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P5 - Forces Key Knowledge - Coggle Diagram
P5 - Forces Key Knowledge
scalar and vector quantities
a scalar quantity has magnitude only, examples include temperature or mass
a vector quantity has both magnitude and direction, examples include velocity
speed is the scalar magnitude of velocity
contact and non-contact forces
forces either push or pull on an object. this is a result of its interaction with another object
contact forces
- the objects are touching e.g. friction, air resistance, tension
non-contact forces
- the objects are not touching e.g. gravitational, electrostatic and magnetic forces
forces are calculated using the formula:
force(n) = mass(kg) x acceleration(m/s2):
gravity
gravity is the natural phenomenon by which any object with mass or energy is drawn together
the mass of an object is a scalar measure of how much matter the object is made up of
- mass is measured in kg
the weight of an object is a vector measure of how gravity is acting on the mass
- weight is measured in newtons(N)
weight(N) = mass(kg) x gravitational field strength(N/kg)
an objects centre of mass is the point at which the weight of the object is considered to be acting
. It does not necessarily occur at the centre of the object.
the mass of the object and its weight are directly proportional
resultant forces
a resultant force is a single force which replaces several other forces. it has the same effect acting on the object as the combination of the other forces it has replaced
a free body diagram shows all the forces acting on an object:
here the car is being pushed to the left by a force of 30N and it is also being pushed to the right by a force of 50N
the resultant force is 50N - 30N = 20N
the 20N is pushing the car to the right, therefore the car will move to the right
when forces are balanced, the object will remain stationary or stay at the same speed
Hooke's Law
Hooke's Law
describes that the extension of an elastic object is proportional to the force applied to the object. However, there is a maximum applied force for which the extension will still increase proportionally. If the
limit of proportionality
is exceeded, then the object becomes
permanently deformed
and can no longer return to its original shape.
forces and elasticity
when work is done on an elastic object, such as a spring, the energy is stored as elastic potential energy
when the force is applied, the object changes shape and stretches. this energy is stored as elastic potential and when the force is no longer applied, the object returns back to its original shape
work done and energy transfer
when a force acts on an object and makes it move, there is
work done
on the object. This movement requires energy - this input energy could be from fuel, food or electricity for example
the energy is transferred to a different type of energy when the work is done. Not all the energy transfers are useful, sometimes energy is wasted.
for example, when car brakes are applied, some energy is wastefully transferred as heat to the surroundings
work done is calculated by:
work done (J) = force (N) x distance moved in the direction of the force (M)
distance v displacement
distance is a scalar quantity. it measures how far something has moved and does not have any associated direction
displacement is a vector quantity. it measures how far something has moved and is measured in relation to the direction of a straight line between the starting and end points
what is terminal velocity?
the terminal velocity is the maximum speed of an object, reached when the forces moving the object are balanced by its frictional forces
braking and thinking distance
the
braking distance
is the distance travelled by a vehicle once the
brakes are applied
and until it reaches a full stop
braking distance is
affected by
weather conditions etc
the
thinking distance
is the distance until the driver thinks and actually starts to apply the brakes
this can be
affected by
alcohol, tiredness, distractions etc.
the stopping distance of a vehicle can be calculated:
stopping distance = thinking distance + braking distance
newtons laws
newtons first law
states that if the resultant force acting on an object is zero:
a stationary object will remain stationary
a moving object will continue at a steady speed and in the same direction
newtons second law
states that if the acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the mass of the object
inertia
- the tendency of an object to continue in a state of rest or uniform motion (same speed and direction)
newtons third law
states that when two objects interact, the forces acting on one another are always equal and opposite
