# physipics # FORCE AND MOTION

NOTE: These Talk-About Frames are samples of a much larger, and growing, collection. Scroll down to see the current list. F1: Introducing velocity

Syllabus / specification points:

• velocity is a measurement of motion

• velocities can have different sizes AND directions

• we can represent velocities by drawing arrows, to compare sizes and directions

• velocities can vary, or an object can have a constant velocity

• an object with a constant velocity of zero is standing still (stationary)

Introducing acceleration, and its cause

F1.B: Introducing acceleration, and its cause

Syllabus / specification points:

• force and acceleration can be represented by arrows

• acceleration is rate of change of velocity

• ’no acceleration’ means velocity is not changing

• Isaac Newton established the concept of force as the cause of acceleration

• ‘rate of change’ is a measure of how quickly something changes

• without a resultant / net / unbalanced force there can be no acceleration; this is the principle of Newton’s first law

• when there is a resultant, net or unbalanced force there is always acceleration

forces and accelerations

F1.B: Forces and accelerations

Syllabus / specification points:

• Newton’s first law is universal: acceleration cannot happen without a force, and a net/resultant/overall/unbalanced force always results in acceleration

• This force and the resulting acceleration are always in the same direction

balanced and unbalanced forces

F2: Balanced and unbalanced forces

Syllabus / specification points:

• forces can oppose each other, and if they are the same size and in opposite directions the resultant or net force is zero; the forces are then balanced

• when resultant force is zero, and forces are balanced, acceleration is zero

• forces can add together to produce a resultant force (net force) that is then increased; the forces are unbalanced

• when resultant force is not zero, and forces are unbalanced, then acceleration is not zero

Balance of driving force and resistive force

F2.B: Balanced of driving force and resistive force

Syllabus / specification points:

• a driving force can be opposed by resistive forces, including friction and air resistance

• if driving force and resistive force are the same size as well as in opposite directions then resultant or net force is zero, the forces are balanced, and there is no acceleration

unbalanced force produces acceleration: forces of falling

F2.C: Unbalanced force produces acceleration

Syllabus / specification points:

• forces can oppose each other, and if they are the same size and in opposite directions the resultant or net force is zero; the forces are then balanced

• when resultant force is zero, and forces are balanced, acceleration is zero

• when resultant force is not zero, and forces are unbalanced, then acceleration is not zero

• gravity produces a downwards force

NOTE: This frame does not address ‘terminal velocity’ – that will come later. It is intended to reinforce the concept, based on Newton’s First law, that force and acceleration are closely linked.

Ground forces, balanced and unbalanced

F2.D: Ground forces, balanced and unbalanced

Syllabus / specification points:

• forces can oppose each other, and if they are the same size and in opposite directions the resultant or net force is zero; the forces are then balanced

• when resultant force is zero, and forces are balanced, acceleration is zero

• when resultant force is not zero, and forces are unbalanced, then acceleration is not zero

• gravity produces a downwards force

• the ground exerts an upwards force on an object on it

Force, acceleration, and deceleration

F3: Force, acceleration, and deceleration

Syllabus / specification points:

• force is the cause of acceleration

• when there is a net force (resultant force, overall force) there is always acceleration

• force, acceleration and velocity are vector quantities and they can be represented by arrows to show their directions

• deceleration requires a force; it’s a kind of acceleration

• net force and acceleration have the same direction, but velocity can have a different direction

Force, acceleration and velocity I

F3.B: Force, acceleration and velocity

Syllabus / specification points:

• acceleration and ‘overall’ force (net force, resultant force) are always in the same direction

• acceleration involves change of velocity

• a car needs a forwards driving force, and there is also a backwards force due to friction and air resistance – resistive force

• when driving force is bigger than resistive force there is forwards acceleration, and velocity increases

• when driving force is the same size as resistive force then the ‘overall’ force (net force, resultant force) is zero, so acceleration must also be zero; velocity is then steady (constant)

• for constant velocity motion a can must use a driving force to balance the resistive forces

Force, acceleration and velocity II: car crash

F3.C: Force, acceleration and velocity II: car crash

Syllabus / specification points:

• velocity is not always in the same direction as force and acceleration

• during a collision, the force and acceleration are in the opposite direction to velocity and are very large

• since acceleration is in the opposite direction to the velocity it acts as negative acceleration, or deceleration

• the large negative acceleration produces a rapid decrease of velocity

• acceleration is equal to rate of change of velocity

introducing velocity

Force and Motion, provisional list, June 2021

F1         Introducing velocity

F1.B      The cause of acceleration

F1.C      Forces and accelerations

F2         Balanced and unbalanced forces

F2.B      Balance of driving force and resistive force

F2.C      Forces of falling

F2.D      Ground forces

F2.E      Gravity and ground

F2.F      Combination of forces in the same and in opposite directions

F2.G      Combination of forces in different directions

F3          Force, acceleration, and deceleration

F3.B      Force, acceleration and velocity I

F3.C      Force, acceleration and velocity II: car crash

F4          Displacement and velocity vectors

F4.B      Displacement is a vector quantity I

F4.C      Displacement is a vector quantity II

F4.D.     Velocity is a vector quantity

F4.E      So what IS velocity?

F5         Forces come in pairs

F5.B      Skateboard forces

F5.C      Water propulsion

F6          Linking force, mass, and acceleration I

F6.B       Linking force, mass, and acceleration II

F6.C      Acceleration equals

F6.D      Force equals

F7         Mass and weight

F7.B     Journey to Mars

F7.C     Life on Mars

F8        Orbit I: escape from the atmosphere

F8.B     Orbit II: acceleration and change in velocity

F9         Typical speeds

F10       Average speeds

F11       Metres per second and kilometres per hour

F12       A displacement-time graph

F13       The gradient of a displacement-time graph

F15       A journey with changing gradients

F15.B    Same journey, two graphs I: constant velocity

F15.C   Same journey, two graphs II: changing velocity

F15.D   A velocity-time graph

F16       Upwards and downwards displacement and velocity

F17       The ABC of a trampoline bounce I: displacement-time

F18       The ABC of a trampoline bounce I: velocity-time

F19       Average velocity and velocity-time graphs

F20       Acceleration and velocity-time graphs

F21       Motion equation malogic I

F22       Motion equation malogic II

F23       Choosing the right one

F24       Predicting the future with equations

Sequence on forces acting on springs                                                      Planned

Sequence on centre of gravity and turning effect of forces                      Planned

Sequence on momentum                                                                          Planned

Sequence on the motions of the Earth and other Solar System objects  Planned

Sequence on stellar evolution                                                                   Planned

Sequence on expansion of the Universe                                                  Planned