Flashcards in 5. Forces Deck (121):
What is a scalar?
Quantities that are fully described by magnitude or size
What are vectors?
Quantities that are fully described by both a magnitude and direction
What is distance?
A scalar quantity that refers to 'how much ground an object has covered'
What is displacement?
A vector quantity that refers to 'how far out of place an object is'
What is magnitude?
How can vectors be drawn?
Is time a vector or scalar quantity?
Is force a vector or scalar quantity?
Is friction a vector or scalar quantity?
Is weight a vector or scalar quantity?
Is displacement a vector or scalar quantity?
Is distance a vector or scalar quantity?
Is density a vector or scalar quantity?
Is energy a vector or scalar quantity?
Is frequency a vector or scalar quantity?
Is mass a vector or scalar quantity?
Is power a vector or scalar quantity?
Is voltage a vector or scalar quantity?
Is volume a vector or scalar quantity?
What is the gradient on a distance-time graph?
What is the gradient on a velocity-time graph?
What is the area under a velocity-time graph?
What is acceleration?
When an object speeds up or changes direction
What is the total area underneath a velocity-time graph?
The total distance travelled
What is the unit of force?
Examples of contact forces?
• Air / Water resistance (drag)
• Normal contact (reaction)
Examples of non-contact forces?
What happens when the resultant force on an object is zero?
The object remains stationary or continues to move at a constant speed and in the same direction.
What is enersher?
The idea that without an unbalanced force, nothing will change in terms of speed, direction etc.
What is Newton's 1st law of motion?
An object will remain at rest or at a constant velocity unless there is a resultant force acting on it.
What is Newton's 2nd law of motion?
The acceleration of an object is directly proportional to the resultant force and inversely proportional to the objects mass.
What equation shows Newtons 2nd law of motion?
F = m a
What is Newton's 3rd law of motion?
If object A exerts a force on object B, then object B exerts a force on object A which is equal and opposite.
How is inertial mass defined?
By the ratio of force over acceleration
(if an object is more inert then it's harder to move)
What is thinking distance proportional to?
What is braking distance proportional to?
What is thinking distance?
The distance travelled during reaction time.
What is braking distance?
The distance travelled while the brakes are applied.
Why does thinking distance increase?
• the speed increases
• the reaction time increases eg. tiredness, distraction, taken drugs / alcohol
When does braking distance increase?
If the initial speed increases or if the size of deceleration is less.
How is thinking distance decreased?
Friction reduced - wet roads, worn tyres or brakes
How is braking distance decreased?
Larger mass of car (more inertia means its harder to stop)
How is thinking distance shown on a graph?
Area of the rectangle underneath the graph
How is braking distance shown on a graph?
Area of the triangle underneath the graph
Typical speed of walking?
Typical speed of running?
Typical speed of driving?
Typical speed of flying?
Speed of sound in air?
Speed of light?
3x10 to the 8
What is the weight of an object?
The force of gravity on it
How to convert from Kg to N?
When does an object reach constant velocity?
When the drag force is equal and opposite to its weight
What is terminal velocity?
When an object reaches a constant velocity because the drag force is equal and opposite to its weight
What happens to the drag force when an object is released in a fluid before its terminal velocity?
The drag force is less than its weight before it reachers terminal velocity
What happens to acceleration when an object is released in a fluid after its terminal velocity?
It is equal to zero
What happens to the resultant force on an object released in a fluid initially?
The resultant force is equal to its weight, initally
When an object is falling, what does it initially accelerate at?
What are plastic objects?
Objects that do not regain their original shape when the forces deforming it are removed
What are elastic objects?
An object that regains its original shape when the forces deforming it are removed
It stores elastic potential energy when work is done on the object
Does an elastic collision lose kinetic energy?
What does Hooke's law state?
That the extension of a spring is in direct proportion with the force applied to it
What is the spring extension?
New length - original length
What does hooke's law only apply to?
What happens to a spring if the force added on the spring is larger?
It not longer obeys hooke's law and does not return to its original length
What is elastic limit?
The limit of proportionality (when hooke's law no longer applies)
What is stopping distance?
Thinking distance + braking distance
What are the two SUVAT equations?
v = u + at
v2 = u2 - 2as
What are the two ways of increasing the turning effect of a force?
• increase the size of the force
• increase the distance from the pivot
What is the turning effect of a force called?
What is the equation to calculate the size of a moment?
What are moments measured in?
