Acknowledgements © 2013 Mark Lesmeister/Pearland ISD This work is licensed under the Creative Commons Attribution- ShareAlike 3.0 Unported License. To view a copy of this license, visit or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA. Selected graphics and problems from OpenStax College. (2012, June 12). College Physics. Retrieved from the Connexions Web site: Cartoons from Looney Tunes Movie Collection, © 2005 Warner Brothers Entertainment. Used under the fair use doctrine for educational purposes. Selected questions from

Pearland High School Physics

FORCE Force is a push or pull exerted on some object. Forces cause changes in velocity such as: Start moving, stop moving or change direction. The SI unit for force is the Newton. 1 Newton = 1 kg m/s 2

Types of Forces  Forces can act through contact or at a distance.  Contact Force – physical contact between two objects  Field Force – does not involve physical contact between two objects.  Example include: electrical forces magnetic forces the force of gravity

Part 1

Observation #1 An object at rest remains at rest, unless something makes it move.

Picture a ball on a table in a moving train car. Is rest the natural state of an object? STOP

Observation #2 An object in motion continues in motion with constant velocity, unless something makes it change its velocity. Constant velocity means constant speed in the same direction.

Combining Observations 1 & 2 An object left alone will not change it’s velocity. Something must cause a change in velocity. A force is something that causes an acceleration or change in velocity Change in speed Change in direction.

Objects and Systems An object is something that has no internal structure, or that we can treat as having no internal structure. Ex: Electron A system is an object or collection of objects grouped together for study. Ex. Atoms

External and Internal Forces An object cannot exert a force on itself. Internal forces have no effect on the motion of a system as a whole. Only external forces are considered in Newton’s Laws.

Observation #3 An object will not change its velocity unless a net external force acts on it.

Newton’s First Law Objects do not change their motion without a cause. Forces are what cause changes in motion. It is the net external force acting on an object that determines whether it will change motion.

Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force. A net external force is required to change velocity.

Force SI unit of force is the Newton (N). 1 N = lb 1 lb. = N A force is a vector. It has a magnitude, measure in N or lbs. It acts in a particular direction.

Common Forces The force of gravity (F g ) pulls straight down. The force of friction (F f ) occurs between two objects that can slide against each other. It opposes the relative motion of the surfaces.

Common Forces The normal force (F N ) is the support force from a surface. It is called “normal” because it is always perpendicular to the surface. The tension (F T ) is the force in a rope or string. The tension is the same in every part of a rope.

Free-body diagram Free-body diagrams consider just one object and the forces that act on it. To draw a free body diagram Draw a dot to represent the object. Draw and label vector arrows representing all the forces acting on the object. All the vectors should be shown as acting at a single point.

Part 2

Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant velocity, unless the object experiences a net external force. A net external force is required to change velocity.

Inertia Another way to say the First Law is to say that objects have inertia. Inertia is the tendency of objects to resist changes in motion. The amount of inertia an object has is determined by its mass.

What is happening? This is the ball in the train car again. The ball appears to start moving for no reason. Does this violate Newton’s First Law?

Inertial Reference Frame An inertial reference frame is one in which Newton’s First Law holds. Accelerating reference frames are not inertial.

Inertial reference frame The Earth is not an inertial reference frame. Large scale motions on the Earth will appear to curve without a force. This is because the Earth is rotating. This includes Large air and ocean currents. Long range missiles.

Forces A force is the interaction of two objects. There are four fundamental interactions, in other words four fundamental forces. PP 2 fm Carbon-14 Nitrogen e-e- PP 2 fm 1 kg 4 kg

Four fundamental forces Gravitational forces Strong nuclear forces Weak nuclear forces Electromagnetic forces

Agent-Object Notation To avoid confusion, we will often identify forces with two subscripts. The first subscript indicates what agent is applying the force. The second subscript indicates to what object the force is applied. F E-M

Types of forces Contact forces- result from physical contact between two objects. Field forces- force that can exist between two objects even in the absence of physical contact. E.g. gravity, electric forces Objects may be in contact, they just don’t have to be.

Weight/Mass Relationship Weight is the magnitude of the force of the Earth’s gravity on an object. The force of gravity is shown in diagrams as F E-O or F g. Mass is a measure of the amount of matter in an object. Weight and mass are proportional. The constant of proportionality near the surface of the Earth is g = 9.81 N/kg. The weight of an object is often written as mg.

Gravitational Mass vs. Inertial Mass The property of an object that determines how much weight it has at a certain place is called its gravitational mass. The property of an object that determines its resistance to changes in motion is called its inertial mass. Experiments have confirmed that these two properties are the same.

Equilibrium model Objects that are at rest or moving with constant velocity are in equilibrium. According to Newton’s First Law, objects in equilibrium have a net external force that equals 0. Δv = 0, ∑ F = 0 ∑ F = 0, Δv = 0

Equilibrium Example A rock-climber who weighs 800 N is held in place by two ropes. One pulls horizontally to the right, and the other pulls up and to the left at an angle of 30 o from the horizontal. What is the tension in the ropes? First, draw a FBD for the rock climber and label your forces (drawing your forces against a backdrop of a coordinate system might help with resolving the forces)

T1T1 T2T2 FGFG θ T1T1 Equilibrium Example Model: Equilibrium

Section 3

Which has more force? When the boxer hits the bag, which has more force, the boxer on the bag or the bag on the boxer?

Newton’s Third Law If Object A exerts a force on Object B, then B exerts a force on Object A that is equal in magnitude but opposite in direction. ©2012 OpenSTAX College

Newton’s Third Law The two forces are called an action-reaction pair. The two forces do not balance each other, since they act on different objects. ©2012 OpenSTAX College

Action-Reaction Pairs Identify all the action-reaction pairs involved in a ball sitting on a table.

Action-Reaction Pairs Identify all the action-reaction pairs involved in a ball sitting on a table.