CHAPTER 3--FORCES

Section 1: NEWTON’S SECOND LAW

REVIEW—NEWTON’S 1ST LAW(LAW OF INERTIA) If an object is moving

at constant velocity, it keeps moving at that velocity unless a net force acts on it; if an object is at rest, it stays at rest, unless a net force acts on it.

INERTIA

Inertia is the tendency of an object to resist any change in motion.

The greater the mass of an object, the greater the inertia.

↑ MASS = ↑ INERTIA

NEWTON’S SECOND LAW The net force acting on an object

causes the object to accelerate in the direction of the net force. The acceleration of an object is determined by the size of the net force and the mass of the object. (LONG VERSION)

OR This law describes how force, mass, and

acceleration are connected. (SHORT VERSION)

FORCE AND ACCELERATION The greater the force that is

applied to an object, the greater the acceleration will be.

Examples…Ball thrown hard = greater force and accelerationBall thrown gently = less force

and acceleration

FORCE AND MASS If you throw a softball and a

baseball as hard as you can…why don’t they have the same speed?

The difference is due to their masses.

MASS of softball= 0.20 kg

MASS of baseball=0.14 kg

NEWTON’S 2ND LAW The acceleration of an object depends on its mass, as

well as the force, exerted on it. Remember—If more than one force acts on an object,

the forces combine to form a net force. NEWTON’S 2ND LAW can be written as the following

equation Acceleration =net force

massOR a = F

m

SI UNITS The unit of MASS is the kg

The unit of ACCELERATION is m/s2

The unit of FORCE is kg x m/s2, which is also called the Newton (N)

PROBLEM A student pedaling a

bicycle applies a net FORCE of 200 N.

The MASS of the rider and the bicycle is 50 kg.

What is the ACCELERATION of the bicycle and the rider?

SOLVING FOR ACCELERATION a = F m

a = 200 N = 50 kg

4 m/s2

PROBLEM MASS of a tennis ball is

0.06 kg

ACCELERATION of the ball leaving the racket is 5,500 m/s2

What is the FORCE that is applied to the racket?

SOLVING FOR FORCE

F = ma

F = (0.06 kg) (5,500 m/s2)

F = 330 N

PROBLEM A student riding a

skateboard applies a net FORCE of 180 N.

The ACCELERATION of the skateboard and the rider is 3 m/s2.

What is the MASS of the skateboard and the rider?

SOLVING FOR MASS m = F

a

m = 180 N (kg x m/s2) 3 m/s2

m = 60 kg

FRICTION The force that opposes motion

between 2 surfaces that are touching each other

The amount of friction depends on 2 factors1. the types of surfaces2. the force pressing the surfaces together

FRICTION What causes friction?

If 2 surfaces are pressed tightly together, welding or sticking occurs in those areas where the highest bumps come into contact with each other. These areas where the bumps stick together are called microwelds and are the source of friction.

MICROWELDS

STATIC FRICTION Friction between 2 surfaces that are

NOT moving past each other. MICROWELDS have formed between

the bottom of the box and the floor.

SLIDING FRICTION Force that opposes the motion of 2

surfaces sliding past each other. Caused by MICROWELDS constantly

breaking and then forming again as the box slides across the floor.

ROLLING FRICTION Friction between a rolling object and

the surface it ROLLS on. Because of rolling friction, the wheels

of a train rotate when they come into contact with the track, rather than sliding over it.

ROLLING FRICTION Rolling friction is usually much less

than static or sliding friction. That is why it is easier to pull a load in a wagon, rather than pushing it on the ground.

AIR RESISTANCE When an object falls

toward Earth, it is pulled downward by the force of gravity; however, another force called air resistance acts on objects that fall through the air.

FORCES THAT OPPOSE MOTION Friction, as well as air resistance,

acts in the direction opposite to that of the object’s motion.

AIR RESISTANCE The amount of air

resistance on an object depends on the speed, size, and shape of the object (NOT MASS).

