Ch. 12Motion & Forces

I. Newton’s Laws of Motion “If I have seen far, it is because I have stood

on the shoulders of giants.”- Sir Isaac Newton (referring to Galileo)

Newton’s First Law

• Newton’s First Law of Motion– An object at rest will remain at rest and an

object in motion will continue moving at a constant velocity unless acted upon by a net force.

Newton’s Second Law

• Newton’s Second Law of Motion– The acceleration of an object is directly

proportional to the net force acting on it and inversely proportional to its mass.

F = ma

Newton’s Third Law

• Newton’s Third Law of Motion– When one object exerts a force on a second

object, the second object exerts an equal but opposite force on the first.

III. Defining Force Force

Newton’s First Law Friction

A. Force• Force

– a push or pull that one body exerts on another– What forces are being

exerted on the football?

Fkick

Fgrav

A. Force• Balanced Forces

– forces acting on an object that are opposite in direction and equal in size

–no change in velocity

A. Force• Net Force

–unbalanced forces that are not opposite and equal

–velocity changes (object accelerates)

Ffriction

W

Fpull

Fnet

NN

Calculating Net Force from Images• Net force is the sum of forces acting on an object

– If forces are acting in the same direction they combine (add together)

– If forces are acting in opposite directions they reduce (subtract)

• If net force is zero the motion of the object will NOT change

• If net force is not zero the object will accelerate (this may be positive or negative acceleration).

What is the net force on the objects below?

Opposite = subtract8N – 3N = 5 N

Same = addition5N + 2N = 7 N

2 N + 5N – 8N = 1 N 7N + 2N – 3N = 6N

B. Newton’s First Law• Newton’s First Law of Motion

–“Law of Inertia”• Inertia

–tendency of an object to resist any change in its motion

–increases as mass increases

• Newton’s First Law of Motion– An object at rest will remain at rest and an

object in motion will continue moving at a constant velocity unless acted upon by a net force.

ConcepTest 1

TRUE or FALSE?

The object shown in the diagram must be at rest since there is no net force acting on it.

FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.

ConcepTest 2

1. Before and after the collision, there is a rightward force pushing you into the door.

2. Starting at the time of collision, the door exerts a leftward force on you.

3. both of the above

2. Starting at the time of collision, the door exerts a leftward force on you.

You are a passenger in a car and not wearing your seat belt.

Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.

Which is the correct analysis of the situation? ...

C. Newton’s Second Law• Newton’s Second Law of Motion

–The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

F = ma

Newton’s Second Law

F = maF: force (N)m: mass (kg)a: accel (m/s2)

1 N = 1 kg ·m/s2

mF

a

CalculationsWhat force would be required to

accelerate a 40 kg mass by 4 m/s2?

GIVEN:

F = ?

m = 40 kg

a = 4 m/s2

WORK:

F = ma

F = (40 kg)(4 m/s2)

F = 160 N

mF

a

CalculationsA 4.0 kg shot-put is thrown with 30 N of

force. What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2

mF

a

D. Gravity• Gravity

– force of attraction between any two objects in the universe

– increases as...• mass increases• distance decreases

B. Gravity• Who experiences more gravity - the

astronaut or the politician?

less distance

more mass

Which exerts more gravity - the Earth or the moon?

D. Gravity• Would you weigh more on Earth or

Jupiter?

greater gravity

greater weight

greater mass

Jupiter because...

Free Fall• When gravity is the

only force acting on an object

• Represented by the letter g

• Near Earth’s surface g = 9.8 m/s2

B. Gravity• Weight

– the force of gravity on an object

MASSalways the same

(kg)

WEIGHTdepends on gravity

(N)

W = mgW: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

W: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

W: weight (N)m: mass (kg)g: acceleration due

to gravity (m/s2)

D. Gravity• Accel. due to gravity (g)

In the absence of air resistance, all falling objects have the same acceleration!

On Earth: g = 9.8 m/s2

mW

g

elephant

m

Wg

feather

CalculationsMrs. J. weighs 557 N. What is her

mass?

GIVEN:

W = 557 N

m = ?

g = 9.8 m/s2

WORK:

m = W ÷ g

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg

mW

g

ConcepTest• Is the following statement true or false?

– An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.

False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

E. Newton’s 3rd Law• For every action

force, there is an equal and opposite reaction force.

3rd law: Action / Reaction• The action force and the

reaction force occur to different objects so the force is not balanced.

• Not all action and reaction forces produce motion.

• Unbalanced forces equal changes in motion

Newton’s third law• Kicking a soccer ball

– Action force = your foot hits the ball– Reaction force = the ball pushes against your foot.

• Ball moves b/c action force is larger than reaction force

• Leaning against a wall– Action force = you pushing against a wall– Reaction force = wall pushing against you

• Nothing moves b/c the action force equals the reaction force.

F. Friction• Friction is a force that opposes the

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

• Friction acts at the surface where object are in contact

• Four main types of friction: static friction, sliding friction, rolling friction and fluid friction.

C. Friction

• Friction– is the force that opposes motion

between 2 surfaces–depends on the:

• types of surfaces• force between the surfaces

C. Friction• Friction is greater...

–between rough surfaces–when there’s a greater

force between the surfaces (e.g. more weight)

• Pros and Cons?

Static Friction• Static friction is the friction

force that acts on objects that are not moving.

• Static friction always acts in the direction opposite to that of the applied force

• Prevents objects from sliding.

• Example: pushing a dresser that does NOT move

Sliding Friction• Sliding Friction is a force

that opposes the direction of motion of an object as it slides over a surface.

• Occurs when there is enough force to overcome the static friction

• There will be a net force in the direction of motion

• Pushing a desk that slides against the floor.

Rolling Friction• Rolling friction is the

force that acts on rolling objects.

• Rolling friction is about 100 to 1000 times less than the force of static or sliding friction

• Skate boarding down a hill

Fluid Friction• Fluid friction opposes the

motion of an object through a fluid.

• Fluid examples: air, water, quick sand, and cake batter

• Air Resistance is the fluid friction action on an object moving through the air.

What are the forces acting on a falling leaf?

Air Resistance• Terminal Velocity

–maximum velocity reached by a falling object

– reached when… Fgrav = Fair

Fair

Fgrav

no net force no acceleration constant velocity still falling

Projectile Motion

• Projectile motion the motion of a falling object after it is given an initial forward velocity.

• this is a curved path• Objects with different mass fall at

the same rate.• The combination of an initial

forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

Momentum • Momentum is the product of an object’s mass

and its velocity, an object with large momentum is hard to stop

• momentum for any object at rest is zero• Law of conservation of momentum if not net

forces acts on a system, then 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