Mechanics Flashcards
Units for Speed + Acceleration
ms-1 , ms-2
Distance Time Graph
Acceleration (curved up) or deceleration (curved down)
* Straight line constant speed ; calculate gradient (rise/run)
* Constant speed means forces are balanced, Fnet = 0.
* Horizontal line stopped/stationary/at rest. Forces are balanced, Fnet = 0.
Speed Time Graph
- Straight line upwards constant acceleration ; calculate gradient (rise/run)
- Horizontal line constant speed, Forces are balanced, Fnet = 0.
- Straight line downwards constant deceleration ; calculate gradient (rise/run); minus sign
before value shows deceleration - Area under speed-time graph = distance travelled. (add up square and triangle). Calculate the
triangle area using ½ base x height!
Accelerations occurs when
- Force is applied to a stationary object: Fnet = ma net means “overall” force
- Force is applied to an object at travelling at constant speed: Fnet = ma
Falling/Dropping Objects
Falling/dropped objects. Before they fall/drop/jump vertical forces are balanced. As they
drop weight force > air resistance so object accelerates. The more it accelerates, the greater
air resistance becomes until weight force = air resistance. Then object falls at constant speed
(terminal velocity) as forces are balanced.
Mass vs Kg
Mass is amount of matter something has and is measured in kg. Your mass is the same in NZ,
France, on the moon and in deep space!
Mass on earth.
Your weight is measured in N and on Earth it is 10x your mass. E.g. a 60 kg student weighs
600N.
Pressure P= F/A
Pressure units Nm-2 or Pa (if area in m2
). If area is in cm2 then pressure is in N cm-2.
e.g. hammer a nail into wood; large force exerted over small area (nail tip) means large
pressure – nail goes into wood easily. Skiing; Your weight (force) exerted over a large area
means pressure is small – you glide over snow instead of sinking in. For the same force, if
area ↓ pressure ↑. For the same force, if area ↑ pressure ↓.
Look at question context – a runner runs on one foot at a time, and a skier normally has 2
skiis on the ground, a car has 4 tyres, a bike two…. Etc Studs when playing sport increase
the pressure exerted into the soft ground so you get better grip and won’t slip over
Gravitational Potential Energy (Change in)Ep = mg(change in)h
Gravitational potential energy – measured in J. where g is 10. Is the energy
you give something you lift up, or the energy you gain when you go up in a lift or up a ski lift.
The height h is the vertical height. It also theoretically equals the work done to lift that
object, go up in that lift or be carried on the ski lift but….in the real world… you need to use
more energy / do more work because of energy “lost” as hear in your muscles, as sound
energy etc.
Kinetic Energy Ek=1/2mv2
Kinetic energy: - measured in J. Movement energy. It is ½ x m x v2
, only the v
gets squared! Use CALCULATOR! If something is dropped, like a ball, all its EP is converted to
EK just before it hits the ground. Air resistance isn’t really significant here. Since mgh = ½ mv2
and so v=�2gh. If you are skiing down a mountain, or rolling down a hill on a bike, then not
all EP will be converted to EK as there would be significant air resistance (drag), friction with
ground, conversion to sound and heat energy. If EP at top isn’t equal to EK at bottom then the
difference in energy was the “lost” energy. If asked to calculate the average frictional force,
calculate energy difference ÷ distance.
Work W=Fd
Work done is change in energy. Work is measured in J. If an object is lifted
vertically the work done on it equals the EP it gains. BUT….. if you struggle and struggle and
just can’t lift that object then NO WORK IS DONE (because the force has not moved the
object any distance). W = F x 0 so W = 0 Also because it hasn’t gained any EP then no work
has been done. Ramps (inclined planes) make things easier because a ramp allows the same
work to be over a greater distance, and so less force is needed. W = F x d (W the same so if
d↑ F↓).
Power P=W/t
Power is measured in Js-1 or W. Power is the rate at which work is done. If you lift
something and take 1 minute and you lift the same thing and take 2 minutes, you were more
powerful the first time. A heavy man climbing a rope slower than a light man could still be
more powerful… you’d need to calculate the work done by each (W = F x d) and divide by the
time it took each, in s.
Force Diagrams
Size of arrows = size of force, so if 2 forces are balanced draw the arrows
the same size! Force arrows should come from the centre of mass of the object.
