Forces and motion definitions Flashcards
Acceleration
The rate of change of velocity
Average speed
Distance over time for the entire region of interest
Braking distance
The distance travelled between the brakes being applied and the vehicle coming to a stop. It is affected by the vehicle and road conditions
Displacement
The direct distance between an object’s starting and ending positions. It is a vector quantity
Displacement-Time Graphs
Plots showing how displacement changes over a period of time. The gradient gives the velocity and curved line represent acceleration
Free fall
when the only force acting on an object is gravity
Instantaneous speed
The exact speed of an object at a specific given point
Projectile motion
The motion of an object that is fired from a point and then upon which only gravity acts
Reaction time
Time taken to process a stimulus and trigger a response to it. Affected by alcohol, drugs and tiredness
Stopping distance
The sum of thinking distance and braking distance for a driven vehicle
Thinking distance
The distance travelled in the time is takes for a driver to react
Velocity-Time graph
Plots showing how velocity changes over a period of time. The gradient gives acceleration
Velocity
The rate of change of displacement. It is a vector quantity
Archimedes’ principle
The upward force acting on an object submerged in a fluid is equal to the weight of the fluid it displaces
Centre of gravity
The single point through which the object’s weight can be said to act
Centre of mass
The point through which all the mass of an object can be said to act
Couple
Two equal and opposite parallel forces that act on an object through different lines of action. It has the effect of causing a rotation without translation
Density
The mass per unit volume of a material
Drag
The frictional force that an object experiences when it moves through a fluid
Equilibrium
For an object to be in equilibrium, both the resultant force and resultant moment acting on the object must be equal to zero
Free-Body diagram
A diagram showing all the forces acting on an object
Friction
The resistive force produced when there is relative movement between two surfaces
Moment of force
The product of a force and the perpendicular distance from the line of action of the force to the pivot
Newton
The unit of force
Newton’s second law
The sum of the forces acting on an object is equal to the rate of change of momentum of the object
Normal contact force
The reaction force between an object and surface
Pressure
The force that a surface experiences per unit area, measured in Pascals(Pa)
Principle of moments
For an object to be in equilibrium, the sum of the clockwise moments acting about a point must be equal to the sum of the anticlockwise moments acting about the point
Tension
The result of two forces acting on an object in opposite, outwards directions
Terminal velocity
The maximum velocity of an object that occurs when the resistive and driving forces acting on the object are equal to each other
Triangle of Forces
A method of determining the resultant force of two forces, by joining them tip to tail. The resultant force is given by the force that would complete the triangle
Upthrust
The upwards force that a fluid applies on an object
Weight
The product of an object’s mass and the gravitational field strength at its location
Conservation of energy
In a closed system with no external forces the total energy of the system before an event is equal to the total energy of the system after the event
Efficiency
The useful output (power/energy)/total input
GPE
Gravitational potential energy - the energy gained by an object when it is raised by a height in a gravitational field
Kinetic energy
The energy an object has due to its motion
Power
The word done or energy transferred by a system divided by the time taken for that to be done
Work done
The energy transferred when a force moves an object over a distance
Brittle
Shows very little strain before reaching its breaking stress
Compression
The result of two coplanar forces forces acting into an object. Compression usually results in a reduction in the length of the object
Compressive deformation
The changing of an object’s shape due to compressive forces
Ductile
It can undergo very large extensions without failure and be stretched into wires
Elastic deformation
If a material deforms with elastic behaviour it will return to its original shape when the deforming forces are removed. The object will not be permanently deformed
Extension
The increase of an object’s length
Force-extension graph
A plot showing how an object extends as the force applied increases. For an elastic object, the gradient should be linear up to the limit of proportionality. The gradient gives the spring constant
Hooke’s law
The extension of an elastic object will be directly proportional to the force applied to it up to the object’s limit of proportionality
Plastic deformation
If a material deforms with plastic behaviour, it will not return to its original shape when the deforming forces are removed. The object will be permanently deformed
Polymeric
A material made from polymers
Spring constant
The constant of proportionality for the extension of a spring under a force. The higher the spring constant, the greater the force needed to achieve a given extension
Strain
The ratio of an object’s extension to its original length. It is a ratio of two lengths and so has no unit
Stress
The amount of force acting per unit area. Its unit is Pascal(Pa)
Tensile deformation
The changing of an object’s shape due to tensile forces
Ultimate tensile strength
The maximum stress that an object can withstand before fracture occurs
Young Modulus
The ratio of stress to strain for a given material. Its unit is the Pascal (Pa)
Conservation of momentum
The total momentum of a system before an event must be equal to the total momentum of the system after the event, assuming no external forces act
Elastic collisions
A collision in which the total kinetic energy of the system before the collision is equal to the total kinetic energy after the collision
Impulse
The change of momentum of an object when a force acts on it. It is equal to the product of the force acting on the object and length of time over which it acts
Inelastic collisions
A collision in which the total kinetic energy of the system before collision is not equal to KE of system after
Linear momentum
The product of an object’s mass and linear velocity
Newton’s first law
An object will remain in its current state of motion unless acted on by a resultant force. An object requires a resultant force to be able to accelerate
Newton’s third law
If an object exerts a force on another object, then the other object must exert a force back that is opposite in direction and equal in magnitude
