physipics ONE:

FORCE AND MOTION

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

the cause of acceleration Frame  F1

F1: The cause of acceleration

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

• 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

Frame  F1.B

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

Frame F2

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

• 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

Frame F2.B

F2.B: 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

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

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.

Force, acceleration, and deceleration

Frame F3

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

Frame F3.B

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

Frame F3.C

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

mass and weight

Frame F7

F7: Mass and weight

Syllabus / specification points:

• Everyday life happens on Earth, but Physics applies throughout the Universe

• In Physics, weight is the force of gravity, and is measured in Newton (just like other forces)

• Mass is measured in kilograms

• The strength of gravity is different in different places in the Universe

• On Earth, each kg of mass has a weight of about 10 N; more precisely about 9.8 N

• The gravitational field strength on the surface of the Earth is approximately 10 N/kg; more precisely, about 9.8 N/kg

• The Moon is small, and the gravitational field strength on its surface is only about 1.6 N/kg

• The same object, with the same mass, has a weight that is about six times smaller on the Moon as it is on Earth

a displacement-time graph

Frame F12

F12: A displacement-time graph

Syllabus / specification points:

• For motion in a straight line, displacement and distance have the same size, so a displacement-time graph and a distance-time graph are the same

• For a person or object with constant velocity, the displacement-time graph shows a simple straight line

• If the velocity (or speed) at the start, when time = 0, is zero, then the line passes through the origin

• A straight line that passes through the origin shows a linear relationship

the gradient of a  displacement-time graph

Frame F13

F13: The gradient of a displacement-time graph

Syllabus / specification points:

• Velocity is rate of change of displacement; that is, it is change in displacement divided by change in time

• Velocity = Δs/Δt

• Velocity is equal to the gradient of a displacement-time graph

•  The size of the gradient, and the velocity, can be found from a right-angled triangle which has a section of the line of the graph as the hypotenuse

• For a straight line graph, all triangles of all sizes and locations give the same answer of the gradient and velocity; it makes sense to use larger triangles to reduce uncertainty of measurement

• For a straight line graph, gradient and velocity or constant

Force and Motion, provisional list, May 2021

Comp: completed and ready, but so far unpublished

F1         The cause of acceleration                                                           FREE

F1.B      Forces and accelerations                                                            FREE

F2         Balanced and unbalanced forces                                                Planned

F2.B      Unbalanced force produces acceleration                                   FREE

F2.C      Opposing forces                                                                         Planned

F2.D      Different directions                                                                      Planned

F3          Force, acceleration, and deceleration                                        FREE

F3.B      Force, acceleration and velocity I                                               FREE

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

F4          Displacement and velocity vectors                                            Comp

F4.B      Displacement is a vector quantity I                                            Comp

F4.C      Displacement is a vector quantity II                                           Comp

F4.D.     Velocity is a vector quantity                                                        Comp

F4.E      So what IS velocity?                                                                    Comp

F5         Forces come in pairs                                                                   Comp

F5.B      Skateboard forces                                                                       Planned

F5.C      Motion with a porpoise                                                               Planned

F6          Linking force, mass, and acceleration I                                      Comp

F6.B       Linking force, mass, and acceleration II                                    Comp

F6.C      Acceleration equals                                                                     Planned

F6.D      Force equals                                                                                Planned

F7         Mass and weight                                                                          FREE

F7.B     Journey to Mars                                                                           Planned

F7.C     Life on Mars                                                                                 Planned

F8        Orbit I: escape from the atmosphere                                            Comp

F8.B     Orbit II: acceleration and change in velocity                                Comp

F9         Typical speeds                                                                             Comp

F10       Average speeds                                                                           Comp

F11       Metres per second and kilometres per hour                                Comp

F12       A displacement-time graph                                                         FREE

F13       The gradient of a displacement-time graph                                FREE

F15       A journey with changing gradients                                              Comp

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

F15.C   A velocity-time graph                                                                   Comp

F15.D   Same journey, two graphs II: changing velocity                          Comp

F16       Upwards and downwards displacement and velocity                Comp

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

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

F19       Average velocity and velocity-time graphs                                 Comp

F20       Acceleration and velocity-time graphs                                        Comp

F21       Motion equation malogic I                                                           Planned

F22       Motion equation malogic II                                                          Planned

F23       Choosing the right one                                                                Comp

F24       Predicting the future with equations                                            Comp

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 motion of the Earth and other Solar System objects    Planned

Sequence on stellar evolution                                                                   Planned

Sequence on expansion of the Universe                                                  Planned

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