Motion and Forces Vocabulary

What Causes Motion?

In the absence of friction, an object in motion stays in motion.
Newton’s First Law (N1): An object with no forces acting on it remains at rest if initially at rest, or continues moving in a straight line at a constant speed if initially moving.
A force is required to stop a moving object.

What is a Force?

A force is a push or pull that acts on an object.
Every force has an agent that acts, pushes, or pulls.
Force is a vector quantity, represented by the symbol \vec{F} .
The magnitude of a force is denoted by F.
Contact forces: Forces that act on an object by touching it.
Long-range forces: Forces that act on an object without physical contact.

Force Vectors

Examples of force vectors include tension, spring force, and weight.
The tails of force vectors are placed on the particle representing the object.

Combining Forces

When multiple forces are exerted on an object, they combine to form a net force, which is the vector sum of all forces: \vec{F}_{net} = \sum \vec{F}.
The net force is also called the resultant force; it replaces the original individual forces.

Catalog of Forces: Four Fundamental Forces

Gravity: Obeys the inverse square law: F{gravity} = G \frac{M1 M_2}{d^2}.
Electromagnetism: Obeys the inverse square law: F{static} = Ke \frac{q1 q2}{r^2} and Maxwell's equations.
Strong Nuclear Force: Holds protons and neutrons together.
Weak Force: Related to radioactive decay.

Weight

Weight: The gravitational pull of the Earth on an object on or near its surface.
The agent for weight is the entire Earth pulling on the object.
An object’s weight vector always points vertically downward.

Spring Force

Springs exert a spring force when deflected (pushed or pulled).

Tension Force

Tension force: The contact force exerted by a string, rope, or wire when it pulls on an object.
The direction of the tension force is always along the string or rope.

Normal Force

Normal Force: The force exerted by a surface against an object pressing against it, perpendicular to the surface.
Responsible for the “solidness” of solids.
Symbol for normal force is \vec{n}.

Friction

Friction is a force exerted by a surface, parallel to the surface.
Kinetic Friction (\vec{f}_k): Acts when an object slides across a surface, opposing the motion.
Static Friction (\vec{f}_s): Keeps an object “stuck” on a surface, preventing its motion relative to the surface; points in the direction necessary to prevent motion.

Drag

Drag: The resistive force of a fluid (air or water) on a moving object.
Like kinetic friction, drag points opposite the direction of motion.
Air resistance can be neglected unless explicitly included in the problem.

Thrust

Thrust: A force that occurs when a jet or rocket engine expels gas molecules at high speed.
Thrust is a force opposite the direction in which the exhaust gas is expelled.

Electric and Magnetic Forces

Electricity and magnetism exert long-range forces on charged particles.
Not considered in basic dynamics problems.

Identifying Forces

Identify all forces acting on the object in a physics problem.
Avoid including non-existent forces.
General force: \vec{F}, Weight: \vec{w}, Spring force: \vec{F}s, Tension: \vec{T}, Normal force: \vec{n}, Static friction: \vec{f}s, Kinetic friction: \vec{f}k, Drag: \vec{D}, Thrust: \vec{F}{thrust}.

Identifying Forces Examples

Bungee Jumper: Identify forces acting on a bungee jumper during their fall.
Skier: Identify forces acting on a skier being towed up a hill.

What Do Forces Do?

A constant force results in constant acceleration.
Acceleration is directly proportional to force.
Acceleration is inversely proportional to an object’s mass.

Finding Mass Example

Using a rubber band to pull a 1.0 kg block with a constant force results in acceleration. Pulling an unknown mass with the same force yields a different acceleration, allowing the unknown mass to be determined.

Newton's Second Law

A force causes an object to accelerate.
Acceleration (\vec{a}) is directly proportional to force (\vec{F}) and inversely proportional to mass (m): \vec{a} = \frac{\vec{F}}{m}.
The direction of acceleration is the same as the direction of the force.
Newton’s Second Law: \vec{F}{net} = m\vec{a}, where \vec{F}{net} is the vector sum of all forces acting on the object.

Units of Force

The basic unit of force is the newton (N).
One newton is the force that causes a 1 kg mass to accelerate at 1 \frac{m}{s^2}.

Racing Down the Runway Example

Boeing 737 (mass = 51,000 kg) accelerates from rest to 70 m/s over 940 m. Determine the thrust of each engine.

Free-Body Diagrams

A free-body diagram represents the object as a particle and shows all the forces acting on it.

Free-Body Diagram Examples

Elevator: Draw a free-body diagram of an elevator speeding up as it moves upward.
Towed Skier: Draw a free-body diagram of a skier being towed up a hill at constant speed.
Block on a Table: Analyze the forces acting on a block being pushed across a table at a steady speed and create a free-body diagram.

Newton's Third Law

Motion often involves interacting objects.
As the hammer hits the nail, the nail pushes back on the hammer.
Examples of interacting objects: bat and ball, foot and soccer ball, Earth and Moon.

Interacting Objects

Interaction: The mutual influence of two objects on each other.
Action/reaction pair: A pair of forces between two interacting objects.
Action/reaction pairs always exist as a pair or not at all.

Reasoning with Newton's Third Law

Every force occurs as one member of an action/reaction pair.
The two members of an action/reaction pair act on two different objects.
Action/reaction pairs point in opposite directions and are equal in magnitude.

Runners and Rockets

To walk, the floor needs friction (static friction) to prevent the foot from slipping.
The rocket pushes hot gases out the back, resulting in a forward force (thrust) on the rocket.