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CHAPTER 4NEWTONS LAWOutlineForces and Newtons LawWeight, Tension and Normal ForceFriction: Static and KineticForces of FrictionApplication of Newtons LawIntroductionIn previous chapters, motion is described in terms of displacement, velocity, and acceleration without considering what might cause that motion.This chapter will look at what causes changes in motion.The two main factors to consider are the forces acting on an object and the mass of the object.MASS

INERTIA is a property of an object that describes how much it will resist change to the motion of the object.5FORCE

Contact forces involve physical contact between two objects.

Field forces involve non-physical contact between two objects but instead act through empty space.Measuring the Strength of a Force

Newtons First Law(Law of Inertia)An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

Balanced and Unbalanced Force

Equal forces in opposite directions produce no motionUnequal opposing forces produce an unbalanced forcecausing motionNewtons 1st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. Once airborne, unless acted on by an unbalanced force (gravity or air), it would never stop!

Newtons 1st Law Wonder why we need to buckle up the seat belt when we are in a moving car?

Law of InertiaNewtons 1st Law

If objects in motion tend to stay in motion, why dont moving objects keep moving forever?

Things dont keep moving forever because theres almost always an unbalanced force acting upon them.What is this unbalanced force that acts on an object in motion?

Newtons Second LawThe acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

F = maThe SI unit of force is the Newton, which is defined as the force that, when acting on a 1-kg mass, produces an acceleration of 1 m/s2.From this definition and Newtons second law, we see that the Newton can be expressed in terms of the following fundamental units of mass, length, and time:

Newtons 2nd Law Newtons 2nd Law

If you want to calculate the acceleration, first you need to modify the force equation to get a = F/m. When you plug in the numbers for force (100 N) and mass (50 kg), you find that the acceleration is 2 m/s2.Newtons 2nd Law

Notice that doubling the force by adding another dog doubles the acceleration. Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 m/s2.Newtons Third LawFor every action there is an equal and opposite reaction.

A man with a mass of 50 kg run into a moving 1000kg bus. Which will have the most force?The man on the bus or the bus on the man?Newtons 3rd Law

Newtons 3rd Law

For example, if a lady slaps his boyfriend for whatever reason, the ladys hand exerts a force on her boyfriends face. The force provided by the lady is called the action force.But the boyfriends thick face also exerts the same force on the ladys hands. The receiving end of the force, which is equal and opposite in magnitude, is called the reaction force.Newtons 3rd Law Action reaction force can only exist when there are two objects interacting.In the previous example, we have the ladys hand and the mans handsome face as the objects of interaction. This is equivalent to saying that a single isolated force cannot exist.Newtons 3rd Law

Tire pushes on roadRoad pushes on tireNewtons 3rd Law

Action: rocket pushes on gasesReaction: gases push on rocketNewtons 3rd Law When it is not an Action-Reaction?

Fg Fg are action and reaction forces. n n are action and reaction forces.Fg n pair are not action reaction force because they act on the same object, the TV.

The weight of an object FW is the gravitational force acting downward on the object.

Force of Gravity and WeightWe are well aware that all objects are attracted to the Earth.The attractive force exerted by the Earth on an object is called the force of gravity Fg .This force is directed toward the center of the Earth, and its magnitude is called the weight of the object.Force of Gravity and Weight (cont.)A freely falling object experiences an acceleration g acting toward the centre of the Earth. Applying Newtons second law:

to a freely falling object of mass m,

Force of Gravity and Weight (cont.)Because weight = Fg = mg, we can compare the masses of two objects by measuring their weights on a spring scale. At a given location, the ratio of the weights of two objects equals the ratio of their massesForces of FrictionWhen a body is in motion, there is resistance to the motion because the body interacts with its surroundings.We call such resistance a force of friction.E.g., movement of fish in the water, or the flight of bird in the air. The former faces friction against water while the latter against air friction.Forces of Friction (cont.)Frictional forces is not constant however.Let us take an example of someone pushing a book.Assume the book was stationary initially.Frictional Forces at workForce applied on a book.

Frictional Forces at work (cont.)Force opposing the book from moving.

Frictional Forces at work (cont.)If we apply an external horizontal force F to the book, acting to the right, the book remains stationary if F is not too great. The force that counteracts F and keeps the book from moving acts to the left and is called the frictional force f.Frictional Forces at work (cont.)Because the book is stationary, we call this frictional force the force of static friction fs.Static friction exist because the rough edges of 2 objects are rubbing against each other as shown in the diagram in page 60. Frictional Forces at work (cont.)If we increase the magnitude of F, as shown in page 61, the magnitude of fs increases along with it, keeping the book in place. The force fs cannot increase indefinitely, however. Eventually the surfaces in contact can no longer supply sufficient frictional force to counteract F, and the book accelerates.Frictional Forces at work (cont.)When the book is on the verge of moving, fs is at maximum, as shown in page 61. When F exceeds fs,max , the book accelerates to the right. Once the book is in motion, the retarding frictional force becomes less than fs,max. Frictional Forces at work (cont.)When the book is in motion, we call the retarding force the force of kinetic friction fk .Experimentally, we find that, to a good approximation, both fs,max and fk are proportional to the normal force acting on the book.

Frictional Forces at work (cont.)Force of static friction fs is given as:

Where s is the coefficient of static friction.Force of kinetic friction fk is written as:

Where k is the coefficient of kinetic friction

Newtons Third Law

Application of Newtons LawsWhen we apply Newtons laws to an object, we are interested only in external forces that act on the object. (Very important!!)Application of Newtons LawsIdentify all forces present for the object below.

Application of Newtons LawsIdentify all forces present for the object below.

Application of Newtons LawsUse the symbol T to denote the force exerted by the rope on the crate. The magnitude of T is equal to the tension in the rope.

This diagram is term free-body diagram. It shows all the forceacting on the object of study.Application of Newtons Laws (cont.)We can now apply Newtons second law in component form to the crate. The only force acting in the x direction is T.

Horizontal Force

Vertical ForceSince the block of crate is not accelerating in the y direction, we have

Horizontal MovementWe can now find the velocity and distance travelled by the crate in the x direction using the following formulas: