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.