FORCE A force is any influence that can change the velocity of a body. Forces can act either through the physical contact of two objects (contact forces: push or pull) or at a distance (field forces: magnetic force, gravitational force). Contact Forces Action-at-a-Distance Forces Frictional Force Gravitational Force Tensional Force Electrical Force Normal Force Magnetic Force Air Resistance Force Applied Force Spring Force

What do we mean by balanced and unbalanced forces? The forces on the book are unbalanced Balanced forces are EQUAL and OPPOSITE in direction

A net force is the vector sum of the forces acting on an object

If all the forces are balanced, we same the object is in EQUILIBRIUM

What are the net forces? These are free-body diagrams

Newton's 1st Law of Motion

FIRST LAW OF MOTION According to Newton's First Law of Motion:   According to Newton's First Law of Motion: " If no net force acts on it, a body at rest remains at rest and a body in motion remains in motion at constant speed in a straight line." Isaac Newton (1642-1727)

MASS and INERTIA   The property a body has of resisting any change in its state of rest or of uniform motion is called inertia. The inertia of a body is related to the amount of matter it contains. A quantitative measure of inertia is mass. The SI unit of mass is the kilogram (kg).

Motion tends to continue unchanged. The elephant at rest tends to remain at rest. Tablecloth trick: Too little force, too little time to overcome "inertia" of tableware.

Newton's 2nd Law of Motion

SECOND LAW OF MOTION   According to Newton's Second Law of Motion, the net force acting on a body equals the product of the mass and the acceleration of the body. The direction of the force is the same as that of the acceleration. In equation form: F = ma

In the SI system, the unit for force is the newton (N): A newton is that net force which, when applied to a 1-kg mass, gives it an acceleration of 1 m/s2.

Weight = mass x gravity W = m x g WEIGHT The weight of a body is the gravitational force with which the Earth attracts the body. Weight (a vector quantity) is different from mass (a scalar quantity). The weight of a body varies with its location near the Earth (or other astronomical body), whereas its mass is the same everywhere in the universe. The weight of a body is the force that causes it to be accelerated downward with the acceleration of gravity g. Weight = mass x gravity W = m x g Newton = kg x m/s2 1 N = 1 kgm/s2

Newton's 3rd Law of Motion

THIRD LAW OF MOTION  According to Newton's third law of motion, when one body exerts a force on another body, the second body exerts on the first an equal force in opposite direction. The Third Law of Motion applies to two different forces on two different objects: "The action force one object exerts on the other, and the equal but opposite reaction force the second object exerts on the first."

Forces occur is pairs…

Net force is sometimes designated F. The second law of motion is the key to understanding the behavior of moving bodies since it links cause (force) and effect (acceleration) in a definite way.

F = ma = 2 m/s2 vf = vo + at = 2(5) = 10 m/s 4.2 A force of 3000 N is applied to a 1500-kg car at rest. a. What is its acceleration? F = 3000 N m = 1500 kg vo = 0 m/s F = ma = 2 m/s2 b. What will its velocity be 5 s later? vf = vo + at = 2(5) = 10 m/s

4. 3 A 1000 kg car goes from 10 to 20 m/s in 5 s 4.3 A 1000 kg car goes from 10 to 20 m/s in 5 s. What force is acting on it? m = 1000 kg vo = 10 m/s vf = 20 m/s t = 5 s = 2000 N F = ma

4.4 A 60-g tennis ball approaches a racket at 15 m/s, is in contact with the racket for 0.005 s, and then rebounds at 20 m/s. Find the average force exerted by the racket. m = 0.06 kg vo = 15 m/s t = 0.005 s vf = - 20 m/s F = ma = - 420 N

= - 3 m/s2 = 10 s x = vot+½at2 = 30(10)+ ½ (-3)(10)2 = 150 m 4.5 The brakes of a 1000-kg car exert 3000 N. a. How long will it take the car to come to a stop from a velocity of 30 m/s? m = 1000 kg F = -3000 N vo = 30 m/s vf = 0 m/s = - 3 m/s2 = 10 s b. How far will the car travel during this time? x = vot+½at2 = 30(10)+ ½ (-3)(10)2 = 150 m

= 1020.4 kg = 3.92 m/s2 vf = vo + at = 0 +3.92(8) = 3.14 m/s 4.7 A net horizontal force of 4000 N is applied to a car at rest whose weight is 10,000 N. What will the car's speed be after 8 s? Fa = 4000 N Fg = 10,000 N t = 8s = 1020.4 kg = 3.92 m/s2 vf = vo + at = 0 +3.92(8) = 3.14 m/s