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How Forces Affect Motion
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6.1 The Concept of Force
a push or a pull; magnitude, unit and direction
6.1.1 Measuring the magnitude
spring balance; mass in kg, weight in N
6.2 Balanced and unbalanced forces
equal and opposite forces do not move the object
Net force
same direction, add; opposite directions, subtract
6.3 The force of friction
acts between two surfaces in contact, opposite to the motion
Depends on the surfaces
smaller friction, larger distance travelled
The three laws of motion
what a net force does, and what it does not do
6.4 First law
no net force, no change in the state of motion
6.5 Second law
F = ma; more force, more acceleration; more mass, less acceleration
6.6 Third law
equal and opposite forces, on two different objects
6.7 A system of objects
a = F / (m₁ + m₂); internal forces are ignored