When will a seesaw balance in terms of moments?
When the anticlockwise moment is equal to the clockwise one
What does the principle of moments state?
For a balanced object, clockwise moments = anticlockwise moments
What is the centre of mass of an object?
The point at which the mass may be thought to be concentrated
Where does the centre of mass lie in symmetrical objects?
Along the line of symmetry
Where will the COM be when an object has more than one line of symmetry?
Where the lines of symmetry intersect
Where will the centre of mass be of a suspended object?
The centre of mass will be directly below the point of suspension
How can the COM of an irregular shape be found?
• by hanging the shape from a point and a plumb line is used to draw a vertical line downwards from the pivot
• this is repeated from different points
What is the stability of an object affected by?
• the width of the base of the object
• the height of it's centre of mass
When will an object not topple over?
When the line of action is within the base
What are the three types of equilibrium when a force is applied to objects?
• stable equilibrium - tilts and falls back to original position
• unstable equilibrium - tilts and falls
• neutral equilibrium - rolls
What happens to the centre of mass when a stable object is tilted?
It is raised
What happens to the centre of mass when an unstable object is tilted?
It is lowered
What happens to the centre of mass when a neutral object is tilted?
It stays at the same level if pushed
What does the pressure of a liquid depend on?
The depth and the density
What is the equation for the pressure at a certain depth due to the column of liquid?
pressure = depth x field strength x density
What is the nature of liquid pressures at different places, but at the same depth?
The pressure acts in all directions and is the same depth
What is the nature of liquid pressures at different depths?
The greater depth, the higher the pressure so the water is pushed down with a greater force and has a bigger horizontal range
What is a gear?
A rotating machine with cogs that mesh with another toothed part to transmit a moment
What do gears act as?
A force multiplier
What is a force multiplier?
Machines designed to increase the size of the force
What is a distance multiplier?
Machines designed to increase the distance or speed with which something moves
What effort and load do force multipliers involve?
• small effort (input force)
• larger load (output force)
Equation for mechanical advantage?
load / effort
Example of a force multiplier?
Lever with a distance from pivot to effort 3 times greater than distance from pivot to load - mechanical advantage is 3
How is a bike an example of a distance or speed multiplier?
Slow movement of pedals produces a much faster rotation of the wheels
What effort and load do distance multipliers involve?
• small movement of effort
• large movement of load
How is energy shown to be conserved in gears?
Work done by input gear = Work done by output gear
Why can a machine not be a force and distance multiplier at the same time?
As energy is conserved, if force is doubled the distance is halved and vice versa
What are levers used to do?
Lift heavy masses with the least amount of effort
What three things can gears do when passing power from one wheel to another?
• increase speed - first gear has more teeth than second so second has to turn faster to keep up
• increase force - second gear has more teeth than first so second turns slower but with more force
• change direction - when gears mesh they always turn in the opposite direction
In a hydraulic system, a smaller force on the master (effort) piston will produce what force on the slave (load) piston?
A larger force as the pressure of the liquid is constant - acting as a force multiplier
What is the ratio of in levers?
What is the ratio of in gears?
Number of teeth
What is the ratio of in hydraulics?
Cross sectional areas
Equation for levers?
Fb/Fa = Lb/La
Equation for gears?
Fb/Fa = Nb/Na
Equation for hydraulics?
Fb/Fa = Ab/Aa
What is the atmosphere?
A thin layer of air around the earth
What happens at a greater altitude?
• the less dense the atmosphere
• the lower the atmospheric pressure
Why is the atmospheric pressure lower at greater altitudes?
• at higher altitudes there is less air above a surface
• so there is a smaller weight of air acting on the surface
• p=F/A and F is reduced so p will be smaller
How is upthrust created?
When the bottom surface of an object is submerged in a liquid it experiences a greater pressure than the top surface
When does an object float?
When its weight = upthrust / weight of fluid displaced
What is upthrust equal to?
The weight of the liquid displaced
What is the equation for momentum?
Mass x velocity
What is momentum measured in?
Why will a ball be harder to stop if it has a large mass and moving at a high speed?
There will be a greater force exerted on the person's hands by the ball. They will have to exert an equal and opposite force on the ball to stop it
What happens to momentum in a collusion of explosion?
Momentum is conserved, provided that there are no external forces having an effect
What are the external forces that could have an effect on momentum?
• friction which decreases momentum
• gravity (e.g. a collision on a slope)