Forces & Moments

What is a force? Is it a scalar or vector quantity, and what is its unit?

A force is a push or pull. It is a vector quantity, meaning it has both magnitude and direction. The unit for force is the Newton (N).

What is the difference between contact and non-contact forces?

Contact forces require two objects to be in physical contact (e.g., friction, normal force).

Non-contact forces can act over a distance without physical contact (e.g., gravity, electrostatic force, magnetic force).

What is the formula for the weight of an object, and what does each variable represent?

The formula is W=mg, where W is the weight in Newtons, m is the mass in kilograms, and g is the gravitational field strength in N kg⁻¹ or acceleration of free fall in m s⁻².

What is the centre of gravity (CG)?

The centre of gravity is the single point through which an object's entire weight appears to act.

What is Hooke's Law?

Hooke's Law states that within the limit of proportionality, the extension of a spring is directly proportional to the applied force. The formula is F=kx.

How is the spring constant (k) determined from an F-x graph?

The spring constant is the gradient of the Force vs. Extension graph.

How do you calculate the effective spring constant for springs connected in series and parallel?

Parallel: keff​=k1​+k2​+...

Series: 1/keff​=1/k1​+1/k2​​+...

What is normal contact force?

It's the perpendicular force that one object exerts on another when they are in physical contact. It is always perpendicular to the surface of contact.

What is frictional force?

Friction is a contact force that acts between two surfaces to oppose motion or attempted motion. It's caused by surface irregularities.

What is the difference between static and kinetic friction?

Static friction acts when there is no relative motion and its magnitude is self-adjusting to prevent motion, up to a maximum value.

Kinetic friction acts when two surfaces are sliding against each other.

What is viscous force?

It is a resistive force experienced by an object moving through a fluid (like air or water). It opposes the motion of the object.

What is upthrust?

Upthrust is the upward force on a body immersed in a fluid due to pressure differences. It is equal to the weight of the fluid displaced by the object.

Define the moment of a force.

The moment of a force is the product of the force and the perpendicular distance from its line of action to the axis of rotation. Its unit is N m.

What is the Principle of Moments?

For an object to be in rotational equilibrium, the sum of clockwise moments about any axis of rotation must equal the sum of anticlockwise moments about the same axis.

What is a couple?

A couple is a pair of equal and opposite parallel forces whose lines of action do not intersect. It produces a turning effect.

What are the two conditions for an object to be in equilibrium?

The resultant force acting on the body must be zero (translational equilibrium).

The resultant torque (or sum of moments) about any axis must be zero (rotational equilibrium).