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