HL Physics - Forces and Momentum

Resolving Force Vectors

1. resolve forces into their horizontal and vertical components using trigonometry

2. calculate the horizontal and vertical resultant forces

3. use trig to find the total resultant force

Free Body Diagrams

• one body only

• arrows originate at the center of mass

• length of arrows represent magnitude

• direction of arrows represent direction

• all forces should be labelled

Newton’s 1st Law

a body at rest will remain at rest and a body in motion will remain in motion unless it is acted on by an external force

Newton’s 2nd Law

the acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass (f = ma)

Terminal Velocity

when air resistance equals weight there is no longer a resultant force, therefore there is no more acceleration, so the object falls at a constant velocity

Field Forces

forces that do not need two bodies to touch to exert a force (eg. gravitational force, magnetic force, etc)

Contact Forces

forces that need two bodies to be touching to exert a force (eg. friction, normal force, etc)

Rules for Friction

1. frictional force is proportional to normal force when static

2. static friction coefficient > dynamic friction coefficient

3. the maximum frictional force (static) is equal to the static friction coefficient times the normal force

4. the dynamic frictional force (which is constant) is equal to the dynamic friction coefficient times the normal force

Hooke’s Law

the restoring force/force applied on a spring is proportional to the extension of the spring

Restoring Force (Hooke’s Law)

the opposing force to the force applied, it is called the restoring force because it acts in the opposite direction to displacement/extension

Viscous Drag Force

a resistive force in fluid (eg. air resistance)

Viscosity

the property of a fluid that describes its resistance to flow (eg. honey has a high viscosity)

Laminar Flow

when flow in a fluid travels in regular paths

Units for Stokes Law

• viscosity: pascal seconds or kgm⁻¹s⁻¹

• radius: meters (m)

• velocity: meters per seconds (ms⁻¹)

• viscous drag: newtons (N)

Buoyancy

the force experienced on a body that is partially or fully submerged by a fluid

Archimedes’ Principle

buoyancy is equal to the weight of the fluid displaced

Momentum

the product of the mass and velocity of a body

When is linear momentum conserved?

in a closed/isolated system where there is no external force or external forces sum up to zero

Impulse

the change in momentum (often caused by collision or impact)

Why does an egg break on the floor but not a trampoline?

the equation (P = Ft) shows that the longer time the egg spends in contact with the surface, the smaller the force applied can be to provide the same impulse (change in momentum)

Collision

when two bodies exert a force on each other, which changes their movement

Elastic Collision

a collision where kinetic energy and momentum are conserved

Inelastic Collision

a collision where momentum is conserved but kinetic energy is not

Explosions

when two bodies are initially at rest and then they exert forces on each other, causing them to move apart rapidly

Period

the time it takes for a body to complete a revolution

Frequency

the number of revolutions a body makes in a second

Relationship Between Frequency and Period

inversely related

Uniform Circular Motion

circular motion with a constant speed

When a body completes a revolution…

distance travelled = 2π and time = period (T)

Angular Velocity

the rate of change of the angle covered by a body

Centripetal Acceleration

when a body moves in a circle, the direction of velocity is always changing, meaning velocity is changing, this acceleration is called centripetal acceleration

Centripetal Force

the force causing a centripetal acceleration (not a force on its own, must be provided by another force, eg. tension or friction)

Centripetal Force…

• is directed toward the center of the circle

• keeps a body moving in a circle

• acts perpendicularly to velocity

• causes the body to change direction, and therefore accelerate

• provided by another force (eg. tension, friction, etc)

Calculating Vertical Centripetal Force

force = mg + contact force (eg. tension, friction, etc)

(at the minimum speed contact force is zero, the centripetal force is fully provided by weight)

Newton’s 2nd Law for Constant vs Changing Mass

constant: f = ma changing: f = p / t

Newton’s Third Law

every action has its equal and opposite reaction