P10 Forces and motion Flashcards
Newtons first law of motion
Newton’s first law states that an object at rest or moving at a constant velocity will continue to do so unless an external force is applied
Newtons 2nd law of motion
The 2nd law links together Resultant Force (F), Acceleration (a) and Mass (m). It states that the acceleration on an object is:
- Directly proportional to the resultant force acting on the object,
- Inversely proportional to the object’s mass
- I.e. Force = mass x acceleration (F = ma)
Newtons 3rd law of motion
Newton’s 3rd law of motion states that for every force there is an equal and opposite reaction force
inertial mass
Inertia is a measure of how much a mass is reluctant to change its speed.
i.e. it is the tendency of a mass to stay at rest or stay moving at a constant speed. MATTER IS LAZY!
The inertial mass is given by:
inertial mass = force/acceleration
what is weight
The Weight of an object is the force acting on it due to gravity. Weight is measured in newtons (N)
what is mass
Mass is a measure of how much matter is contained in an object. Mass is measured in kilograms (kg).
how are mass and weight linked
If we know the mass of an object in kg we can use the equation that links weight (w) and mass (m) to find its weight in N, i.e.:
Weight (N) = Mass (kg) x gravitational field strength (N/kg)
In symbols: W = mg
what is terminal velocity
The maximum acceleration on the object is at the very start that it enters the fluid.
As the object speed increases, the resisting frictional force acting on the object increases which acts to lower the size of the resultant force. As a = F/m If the resultant force decreases, so does the acceleration of the object.
The weight (downwards force) is constant, but as the speed increases, then so does the frictional drag force. Eventually a speed is reached where the forces of weight and frictional drag force are balanced and a = zero. When this happens the size of the resultant force is zero and the object now moves with a constant velocity. The velocity is called the terminal velocity.
(the velocity object eventually reaches when it is falling)
what is momentum
Momentum (symbol p) is defined as the product of an objects mass m and its velocity V.
p = M x V
As mass has units kg and velocity m/s, the units of momentum are kgm/s
Momentum has both magnitude (size) and direction Therefore, momentum is a VECTOR quantity.
what is the law of conservation of momentum
In a closed system (i.e. with no external resultant forces acting) the total momentum before an event is equal to the total momentum after the event.
In other words, momentum is always conserved
how do we know different parts of an exploding body move in different directions
Before an explosion the total momentum is zero.
As momentum is conserved, the total momentum afterwards must also be zero.
This means that the different parts of the exploding body must move off in different directions.
what is the resultant force acting on an object
F = m a
what is the resultant force of an object moving at terminal velocity
zero
what opposes the driving force of a vehicle
friction and air resistance
what does the stopping distance of an object depend on
thinking distance and breaking distance
what increases the braking distance
- high speed
- poor weather conditions
- poor vehicle maintenance
what increases the thinking distance
- poor reaction time (our to tiredness, drugs, alcohol, or use of mobile phone)
- high speed
what is the momentum of a moving object
p = m v
what is momentum defined as
mass x velocity
what is the total momentum of 2 object pushing away from each other
zero - they move with different speeds if they have different masses and with opposite momentum
equation for when 2 objects recoil
m(A) v(A) + m(B) v(B) = 0
what does the force of the impact depend on when two vehicles collide
- mass
- change in velocity
- length of impact time
what does a longer impact time mean
the more the impact force is reduced
how do we prove the shorter the impact time, the greater the impact force
impact force = change in momentum / impact time
what do cycle helmets / cushioned surfaces do
they reduce impact forces by increasing the impact time
what do seat belts and air bags do
they spread the force across the chest and increase the impact time
at do side impact bars and crumple zones do
they give way in an impact, so increase the impact time
when is an object called elastic
if it returns to its original shape after removing the force deforming it
what is the extension
the difference between the length of an object and its original length
what is the extension of a spring proportional to
the extension of a spring is directly proportional to the force applies to it, as long as the limit of proportionality is not exceeded
what happens beyond the limit of proportionality
the extension of a spring is no longer proportional to the applied force so the relationship becomes non linear
