4.1 - 4.9 Flashcards
define the Newton
the force required to give a 1-kilogram mass an acceleration of 1 m/s^2
what is Newton’s first law
an object will maintain its current velocity unless acted upon by a resultant force
what is Newton’s second law
the rate of change of momentum is directly proportional to the resultant force and takes place in the direction of that force
what is Newton’s third law
when two objects interact they exert an equal but opposite force on the other
define ‘weight’
the gravitational force on an object measured in newtons
what is the centre of mass
the point through which any externally applied force produces a straight line motion and no rotation
what is the centre of gravity
an imaginary point where all of the weight of an object appears to act
how can you find the centre of mass of a 2d object
make a hole in the object and hang it on a pin. draw a straight line down using a plumb-line hung from the pin and then do the same thing after rotating the object. where these two lines intersect is the position of the centre of mass
how is a free-body diagram drawn
each force is represented by a labelled arrow pointing in the direction the force acts. each arrow is drawn to scale (longer arrow’s show a greater force)
define ‘terminal velocity’
the constant speed reached by a falling object when the drag force is equal but opposite to the weight
factors affecting drag
speed,
cross-sectional area,
density of fluid
relationship between drag and speed
drag is directly proportional to the speed squared
define the moment of a force
the product of force and the perpendicular distance between the line of action of the force and the point of rotation
what is the principle of moments
for an object in rotational equilibrium the sum of the clockwise moments equals the sum of the anticlockwise moments about a point
define a ‘couple’
a pair of equal and opposite coplanar forces acting on an object but not in the same straight line
what is the torque of a couple
the product of one of the forces of the couple and the perpendicular distance between them
what does it mean when three forces add to form a triangle
the net force is zero so the object they are acting on is in equilibrium
define density
the mass per unit volume of a substance
define pressure
the force exerted per unit area. measured in pascals
what is Archimedes principle
the upthrust on an object in a fluid is equal to the weight of the fluid displaced
how can you find the volume of an irregular solid
partially fill a measuring cylinder with water. place the irregular solid inside it (ensure it’s fully submerged). find the difference between the initial and final volume displayed on the cylinder. that is the volume of the object.
what is a fluid
a substance that can flow. it includes gases and liquids
how can the equation for the pressure exerted by a fluid be derived
start with Weight = mass of column * g. mass is the densityvolume and volume is the cross-sectional areaheight so W=ρAhg. the pressure this exerts equals the weight/cross-sectional area so
P= W/A= ρAhg/A= ρhg. so Pressure=ρhg
how can you derive the equation for the upthrust on a submerged object
start with pressure=ρgh at the top of the object. the pressure at the bottom of the object is (x+h)ρh where x is the object’s height. since P=F/A, F=PA. therefore the resultant upthrust on the object is
(h+x)ρgA - hρgA= xρgA.
