Forces Newton 1 What Is a Force A

Forces & Newton 1

What Is a Force? • A Force is an interaction between two bodies. – Convention: Fa, 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.

Forces - 1151 • Contact (fundamentally E+M) – Normal: Perpendicular to surface – Friction: Parallel to surface – Anything touching the object • Rope: Tension • Spring F = -kx • Person Physics • Non-Contact or Action at a Distance (these are field forces) – Gravity g

• Force & Mass – 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 On Earth – Depends on the gravitational W = (9. 8 N/kg) m 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

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. 2000 N B. 500 N C. 1000 N D. 0 N E. 2000 kg

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 l Identify all forces acting on the object. l 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. l Represent the object as a dot at the origin of the coordinate axes. This is the particle model. l Draw vectors representing each of the identified forces. Be sure to label each force vector.

Examples of Force Vectors Pull (contact force) Push (contact force) Gravity (long-range force)

Identifying Forces l Identify “the system” and “the environment. ” The system is the object whose motion you wish to study; the environment is everything else. l 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. l Draw a closed curve around the system. Only the object is inside the curve; everything else is outside. l 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. l 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. l 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

The Forces on a Skier y n T fk w x

The Forces on a Rocket Fthrust 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!

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.