Dynamics Flashcards
Define Newton’s First Law.
An object in motion continues to move with uniform velocity; an object at rest stays at rest unless object is acted upon by a resultant force.
Objects are reluctant to change their state of motion = inertia
Inertia measured in kg.
An object can move without forces acting on it.
If object is at rest or moving with constant velocity, then all forces acting in any direction have a resultant of zero.
Define Newton’s Second Law.
The resultant force is proportional to the product of mass and acceleration of an object.
Alternatively, the resultant force is proportional to the rate of change of momentum.
Fnet = ma
Description of motion of objects in a fluid.
When an object moves in a fluid, its motion is affected by friction (viscous force/drag)
Friction in air = air resistance
In general, friction increases with speed.
When an object is at rest in a still fluid, friction is zero.
Once an object starts to move, its friction will increase.
The forward force and friction will be the 2 forces acting upon it.
So Newton’s Second Law can be applied as:
Forward force - R = ma
If forward force > R, object is accelerating.
As speed increases, R increases and acceleration decreases since net force is decreasing.
Eventually forward force = R, then object has reached top speed, Fnet = 0, a = 0
Define force.
Force is a rate of change of momentum.
What are the relationships between Fnet, m and a?
Fnet is proportional to a if m is constant.
Acceleration is inversely proportional to m if Fnet is constant.
Define Newton’s Third Law.
When objects interact, the force that each object on the other is equal in size and opposite in direction.
The 2 forces must:
Act on 2 different objects
Be equal in magnitude
Be opposite in direction
Be the same type of force
Define momentum
Vector quantity
Product of mass and velocity
p = mv
Unit p: kgm/s or Ns
Describe Newton’s First Law in terms of momentum.
If object maintains state of motion, then momentum stays the same.
p is constant.
Describe Newton’s Second Law in terms of momentum.
The rate of change of momentum is equal to the net force.
Impulse = change in momentum.
Describe Newton’s Third Law in terms of momentum.
Expressed as the rate of change of momentum due to the action force on one body is equal and opposite to the rate of change of momentum due to the reaction force of the other body.
Describe the Principle of Conservation of Momentum.
In all isolated collision of bodies, momentum is conserved.
This means that the sum of momentum before collision is equal to the momentum after collision for an isolated system of bodies.
m1 u1 + m2 u2 = m1 v1 + m2 v2
What are the two types of collisions?
Elastic and Inelastic.
Describe elastic collision.
Kinetic energy is conserved.
Sum of initial kinetic energy = sum of final kinetic energy.
The relative speed of approach is equal to the relative speed of separation of the bodies.
Sum of momentum before = sum of momentum after (momentum conserved)
Describe inelastic collision.
Kinetic energy is not conserved.
Total energy is conserved.
Momentum is conserved.
Relative speed of approach is not the same as relative speed of separation.
