Physics: Principles with Applications, 6th edition Lecture PowerPoints Chapter 4 Physics: Principles with Applications, 6th edition Giancoli © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.

Inertia and Free Particle Model Force – Push or pull on an object Unit is newton (N) Force is a derived SI unit – dimensions are kg m/s2 Types of Forces Fundamental forces (all field forces) Field forces Contact forces

Inertia and Free Particle Model Fundamental forces (weakest to strongest) Gravity* Acts between any two objects with mass Acts at infinite distances Weak interactive Involved in some types of nuclear phenomena Acts at distances up to 10-18 m Electromagnetic Operates between electrically charged particles Strong interactive Holds nucleus of an atom together Acts at distances up to 10-15 m *Einstein’s General Theory of Relativity states that gravity bends space around the object rather than attracting it

Inertia and Free Particle Model Field force a force acting on a particle at various positions in space Field forces can affect an object without touching it These are the fundamental forces

Inertia and Free Particle Model Field force

Inertia and Free Particle Model Contact force Types of contact forces (continued) Normal force – contact force that acts perpendicular to the common surface of contact Example: Gravity acts on a book on a table, but the book does not fall through the table because of the normal force of the table pushing upward on the book

Inertia and Free Particle Model

Inertia and Free Particle Model Contact force Types of contact forces (continued) Buoyancy – an upward force exerted by a fluid that opposes the weight of an immersed object. Example: Boat floating in water

Inertia and Free Particle Model Contact force Buoyancy Source: https://en.wikipedia.org/wiki/Buoyancy

Inertia and Free Particle Model Contact force Types of contact forces (continued) Elastic Restoring –

Constant Force Tension Exists in a flexible cord connecting two masses Force of tension the same at all points in cord Tension is a pull-only force (can’t push a cord) The net force of tension is 0N because each mass pulls on the other with equal tension

Inertia and Free Particle Model Contact force Tension example – Source: https://en.wikipedia.org/wiki/Tension_(physics)

Constant Force Example Two boxes connected by cord FP acts on Box A, FT acts on Box B Can treat as one system - both boxes accelerate at same rate [FP / (mA + mB)]

Inertia and Free Particle Model Contact force Types of contact forces (continued) Tension – Present in objects that have the property of elasticity (when you stretch them and let go, they return to their original size (example: rubber band) Tension is high when object stretched Tension is low when object not stretched Tension is the same throughout the object

Inertia and Free Particle Model Contact force Acts between objects that are touching Types of contact forces Friction – Exists between two solid surfaces because even the smoothest surface is quite rough on a microscopic scale. Dry friction arises from a combination of inter-surface adhesion surface roughness surface deformation surface contamination

Inertia and Free Particle Model Contact force Types of contact forces (continued) Examples of friction Dry friction – includes static friction and kinetic (sliding) friction Static friction - on between two or more solid objects that are not moving relative to each other.  Kinetic friction (sliding friction) - two objects are moving relative to each other and rub together  Rolling resistance – force that resists the rolling of a wheel or other circular object along a surface caused by deformations in the object and/or surface Air resistance - Opposes the motion of objects moving in air (examples: open parachute, sail on sailboat)

Constant Force Friction Exists between two solid surfaces because even the smoothest surface is quite rough on a microscopic scale. The vector friction is always opposite the direction of the applied force or motion

Constant Force Friction Types of friction: If no motion occurs, referred to as static friction Example: You push on a refrigerator, and it finally starts to move. Static friction kept it from moving

Constant Force Friction Types of friction: If motion occurs, referred to as kinetic (sliding) friction Example: You let go of the moving refrigerator, and it starts to slow down then stops due to sliding friction

Constant Force Friction Types of friction: When object rolls on surface, referred to as rolling resistance Example: One reason your bicycle slows if you stop pedaling

Constant Force Friction Types of friction: Air resistance opposes the motion of objects moving in air (examples: open parachute, sail on sailboat) Terminal velocity* is highest velocity attainable by an object as it falls through a fluid (gas or liquid). At terminal velocity, resistance = weight of object. Terminal velocity in air for a skydiver in freefall is 195 km/h (122 mph or 54 m/s). *Source: https://en.wikipedia.org/wiki/Terminal_velocity

Constant Force Friction What causes friction? Surfaces are not smooth even if they appear to be (microscope) Dry friction (static friction and sliding friction) arises from a combination of Inter-surface adhesion Surface roughness Surface deformation Surface contamination

Constant Force Friction Coefficient of static friction (μs) If you know Fs, you know only one of the infinitely many possible static frictional forces that could be exerted between the two bodies. Which force do you know? The maximum static frictional force.

Constant Force Friction Coefficient of kinetic (sliding) friction (μk) The amount of kinetic friction Fk a body experiences is proportional to the size of the normal force N exerted on the body by the structure it slides against.

Constant Force Friction Coefficient of kinetic (sliding) friction (μk) Note: The statement “kinetic friction is a function of normal force only-- surface area has nothing to do with it” is true ONLY as long as you are dealing with two rigid bodies that are sliding relative to one another.

Constant Force Friction

Constant Force Friction

Inertia and Free Particle Model

Inertia and Free Particle Model Contact force Coefficients of friction (μk and μs)