Chapter 11

Newton’s Laws of Motion

Chapter 10 breakdown

Position Motion Speed Velocity Vector Acceleration

11.1: Forces change motion

Force: A push or pull Anytime you are changing the motion of an

object you are using a force Force is a vector

Has both size and direction Three types of forces:

Contact force Gravity Friction

Contact force: Force created when one object pushes or

pulls another object by touching it The first object is applying a contact force

Gravity: The force of attraction between two

masses Strength of the force depends on their

masses Larger the mass = larger force

Friction: A force that resists motion between two

surfaces that are pressed together

Balanced and Unbalanced Forces

Net force: The overall force acting on an object

when all the forces are combined If the net force is zero – the forces

are balanced Balanced forces have the same effect as

no force at all Only an unbalanced force can

change the motion of an object

An object with forces acting upon it can move at a constant velocity as long as the forces are balanced. Balanced forces cannot change an

object’s speed or its direction.

Newton’s First Law of Motion

Objects at rest remain at rest and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force. The “motion” part of the Law will hold

true in environments with no friction. The “rest” part of the Law will hold true

in any environment. AKA: Law of Inertia

Inertia

Inertia: The resistance of an object to a change

in the speed or the direction of its motion

Related to mass: When you measure the mass of an object

you are also measuring the inertia of the object

11.2: Force and mass determine acceleration

If you want to give two objects with different masses the same acceleration you must apply different forces to them.

Newton’s Second Law of Motion: The acceleration of an object increases

with increased force and decreases with massM =

AF = A

Newton’s Second Law contiued

• F = ma• Mathematical formula of the 2nd law• Standard units: – force = newton (N)–mass = kilograms (kg)– acceleration = meters per second per

second (m/s2)• a N is the amount of force needed to

accelerate one kg of mass one m/s2 – 1N = 1kg m/s2

You can rearrange the formula to find acceleration or mass: Acceleration = force divided by mass a = F ÷ m

Mass = force divided by acceleration m = F ÷ a

Force can make an object change direction You can change the direction of an

object without changing its speed

Centripetal Force

Centripetal force: Any force that keeps an object moving

in a circle The force ALWAYS points toward the

center of the circle No matter the mass of the object, if it moves

in a circle, its force and acceleration are directed toward the center of the circle.

11.3: Force acts in pairs

Newton’s Third Law of Motion: Every time one object exerts a force on

another object, the second object exerts a force that is equal in size and opposite in direction back at the first force

For every action there is an equal but opposite reaction

Think about a balloon….what happens when you blow up a balloon and then release it?

Action and reaction pairs

Action / reaction force pair: Force exerted upon one object and the

force that the objects exerts back One force is the “action” force One force is the “reaction” force

Action and reaction forces do not always result in motion If you press against the wall, the wall

resists the push with the same amount of force

Action & reaction forces vs. balanced forces

Action & reaction forces act on different objects: If you pull a backpack across the floor, you

can feel the backpack pull on you with an equal force

One force is acting on you, the other on the backpack

Balanced forces act upon a single object If you and your friend both pull on the

backpack in opposite directions with equal forces, the backpack doesn’t move-both forces are exerted on the backpack

You can use Newton’s 3 laws to describe and predict motion: You can describe why something moves

in the way that it does You can predict how an object’s motion

will change by knowing the force applied

Newton’s 3 laws are not independent of one another – they are used together to explain the motion of objects

11.4: Forces transfer momentum

Momentum (p): The measure of mass in motion Product of its mass and velocity (product

means what in math?) Similar to inertia in regard to mass (the

more mass the more momentum and inertia) BUT momentum also depends on how fast the object is moving

momentum = mass times velocity p = mv

Standard units of measure: mass = kg velocity = m/s momentum = kilogram-meter per

second

Momentum is a vector has both size and direction (from the

velocity)

kg m/s

Transfer of momentum Collision:

a situation in which two objects in close contact exchange energy and momentum

Generally action / reaction forces The object with the greater mass

will have the greater change in velocity

During a collision between two objects, each object exerts a force on the other

Conservation of momentum

Conservation of momentum: the total momentum of a system of objects

does not change as long as no outside forces are acting on the system

System: a collection of objects that affect one another the colliding objects make up a system as they collide they both change velocity and

momentum total momentum of the system remains

unchanged as momentum is transferred not lost

Finding total momentum

objects moving in the same direction: add the momenta of the objects

objects moving in opposite directions: subtract one momentum from the other