Chapter 12 - Unit 10

Forces and Motion

I. Force

A. Is a push or pull

B. Can cause objects to change their velocity

C. Forces are represented with vectors

D. Forces are measured in Newtons (N)

II. Friction

Friction – a force that opposes motion

Four Main Types of Friction

1. Static

2. Sliding

3. Rolling

4. Fluid

A. Static Friction

1. Force that acts on Stationary objects

2. Prevents things from moving easily

3. Strongest friction force

Friction Coefficient

A close up of the roughness of the two surfaces.

The force of gravity.

B. Sliding Friction

1.Two solid surfaces slide over each other

2. Depends on weight and type of surface.

The friction between the glass and the wood is considered sliding friction

C. Rolling Friction

1. One object rolls over a solid surface

2. Produced by wheels or ball bearings

Ball Bearings

Rolling Friction

D. Fluid Friction

1. When an object moves through a fluid.

2. The fluid opposes motion

3. Air resistance or water resistance

The friction between the board and the water is considered fluid friction

What forces are acting on this person?

What forces are acting on the Storm Runner?

What forces are acting on the wind surfer?

III. Finding the Net Force

A. Length of the vector shows the strength

Stronger Force Weaker Force

B. Forces that are going the same direction can be added

+10 N 5 N = 15 N

C. Forces that are opposite are subtracted.

10 N

+5 N

=5 N

D. Perpendicular forces do not affect each other.

IV. Balanced ForcesA. Forces which are opposite and equal

B. Example:

Net Force = Zero

B. Example:

600 N800 N

Net Force = 200 N

V. Unbalanced ForcesA. Forces that cause acceleration

VI. Gravity

A. Force of attraction between 2 masses

B. Causes an acceleration of 9.8 m/s/s.

(On Earth)

C. Mass does not affect the acceleration due to gravity.

VII. Free Fall

A. Air resistance affects all falling objects.

B. When the air resistance equals the force of gravity, the object no longer accelerates.

1. This is the maximum speed an object can fall

2. We call this terminal velocity

VIII. Projectile Motion

A. An object moving through the air is a projectile

B. Gravity pulls down

C. It is moving forward due to inertia

D. Follows curved path.

IX. Newton’s Laws of Motion

A.Newton’s First Law

1. An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an unbalanced force.

2. This is called inertia.

B. Newton’s Second Law

1. An unbalanced force will accelerate an object in the direction of that force.

2. Force equals Mass times Acceleration

F = ma

3. Applied to weight, the equation becomes:

W = mg (Weight = mass X 9.8 m/s/s)

4. The unit used for force is called a Newton (N)

C. Newton’s Third Law

1. For every action there is an equal and opposite reaction.

C. Newton’s Third Law

1. For every action there is an equal and opposite reaction.

X. Momentum

A.Momentum = mass X velocity

B. The more mass an object has, the more momentum it will have.

C. The faster an object moves, the more momentum it will have.

D. Units: kg x m/s or kg-m/s

E. Law of Conservation of Momentum

1. When a collision occurs, momentum is conserved

2. The loss of momentum from one object is gained by another object.

3. momentum before = momentum after

4. m1v1 = m2v2

XI. Universal Forces

A. Electromagnetic Forces

1. Only force that can retract and repel2. Force between electrons and protons3. Force between poles of a magnet

B. Nuclear Forces

1. Strong force – holds the nuclear together2. Weak force – attraction between all subatomic particles3. Strongest universal force

C. Gravitational Force

1. Attraction between two masses

2. Describes by Newton

3. Weakest Universal force

4. Can act over large distance