Dynamics 2 N1&2 L
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Transcript of Dynamics 2 N1&2 L
DYNAMICSNewton’s 1st Law of MotionNewton’s 2nd Law of Motion
Objectives
State Newton’s 1st Law of Motion Show an understanding that mass is the property of a
body which resists change in motion Define linear momentum as the product of mass and
velocity Define force as the rate of change of momentum State Newton’s 2nd Law Identify forces acting on an object and draw free body
diagram(s) representing the forces acting on the object (for cases involving forces acting in at most two dimensions)
Recall and solve problems using the relationship F=ma, appreciating that force and acceleration are always in the same direction
Apply the relationship between resultant force, mass and acceleration to new situations and solve related problems
Newton’s 1st Law - Fnet = 0
Definition A body at rest will remain at rest and a
body in uniform motion in a straight line will remain so unless a net force acts on it.
For a body at rest OR travelling with constant velocity the net force acting on the body is zero.
Newton’s 1st Law - Fnet = 0
Also called the Law of Inertia. Inertia - the reason for the reluctance of
a body to change it’s state of motion. The larger the mass, the larger the inertia.
A body at rest will tend to be at rest. A body in motion will tend to be in
motion. “Tend” means continue.
Newton’s 1st Law - Demo 1
The Magic Table Cloth
Newton’s 1st Law – Demo 2
BonnetSeat Belt
Newton’s 1st Law
Newton’s 1st Law
Newton’s 2nd Law - Moving Bullet vs The Mad Elephant
0.02kg moving at 400ms-1 2000kg moving at 5ms-1
Which one is harder to stop?
Newton’s 2nd Law - Linear Momentum
Definition Product of the mass of an object and its
velocity p = mv
Vector quantity Has the same direction as the velocity Units: kg m s-1
Newton’s 2nd Law - Fnet 0
Recall: When a resultant force acts on on object, its velocity changes.
Velocity changes Momentum changesN2L
Formal Definition The rate of change of momentum of an object
is directly proportional to the resultant force acting on it. The rate of change of momentum takes place in the direction of the force.
Newton’s 2nd Law - Fnet 0
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Resultant Force α Change in Momentum
Time taken
⇒ F =(mv −mu)
Δt=
Δ(mv)
Δt
Newton’s 2nd Law - Fnet 0
If the mass of the object does not change, then:
Force (N) = mass (kg) x acceleration (ms-2)
Definition can be simplified into: The resultant force acting on a body is
proportional to the acceleration, given that the mass of the body remains constant.
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F =m(v − u)
Δt= mass ×
change in velocity
time taken= mass × acceleration
Newton’s 2nd Law - Fnet 0
Mass has been constant in all these Mass has been constant in all these cases.cases.
Are there cases when mass changes?Are there cases when mass changes?
If mass changes, can we still use N2L?If mass changes, can we still use N2L?
Newton’s 2nd Law - Fnet 0
Changing mass
Missiles
Space
Shuttle