9709 · 4.1
Forces and equilibrium flashcards
Revision flashcards for Cambridge 9709 Forces and equilibrium (syllabus 4.1). Flip, recall, then mark a real past-paper question.
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What is a 'force' in mechanics?
A push or a pull on an object that can cause it to accelerate (change its velocity). It is a vector quantity, possessing both magnitude (in Newtons, N) and direction.
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What does it mean for a body to be in 'equilibrium'?
The body is either at rest or moving with a constant velocity. This occurs when the vector sum of all forces acting on it is zero (the resultant force is zero).
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What is a 'free-body diagram'?
A diagram showing a single, isolated body and all the external forces acting upon it. Internal forces are not shown.
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What is the 'Normal Reaction' force?
A contact force exerted by a surface on an object. It always acts perpendicular to the surface at the point of contact.
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A common mistake is to assume the Normal Reaction, R, is always equal to the weight, mg. When is this NOT true?
R is not equal to mg if the surface is inclined, or if there are other vertical forces acting on the object (e.g., someone pushing down on it, or a rope pulling up).
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What is 'friction'?
A force that opposes the motion or attempted motion between two surfaces in contact. It always acts parallel to the surfaces.
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What is the 'coefficient of friction', $\mu$?
A dimensionless number that represents the 'roughness' between two surfaces. It's the ratio of the maximum frictional force to the normal reaction force: $\mu = F_{max} / R$.
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What is 'limiting equilibrium'?
The state where an object is on the point of sliding. In this specific case, the frictional force has reached its maximum possible value, $F_{max} = \mu R$.
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Is the force of friction always equal to $\mu R$?
No. The force of friction $F$ satisfies the inequality $F \le \mu R$. It is only equal to $\mu R$ when the object is in limiting equilibrium (on the verge of sliding).
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How do you resolve the weight, $mg$, of an object on a plane inclined at angle $\theta$?
The component of weight acting perpendicular to the plane is $mg \cos\theta$. The component of weight acting parallel to the plane is $mg \sin\theta$.
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What is the difference between 'tension' and 'thrust'?
Tension is a pulling force in a string, rope, or light rod. Thrust is a pushing force, usually associated with a rigid rod or an engine.