1. Pearson Prentice Hall Physical Science: Concepts in Action
Chapter 12
Forces and Motion
Pearson Prentice Hall Physical Science: Concepts in Action

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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?

11. 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)

12. 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 = 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 N/ 1000 kg = 0.5 m/ s2

13. 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

14. 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

15. 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

16. 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

17. Let’s try a problem
Q: Which has more momentum, a 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 = 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?