L7-8: Motion and Forces – Foundations of Force in Physics

Section 4.1 Motion and Forces

What Causes Motion?

Understanding what causes objects to move or stop is fundamental in physics. Everyday experiences suggest that continuous force is needed to keep objects moving, but experiments reveal the role of friction and other forces.

• Friction slows down moving objects. For example, a book sliding across a table eventually comes to a stop due to friction.

• Objects slow down at different rates depending on the amount of friction present.

• In the absence of friction or other significant forces, an object in motion will continue moving indefinitely. Example: The Voyager space probe continues its motion through space for billions of years due to negligible friction.

Friction and Motion

Friction is a resistive force that opposes motion between surfaces in contact. The effect of friction can be observed in various scenarios:

• On smooth snow, a sled slows down quickly and comes to rest.

• On slick ice, the sled slides farther before stopping.

• On a frictionless surface, the sled would never stop moving once set in motion.

Key Principle: In the absence of friction, a moving object will remain in motion.

What is a Force?

A force is a fundamental concept in physics, defined as a push or a pull exerted on an object.

• Definition: A force is a push or a pull.

• Agent: Every force has an agent, which is the source that exerts the force.

• Object: The force acts on an object.

Properties of Forces

• Vector Quantity: Force is a vector, meaning it has both magnitude and direction.

• Symbol: The general symbol for force is the vector ${F}$.

• Magnitude: The size or strength of a force is its magnitude, denoted as $F$.

Types of Forces

• Contact Forces: Forces that act on an object by touching it at a point of contact (e.g., friction, tension, normal force).

• Long-Range Forces: Forces that act on an object without physical contact (e.g., gravity, electromagnetic force).

Drawing Force Vectors

Force vectors are used to visually represent the direction and magnitude of forces acting on an object.

• Represent the object as a particle (a dot).

• Place the tail of the force vector on the particle.

• Draw the force vector as an arrow pointing in the direction of the force, with length proportional to the magnitude.

• Label the vector with an appropriate symbol (e.g., ${F}$, ${T}$, ${w}$).

Examples of Force Vectors

• Tension (${T}$): The pulling force exerted by a rope or string.

• Spring Force (${F}_{s}$): The force exerted by a compressed or stretched spring.

• Weight (${w}$): The gravitational force exerted by the Earth on an object.

All force vector tails are placed on the particle representing the object, regardless of where the force is applied.

QuickCheck 4.1: Net Force and Vector Addition

When multiple forces act on an object, the net force is the vector sum of all individual forces. If two of three forces are known, the third can be determined by ensuring the net force points in the specified direction.

• Vector Addition: Forces are added using vector addition rules.

• Example: If ${F}_1$ and ${F}_2$ are known, ${F}_3$ can be found such that ${F}_1+{F}_2+{F}_3={F}_{{net}}$.

Summary Table: Common Force Types and Notation

Force Type	Symbol	Description
General Force	${F}$	Any push or pull
Weight	${w}$	Gravitational force
Spring Force	${F}_{s}$	Force from a spring
Tension	${T}$	Force from a rope or string
Normal Force	${n}$	Perpendicular contact force from a surface
Static Friction	${f}_{s}$	Friction preventing motion
Kinetic Friction	${f}_{k}$	Friction opposing motion
Drag	${D}$	Resistive force from a fluid
Thrust	${F}_{{thrust}}$	Force from expelled mass (e.g., rocket)

Key Equations

• Force as a Vector: ${F}=F_{x}{}+F_{y}{}+F_{z}$

• Net Force: ${F}_{{net}}={F}_1+{F}_2+{F}_3+ext{...}$

Example Application

Example: If a sled is moving on a frictionless surface, it will continue moving indefinitely unless acted upon by another force. If friction is present, the sled will slow down and eventually stop.