Dynamics and Forces

https://faculty.uml.edu/andriy_danylov/Teaching/documents/preLECTURE7Ch5F17Forces.pdf

What does dynamics study?

Why things move

What was force introduced to describe?

Interactions between two objects

What is a force?

A push or a pull

What does force have?

Magnitude and direction

Is force a scalar or vector quantity?

Vector

Why is force a vector quantity?

It has both magnitude and direction

What do we need to do to quantify a push or a pull?

Specify both magnitude and a direction

What does an object at rest need to get it moving?

A force

What does a moving object need to change its velocity?

A force

2 main types of forces:

Contact and non-contact forces

Forces that act on an object by touching it at a point of contact

Contact forces

Examples of contact forces:

Frictional Force Tension Force Normal Force Spring Force

What are non-contact forces also called?

Long-range forces

Forces that act on an object without physical contact:

Non-contact forces

Examples of non-contact forces:

Gravity, electric force, magnetic force

What are the major forces?

Gravity, spring force, normal force, friction force, tension force

The pull of a planet on an object near the surface:

Gravitational force

What does the gravitational force of a planet on an object near its surface come from?

The entire planet

What objects does gravitational force act on?

All, whether moving or at rest

How does the gravitational force vector always point?

Vertically downward

What can a spring do?

Push or pull

When can a spring push?

When compressed

When can a spring pull?

When stretched

Diagram of compressed spring exerting pushing force:

Representation of compressed spring exerting pushing force using force vector:

Diagram of stretched spring exerting pulling force:

Representation of stretched spring exerting pulling force using force vector:

What does a table surface do when an object sits on the table?

Exerts an upward contact force on the object

How is normal force directed?

Perpendicular to a surface

Why is normal force called normal force?

It is always directed perpendicular to a surface

Draw force vector of normal force on a frog on a slanted surface:

What is a table made of?

Atoms joined together by molecular bonds

What can atoms joined together by molecular bonds that make up a table be modeled as?

Springs

What is normal force on an object sitting on a table a result of?

Many molecular springs being compressed ever so slightly

Diagram of normal force of object on table:

Two types of frictional force:

Kinetic and static

What can a surface do when an object slides along it?

Exert a contact force which opposes the motion

The contact force exerted by a surface on an object that opposes the motion of the object:

Kinetic friction

Diagram of kinetic friction:

How is the kinetic friction force directed?

Tangent to the surface and opposite to the velocity of the object relative to the surface

What does kinetic friction tend to do?

Slow down the sliding motion of an object in contact with a surface

The contact force that keeps an object “stuck” on a surface, and prevents relative motion

Static friction

Diagram of static friction:

How is the static friction force directed?

Tangent to the surface

How does the static friction force point?

Opposite the direction in which the object would move if there were no static friction

The contact force exerted by a string or rope or wire pulling on an object:

Tension force

The tension force is in what direction?

Direction of the string or rope

Diagram of a rope exerting tension force on a sled by being pulled by a man standing up:

Diagram of a rope exerting tension force on a donkey by being pulled by a man with his hands at about the same height as the horse:

What is the net force given by when multiple forces are present?

Vector sum of all the forces

Symbol for net force:

Draw the net force on this box: