector). Adding forces means adding vectors. 1

2 Forces Contact (fundamentally E+M) Normal: Perpendicular to surface Friction: Parallel to surface Anything touching the object Rope: Tension Spring F = -kx Person Non-Contact or Action at a Distance (these are field forces) Gravity g Physics Force & Mass Force Symbol: F or F Units: SI Newton (N) (~0.22 lb) English Pound (lb) cgs dyne (10-5 N) Mass Symbol: m or M Units: SI kilogram (kg) English slug (~14.5 kg) cgs gram (g) Weight Compared to Mass Weight is a Force Depends on the gravitational On Earth field strength at the location Mass is the amount of stuff Mass is independent of location! (although it is often measured by comparing weights) Mass is also the inertia of the object Inertia is resistance to a change in motion See Newton 1 W = (9.8N/kg) m 2

3 Remember 1. Every force must have an agent that produces it; 2.Every contact force must act only at the point of contact; 3.The normal force acts only perpendicular to the surface in contact; 4.The friction force acts only parallel to the surface in contact; 5.The tension force from a string or rope acts only along the line of the string or rope. Tension Spring scale reads the force exerted on each end. Tension is force transmitted by rope and is the force exerted by each end of rope. Tension ACT A pair of tug-of-war teams are pulling on the ends of a rope, each team with a force of 1000 N. The tension in the rope is: A N B. 500 N C N D. 0 N E kg 3

4 Tension ACT a. T 1 >T 2 b. T 1 =T 2 c. T 1 <T 2 d. depends on pulley radius Free Body Diagrams Drawing a FBD Identify all forces acting on the object. Draw a coordinate system. Use the axes defined in your pictorial representation. If those axes are tilted, for motion along an incline, then the axes of the free-body diagram should be similarly tilted. Represent the object as a dot at the origin of the coordinate axes. This is the particle model. Draw vectors representing each of the identified forces. Be sure to label each force vector. 4

5 Examples of Force Vectors Pull (contact force) Push (contact force) Gravity (long-range force) Identifying Forces Identify the system and the environment. The system is the object whose motion you wish to study; the environment is everything else. Draw a picture of the situation. Show the object the system and everything in the environment that touches the system. Ropes, springs, and surfaces are all parts of the environment. Draw a closed curve around the system. Only the object is inside the curve; everything else is outside. Locate every point on the boundary of this curve where the environment touches the system. These are the points where the environment exerts contact forces on the object. Name and label each contact force acting on the object. There is at least one force at each point of contact; there may be more than one. When necessary, use subscripts to distinguish forces of the same type. Name and label each long-range force acting on the object. For now, the only long-range force is weight. Forces on a Bungee Jumper y T w x 5

6 The Forces on a Skier f k n w T The Forces on a Rocket F thrust y D w x Inertial Reference Frame The following statements can be thought of as the definition of inertial reference frames. An IRF is a reference frame that is not accelerating (or rotating) with respect to the fixed stars. If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector! 6

7 Newton 1 Newton s First Law An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame. If no net forces act, there is no acceleration. F net = 0 a= 0 7