Forces Flashcards
Define resultant force
The total/overall force applied to a point (replaces a number of forces by simplification)
What happens when the resultant force on an object is zero? Not zero?
Zero: The acceleration will be 0 - it will stay stationary if already stationary OR continue to move at the same velocity if already moving
Not zero: The object will accelerate in the direction of the resultant force
Equation linking acceleration, force and mass?
In terms of proportion?
a = F ÷ m OR F = m x a [when F is in newtons, N]
a ∝ F AND a ∝ 1/m SO m x a ∝ F
What happens when two objects interact?
They always exert equal and opposite forces on each other (when you punch someone it hurts you as well as that person)
What effects might a force have on an object?
Change its shape, its motion or its state of rest
Describe the forces present when a vehicle travels at constant speed
The resistive forces (mostly air resistance-caused, otherwise friction between car parts) balance the driving force (caused by friction between the wheels and the road)
Define thinking distance, braking distance and stopping distance
Thinking: the distance travelled while during the driver’s reaction time
Braking: the distance travelled under the braking force (i.e. while the brakes are held)
Stopping: thinking distance + braking distance
What might slow a driver’s reaction time? What else might increase the thinking distance?
Tiredness, drugs, alcohol
Distractions (children talking, music, looking out the window)
What conditions might increase the braking distance?
Speed, road conditions (icy or wet) or car conditions (quality/condition of brakes and tyres)
Equation linking momentum, mass and velocity?
Unit of momentum?
momentum = mass x velocity
P = m x v
Kgm/s
What would a positive sign indicate when talking about momentum? Negative? Why is this?
+ve: movement from left to right
-ve: movement from right to left
Why?: momentum is a vector quantity
Equation linking momentum, force and time?
force = change in momentum ÷ time taken F = mv – mu ÷ t OR F = Δ(mv) ÷ t [Δ means change in]
What does conservation of momentum mean?
In any collision in which no external forces act, momentum is conserved
Why does a crumple zone on a car/cycle helmet/air bag prevent injury?
The time over which the force of impact is applied increases. t ∝ 1/F . When t increases, F decreases
Cushioning is NOT precise enough and NOTHING is absorbed
Define weight and give its unit
Mass of an object x the gravitational field strength (W = mg)
Newtons, N
What can be said about acceleration due to gravity?
It is numerically equal to the strength of the gravitational field and is independent of mass, IF gravity is the only active force
How does speed relate to frictional force (friction/air resistance/drag)?
The higher the speed (the faster), the more frictional force
Why does a person, falling through the air, stop accelerating after some time? What happens when they open a parachute?
Terminal velocity. Air resistance becomes equal to the weight, so the resultant force is 0.
Parachute: air resistance increases, so velocity (and therefore the air resistance) decreases, until the weight is equal to the AR again.
What happens to a spring or elastic object when force is applied?
It will stretch and store elastic potential energy
If an object can recover its original shape (e.g. a spring), when is elastic potential energy stored and what does it do to the object?
When work is done on it
It changes its shape
What does Hooke’s law state (about proportionality with force and extension of a spring)?
Up to some point (limit of proportionality) the extension of the spring is directly proportional to the applied force.
F ∝ x therefore F = kx [k is the spring constant in newtons per metre, N/m]
What happens if a spring is loaded beyond its limit of proportionality?
The spring is damaged/deformed so that its behaviour is no longer predictable.