CHAPTER 3: FORCES

Section 2--GRAVITY

GRAVITY Anything that has mass is attracted by

the force of gravity.

According to the law of gravitation, any 2 masses exert an attractive force on each other

GRAVITY The attractive force depends on the

mass of the 2 objects and the distance between them.

Gravity is 1 of the 4 basic forces. (The other basic forces are the electromagnetic force, the strong nuclear force, and the weak nuclear force.

MASS(M) VS. DISTANCE(D)

If the mass of either of the objects increases, the gravitational force between them increases. (Left pic) ↑M = ↑GF

If the objects are closer together, the gravitational force between them increases. (Right pic)↓ D=↑GF

GRAVITATIONAL ACCELERATION Near Earth’s surface, the gravitational

attraction of Earth causes all falling objects to have an acceleration of 9.8 m/s2.

As a result, all objects fall with the same acceleration rate, regardless of their mass.

GRAVITATIONAL ACCELERATION According to the second law, the force

on an object that has an acceleration of 9.8 m/s2 is as follows:

F = m x 9.8 m/s2

A force has a direction. The force of gravity is always directed

downward.

WEIGHT (W) The gravitational force exerted on an

object is called the object’s weight.

You can find the gravitational force or weight, by using Newton’s 2nd Law…

EQUATIONS Gravitational Force(F)=mass(m) x

acceleration due to gravity(a=9.8 m/s2)OR Weight(W)=mass(m) x acceleration

due to gravity(a=9.8 m/s2)

***Because gravitational force = weight;mass ≠ weight

***Massstays the same; WEIGHTchanges due to gravity

WEIGHT AND MASS ARE NOT THE SAME.

WEIGHT is a force due to gravity.

WEIGHT changes when gravity changes.

MASS is a measure of the amount of matter in an object.

MASS always stays the same.

PROBLEM Mass of a person = 50 kgWhat is the weight of this person?

Weight (W)=mass(m) x 9.8 m/s2

W = 50 kg x 9.8 m/s2 = 490 kg x m/s2 or 490 N

PROBLEM What is my MASS? Mass = 150 poundsConvert to

kilograms 1 kg = 2.2 lbs (CONVERSION

FACTOR) 150 lbs x 1 kg

2.2 lbs = 68.2 kg

PROBLEM What is my WEIGHT? W = mass x 9.8 m/s2 ( accel. due to

gravity)

W = 68.2 kg x 9.8 m/s2

W = 668.4 kg x m/s2

OR 668.4 N

PROJECTILE MOTION Anything that is thrown or shot through

the air is called a projectile. Projectiles follow a curved path, because

they have horizontal and vertical velocities.

Example—Force from a thrown ball—horizontal

velocityForce from gravity—vertical velocityRESULT: curved path

CENTRIPETAL FORCE Centipetal force is a force directed

toward the center of the circle for an object moving in a circular motion.

The word “centripetal” means to move toward the __________.

Examples—amusement park rides, bucket/water

CHAPTER 3: FORCES

Section 3—Newton’s Third Law of Motion

NEWTON’S THIRD LAW OF MOTION When one object exerts a force on a

second object, the second one exerts a force on the first that is equal in size and opposite in direction.

OR Another way to say this is “to every

action force there is an equal and opposite reaction force.”

EXAMPLES

Trampoline—you exert a force downward and the trampoline exerts an equal force upward; Rocket propulsion

***Even though the forces are equal, they are not balanced, because they act on different objects

MOMENTUM Momentum is the property of a moving

object. The momentum of an object is the

product of its mass and velocity. Momentum is represented by the symbol

(p) and can be calculated as follows: Momentum (p) = mass (m) x velocity

(v) The SI unit for momentum is kg x m/s

THE LAW OF CONSERVATION OF MOMENTUM The law of conservation of

momentum states that the total momentum stays the same unless an outside force acts on the objects. Momentum, however, can be transferred from one object to the other.

(P) BEFORE VS. (P) AFTER