Chapter 4 Force and The Laws of Motion Physics Teacher: Mrs. R. Williams

Background Sir Isaac Newton ( ) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.

Newton’s Laws of Motion 1. An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. 2. Force equals mass times acceleration (F = ma). 3. For every action there is an equal and opposite reaction.

SECTION 4.1--Force ◦ Definition: Force is the cause of an acceleration or the change in an object’s velocity ◦ A force can be a push, a pull, and pressure; force can act directly through physical contact or at a distance like a magnetic field. ◦ We measure force in NEWTONS.  A newton is the force needed to accelerate a 1 kilogram mass by 1 meter per second squared 1N = 1kg x 1m/s 2

Newtons ◦ Newtons ARE NOT units of weight or mass! ◦ Weight: What you weigh is a measure of your mass on the planet earth. Your weight can change relative to your environment! Mr. Price weighs 200 pounds on earth, but on the moon he would weigh only 33 pounds! ◦ While your weight can change your mass is constant regardless of your environment. Although Mr. Price would weigh 33 pounds on the moon his MASS would still be 200 pounds on the moon, Jupiter or in a black hole! ◦ Newtons are a measure of force—we can use newtons to measure how much force the earth is exerting on your body (your weight) but newtons are units of force! ◦ Conversion Factors  1 pound = newtons  1 newton = pounds

Forces Two Types of Forces 1. Contact Forces: The force that arises from the physical contact of two objects. 2. Field Forces: The force that can exist between objects even in the absence of physical contact (like gravity and magnetism) The 4 Fundamental Forces of the Universe ◦ Gravity ◦ Electromagnetism ◦ The Strong Nuclear Force ◦ The Weak Nuclear Force

Force Diagram Force Diagrams ◦ FORCE IS A VECTOR with both a magnitude and a direction ◦ Forces are represented by arrows which indicate the direction of the force. The length of the arrow indicates the magnitude of the force. ◦ Diagrams are often used to analyze situations where more than one force is acting on an object. These are known as “force diagrams” ◦ Simple force diagrams of single objects and the forces acting on them are called “free-body diagrams.”

Section 4-2—Newton’s First Law ◦ Inertia: “An object at rest remains at rest, and an object in motion continues in motion unless the object experiences a net external force.” ◦ Acceleration is determined by net external force.  Newton’s first law implies that the net external forces on an object (with a constant velocity) must be equal to zero.  “net external forces” means the sum of all of the individual forces—add those vectors!  The Greek letter sigma ( Σ ) is used to designate a sum.

What does this mean? Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

What is meant by unbalanced force? If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

Some Examples from Real Life A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion. Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

Newton’s First Law ◦ Mass is a measurement of inertia  The inertia of an object is proportional to its mass— bigger objects are harder to move and harder to stop.  Don’t confuse “size”, “mass” and “density” ◦ Equilibrium  Equilibrium exists when the net forc3es on an object equal zero.  Equilibrium can occur when an object is at rest or moving with a constant velocity.

Section 4-3--Newton’s 2 nd and 3 rd Laws Newton’s 2 nd Law: Force = Mass x Acceleration Σ F = ma  For objects at equilibrium a=0 Newton’s 3 rd Law: For every action there is an equal and opposite reaction.  Newton’s third law implies that forces always exist in pairs.  We often split these pairs up into the action force and the reaction force, which are always have the same magnitude but opposite directions.  Action/reaction pairs can cancel each other out which results in equilibrium.  Action/reaction pairs can also accelerate objects (see action/reaction handout) ◦ Field Forces also Exist in Pairs

Section 4-4—Everyday Forces ◦ Weight  “Weight” is defined in physics as the force of gravity exerted on a mass. Weight has magnitude but no direction making it a scalar quantity.  Remember the force of gravity is g = 9.81m/s 2  This value changes inversely to the distance from the center of the earth (you weigh less at higher altitudes!)

THE NORMAL FORCE ◦ The Normal Force  The “Normal Force”: A force exerted by one object on another in a direction perpendicular to the surface of contact. (“The Floor Force”)  *The mathematical definition of normal is “perpendicular”.  The normal force is always perpendicular to the surface of contact…NOT always opposite the force of gravity.  F n = m g cos Θ

FRICTION ◦ The Force of Friction  Whenever two objects come into contact some energy is lost to friction.  Friction is defined as the resistive force that opposes the relative motion of two contacting surfaces. ◦ Two Types of Friction  Static Friction is the friction experienced by two or more objects that are in contact and at rest.  Kinetic Friction is the friction experienced by two or more objects that are in contact and are moving relative to each other.  Kinetic friction is less than static friction  Friction must always be considered in order to exactly calculate net forces.

Force of Friction ◦ The force of friction is proportional to the normal force (i.e. more massive objects experience more friction and a larger normal force) ◦ Friction depends on the surfaces in contact (i.e. smooth surfaces experience less friction than rough surfaces.) ◦ The Coefficient of Friction (the Greek letter mu – μ )  The coefficient of static friction ( μ s ) is the ratio of the maximum value of the force of static friction to the normal force.  The coefficient of kinetic friction ( μ k ) is the ratio of the force of kinetic friction to the normal μ = Friction force Normal force