PEARSON PRENTICE HALL PHYSICAL SCIENCE: CONCEPTS IN ACTION

Chapter 12Forces and Motion

12.1 Forces Objectives: 1. Describe examples of force and identify

appropriate SI units used to measure force 2. Explain how the motion of an object is

affected when balanced and unbalanced forces act on it

3. Compare and contrast four kinds of friction 4. Describe how Earth’s gravity and air

resistance affect falling objects 5. Describe the path of a projectile and

identify the forces that produce projectile motion

Force and SI Units In its most basic form, a force is a push

or a pull A force can cause a resting object to

move or it can accelerate a moving object by changing the object’s speed or direction

The SI unit of force is the Newton (N) 1 N causes a 1 kg mass to accelerate at

a rate of 1 m per second each second 1 N = 1 kg-m/s2

Use an arrow to represent the direction & strength of a force

Balanced & Unbalanced Forces

You can combine force arrows to show the result of how forces combine

Forces in the same direction add together while opposing forces are subtracted

Definition: net force is the overall force acting on an object after all forces are combined

When the forces on an object are balanced the net force is zero & there is no change in the object’s motion

When an unbalanced force acts on an object, the object accelerates

Friction Definition: friction is a force that opposes

the motion of objects that touch as they move past each other

Four types of friction are static friction, sliding friction, rolling friction and fluid friction

Definition: static friction is the friction force that acts on objects that are not moving

Static friction acts in the direction opposite that of the applied force

Definition: sliding friction opposes the direction of motion of an object as it slides over a surface

Sliding friction is less than static friction & less force is needed to keep an object moving or start it moving

Definition: rolling friction is the force that acts on rolling objects

When a round object rolls across a flat floor, both the object and the floor are bent slightly out shape

Definition: fluid friction opposes the motion of an object through fluid Ex: spoon through batter

Definition: a fluid is a mixture of water and gases Definition: air resistance is fluid friction acting on

a moving object through the air At high speed air resistance becomes important This is why bicyclists, skaters and other athletes

wear slick racing suits

Gravity and Falling Objects Definition: gravity is a force that acts

between any two masses Earth’s gravity acts downward toward the

center of the earth Gravity causes objects to accelerate

downward Air resistance acts in the direction

opposite of gravity, opposing the motion and reducing acceleration

Definition: terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity

Terminal velocity equals 9.8 m/s2

Projectile Motion

Definition: projectile motion is the motion of a falling object after it is given an initial forward velocity

The combination of an initial forward velocity and the downward vertical force of gravity causes the object to follow a curved path

12.2 Newton’s First & Second Laws of Motion

Objectives: 1. Describe Newton’s first law

of motion & its relation to inertia

2. Describe Newton’s second law of motion and use it to calculate acceleration, force and mass values

3. Relate the mass of an object to its weight

Newton’s first law In the mid 1600s Isaac Newton expanded on the

work of Galileo (Galileo’s work from late 1500s to early 1600s)

Newton’s first: the state of motion of an object does not change if the net force acting on it is zero

Put another way: an object at rest tends to stay at rest & an object in motion tends to stay in motion with the same direction and same speed

Definition: inertia is the tendency of an object to resist a change in its motion

Newton’s first law is also called the Law of Inertia Q: How does a zero net force affect an object’s

motion?

Newton’s Second Law Newton’s second: the acceleration of an object is

equal to the net force acting on it divided by the object’s mass

Acceleration = net force or a = F Mass m Cross multiplying the equation tells us that F=ma It is useful to know that N & m are equivalent kg s2

The acceleration of an object is always the same direction as the net force but..

The second law applies when net force acts in a direction opposite to the object’s motion such as seat belts opposing a passenger’s forward motion in a collision (causing deceleration)

Let’s try a couple problems… Q: A 20 N net force acts on an object with

a mass of 2.0 kg. What is the object’s acceleration?

A: F=ma F=20 N m= 2.0 kg so 20=(2.0)(a)

Divide both sides by 2.0 20/2.0 = 2.0/2.0 (a)

A= 10 m/ s2 Q: What is the acceleration of a 1000 kg

car subject to a 550 N net force? A: F=ma or a=F/m

550 N/ 1000 kg = 0.5 m/ s2

Mass and Weight Definition: weight is the force of gravity acting on

an object Weight formula: weight = mass x acceleration

due to gravity or W=mg F or W must be written in Newtons, acceleration

in meters per second squared and the mass in kilograms

Acceleration due to gravity = 9.8 m/ s2 Q: If an astronaut has a mass of 112 kg, what is

his weight on Earth: W= mg so 112 kg x 9.8 m/ s2 = 1100 N Mass is a measure of inertia of an object Weight is a measure of the force of gravity acting

on an object

12.3 Newton’s Third Law of Motion and Momentum

Objectives: 1. Explain how action and

reaction forces are related according to Newton’s third law of motion

2. Calculate the momentum of an object and describe what happens when momentum is conserved during a collision

Action and Reaction Forces Newton’s third law: Whenever one object exerts a

force on a second object, the second object exerts an equal and opposite force on the first object

Forces come in pairs: push and pull, action & reaction

Ex: swimmer pushes against the water (creating an action force) & the water produces a reaction force that pulls the swimmer along (action on the water, reaction on the swimmer)

When action & reaction forces do not act on the same object as in the water & swimmer, the net force is not zero

When the action & reaction forces act on the same object the net force is zero

Momentum Definition: momentum is the product of an

object’s mass and its velocity If the object has a lot of momentum, it is

hard to stop An object has a lot of momentum if the

product of its mass and velocity is large Momentum = mass x velocity Law of Conservation of Momentum: if no

net force acts on a system, the total momentum of the system does not change

In a closed system, the loss of momentum of one object equals the gain in momentum of another object & momentum is conserved

Let’s try a problem Q: Which has more momentum, a 0.046

kg golf ball travelling at 60 m/s, or a 7.0 kg bowling ball going 6.0 m/s? **notice the units for momentum are kg-m/s**

A. Golf ball = 0.046 kg x 60 m/s = 2.8 kg-m/s

Bowling ball= 7.0 kg x 6 m/s = 42 kg-m/s So the bowling ball has more momentum Q: If an eagle and a bumblebee are

traveling at 8 km/h, which has more momentum & why?