Chapter 12

Forces and Motion

What is a Force? Force: A push or a pull that acts on an object

Can cause an object to move, or accelerate a moving object by changing speed or direction

We measure force using newtons (N) Named after Sir Isaac Newton

Combining the Force Much like speed and acceleration, forces can

combine

Forces in the same direction add Forces in the opposite directions subtract

Net Force: Overall force acting on an object

Bringing Balance to the Force The net force can be zero

When this happens, an objects movement is zero

Examples: Tug o’ war Arm Wrestling

Unbalanced Forces When an unbalanced force acts on an object,

the object accelerates

Examples:

Friction Friction: Force that opposes the motion of

objects that touch as they move past each other.

All objects are subject to some kind of friction

Four types of Frictions: Static, Sliding, Rolling, Fluid

Static Friction Static Friction: Force that acts on objects that

are not moving.

Always act in the direction opposite to the applied force

Examples: Ground, Heavy Item

Sliding Friction Sliding Friction: Force that opposes the

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

Sliding friction is less than static friction, this means it takes less force to keep the object moving then to stop and start over.

Rolling Friction Rolling Friction: Force that acts on rolling

objects

Less force than static or sliding

Examples: Dollies, Ball bearings

Fluid Friction Fluid Friction: The force against an object

through fluid

Example: Cake Batter

Air Resistance: Fluid friction acting on an object moving through the air

Gravity Gravity: Force that acts between any two

masses

Earth’s Gravity acts down toward center of Earth

Falling Objects Gravity causes objects to accelerate down Air Resistance acts opposite and reduces accel

If an object falls long enough, these two forces equal themselves

Terminal Velocity: constant velocity of a falling object when gravity=air resistance

Projectiles Projectile motion: the motion of a falling

object (projectile) after it is given an initial forward velocity.

The combination of a forward velocity and downward force of gravity causes a projectile to take a curved path.

Sir Isaac Newton and his Laws 1st Law of Motion: The state of motion of an

object does not change as long as the net force acting on the object is zero

Simplified: An object in motion stays in motion, and an object at rest stays at rest unless acted upon by a force.

1st Law also called Law of Inertia.

Inertia: tendency of an object to resist a change in its motion.

1st Law: Inertia

2nd Law of Motion The acceleration of an object is equal to the

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

Mass: measure of the inertia of an object and depends on the amount of matter an object has.

2nd Law put into a formula Acceleration= Net Force/ Mass

A= F/m

Conceptual 2nd Law The acceleration is always in the same

direction as the net force.

The net force can be a result when forces act in different directions Example: Seat Belt

Weight and Mass: What’s the difference?

Weight: Force of gravity acting on an object

Weight= mass x gravity W= mg

Basically Newton’s 2nd Law

Mass is a measure of the inertia of an object Weight is a measure of gravity on an object

Newton’s 3rd Law When one object exerts a force on a second

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

For every action there is an equal and opposite reaction

Action/Reaction Force Examples

Action-Reaction forces don’t cancel out because the forces do not act the same

Momentum Momentum: product of an object’s mass and

its velocity

An object has a large momentum if the product of its mass and velocity is large

When an object is at rest, momentum is zero

Momentum Formula Momentum= Mass x Velocity

Conservation of Momentum Law of Conservation of Momentum: if no net

force acts on a system, total momentum does not change

The loss of momentum of one object equals the gain in momentum of another object

Electromagnetic Forces Electric Forces: force that acts between charged

objects or particles Attraction and Repel (Positive and Negative)

Magnetic Forces: force that acts on certain metals, on the poles of magnets, and on moving charges Magnets can also attract and repel (North and South)

Nuclear Forces Nuclear Force: The force that is strong enough

to hold the nucleus together in an atom

Strong Nuclear: Powerful force that acts only on holding neutrons and protons together

Weak Nuclear: Not as strong, acts only on a short range

Gravitational Force Gravitational force: attractive force that acts

between any two masses

Every object in the universe attracts every other object

The reason why we don’t see these is because our masses are so small

Gravity over distances The greater the mass of the objects, the

greater gravitational force

It is the weakest force but is the most effective force over long distances

Earth, Moon, and Tides The moon has inertia, so it should go in a straight line

But the Earth holds it in place because of gravity

Centripetal force: center-directed force that continuously changes the direction of an object to make it move in a circle

The gravity the moon exerts on the Earth causes bulges in the oceans, causes high tides and low tides

Satellites Satellites work very similar to the moon, by

using the Earth’s gravity and centripetal force to stay in orbit

Today, we use satellites from monitoring the weather to getting and receiving cell phone calls