Forces Ch. 6 Milbank High School

Sec 6.1 Force and Motion ► Objectives  Define a force and differentiate between contact forces and long-range forces  Recognize the significance of Newton’s second law of motion and use it to solve motion problems  Explain the meaning of Newton’s first law and describe an object in equilibrium

The Forces of Nature I. I. Gravitational: attraction bet. masses tides, gravity, weight II. II. Electromagnetic: friction tension adhesion lift electrostatic drag buoyant magnetic III. III. Weak Nuclear: helps to explain atomic collisions IV. IV. Strong Nuclear: binds atomic nuclei

Force ► How do forces influence motion? ► Force– a push or pull exerted on an object having magnitude and direction ► System—object that experiences the force ► Environment—world around the system that exerts the force

Two Categories of Forces… ► Contact Force  Acts on an object only by touching it ► Long-Range Force  Exerted without contact ► Magnets ► Gravity Agent: a specific, identifiable, immediate cause of a force

Types of Forces ► F f - - Friction (opposes sliding) ► F N - - Normal (surface) ► F sp - - Spring (push or pull of a spring) ► F T - - Tension (spring, rope, cable) ► F thrust - - Thrust (rockets, planes, cars) ► F g - - Weight (force due to gravity)

Representing Forces... ► Forces are vectors  Forces are drawn as arrows (vectors)  forces add like vectors.  the sum of all the forces is called the net force. ► A picture of a body with arrows drawn representing all the forces acting upon it is called a FREE BODY DIAGRAM.

Free Body Diagrams

Try it... Draw a picture of your book sitting on the desk. Identify all the forces acting on it.

Free Body Diagrams... Book T (table) W (weight)

Free Body Diagrams... What forces are acting on a skier as she races down a hill?

The Answer... FNFN d & f W

The Answer... FNFN f and d W

Draw free body diagrams for the following ► An egg is free-falling from a nest in a tree. Neglect air resistance. ► A skydiver is descending with a constant velocity. Consider air resistance. ► A car is coasting to the right and slowing down.

Newton’s Second Law ► F = ma ► a = F net / m ► Expressed in Newtons (N)  Force required to give 1kg mass a 1m/s 2 acceleration

Example ► A race car has a mass of 710 kg. It starts from rest and travels 40.0 m in 3.0 s. The car is uniformly accelerated during the entire time. What net force is exerted on it?

Newton’s First Law of Motion ► “An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero.”

Newton’s First Con’t ► Inertia—the tendency of an object to resist change. ► Equilibrium—object at rest or moving at a constant velocity

Finally…Misconceptions about forces ► When a ball has been throw, the force of the hand that threw it remains on it. ► A force is needed to keep an object moving ► Inertia is a force ► Air does not exert a force ► The quantity ma is a force

Sec. 6.2 Using Newton’s Laws ► Objectives  Describe how the weight and the mass of an object are related  Differentiate between the gravitational force weight and what is experienced as apparent weight  Define the friction force and distinguish between static and kinetic friction  Describe simple harmonic motion and explain how the acceleration due to gravity influences such motion.

Mass and Weight ► The weight force, F g, is used to find the downward force of an object. ► Both the net force and acceleration are downward. F g = mg

Example Problems ► Pg. 128 ► Practice Problem 12.  Pg. 129

Friction ► Static friction force  The force that opposes the start of relative motion between the two surfaces in contact ► Friction force with object isn’t in motion ► Kinetic Friction Force  The force that opposes relative motion between surfaces in contact ► Friction force when object is in motion

Calculating Friction Kinetic Friction Force F f, kinetic = µ k F n Static Friction Force 0< F f, static < µ s F N

Typical Coefficients of Friction Surfaceµ s µ k Rubber on concrete Rubber on wet concrete Wood on wood Steel on steel (dry) Steet on steel (with oil) Teflon on steel

Example Problems ► Pg  Balanced Friction Forces  Unbalanced Friction Forces

Terminal Velocity ► The constant velocity that is reached when the drag force equals the force of gravity ► Objects can only fall so fast due to their size and shape and density of the air/fluid  Ping-pong ball – 9 m/s  Basketball – 20 m/s  Baseball – 42 m/s  Skydiver: >62 m/s w/o chute 5 m/s w/ chute 5 m/s w/ chute

Periodic Motion ► Pendulums, springs, strings ► Simple Harmonic Motion  Motion that returns an object to its equilibrium position as a result of a restoring force that is directly proportional to the object’s displacement ► Period (T)  Time needed to repeat one complete cycle of motion ► Amplitude  Maximum distance the object moves from equilibrium

Amplitude, Frequency, Period The Amplitude is the displacement. The Frequency is the number of cycles/sec. The Period is the time for one cycle T = 1/f

Period of a Pendulum

Problems ► Pg. 136 ► 17-19

Sec. 6.3 Interaction Forces ► Objectives  Explain the meaning of interaction pairs of forces and how they are related by Newton’s third law  List the four fundamental forces and illustrate the environment in which each can be observed.  Explain the tension in ropes and strings in terms of Newton’s third law

Interaction forces ► Two forces that are in opposite directions and have equal magnitude ► Newton’s Third Law—all forces come in pairs ► F A on B = -F B on A