1.5 - Forces
Newton’s laws
1. Unless acted upon by a non-zero resultant force, an object will maintain its velocity
2. If there is a resultant force acting on an object it will accelerate, ie ∵ F = 0,
m > 0, ∴ a = 0; acceleration of an object depends on its mass and
magnitude of applied force
3. Forces act in opposite directions to each other with the same magnitude and type
Terminal velocity
● Happens when weight of object = air resistance (mg = Fdrag)
○ Terminal velocity depends entirely on mass of object
1. (Moment when object is dropped) mg is only force acting downwards
2. As object accelerates downwards due to gravity, Fdrag increases
3. In the end, mg = Fdrag, terminal velocity reached (v > 0, a = 0)
Free-body force diagrams
● Eg
● ***When solving unknown forces, equation must resolve ⟂ or ∥ to plane
Moments (≠ momentum)
● ***Moment (singular): a turning effect/action of force
○ ***moment ( Nm)=F (force [ N ])× d (⊥ distance [m])
● ***Centre of gravity/mass: a point representing the mean position of the matter in a
body/system
○ Exceptions: eg doughnut’s centre of mass NOT part of object
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Newton’s laws
1. Unless acted upon by a non-zero resultant force, an object will maintain its velocity
2. If there is a resultant force acting on an object it will accelerate, ie ∵ F = 0,
m > 0, ∴ a = 0; acceleration of an object depends on its mass and
magnitude of applied force
3. Forces act in opposite directions to each other with the same magnitude and type
Terminal velocity
● Happens when weight of object = air resistance (mg = Fdrag)
○ Terminal velocity depends entirely on mass of object
1. (Moment when object is dropped) mg is only force acting downwards
2. As object accelerates downwards due to gravity, Fdrag increases
3. In the end, mg = Fdrag, terminal velocity reached (v > 0, a = 0)
Free-body force diagrams
● Eg
● ***When solving unknown forces, equation must resolve ⟂ or ∥ to plane
Moments (≠ momentum)
● ***Moment (singular): a turning effect/action of force
○ ***moment ( Nm)=F (force [ N ])× d (⊥ distance [m])
● ***Centre of gravity/mass: a point representing the mean position of the matter in a
body/system
○ Exceptions: eg doughnut’s centre of mass NOT part of object
1
