Lesson 3.2 Applied forces: Friction and tension

What is friction?

The resistance to motion of a surface moving relative to another, caused by electromagnetic attraction between charged particles in two touching surfaces. Symbol: Fᶠ, measured in newtons (N).

What happens to friction when an object moves at constant speed on a horizontal surface?

Net force is zero, so frictional force equals the applied horizontal force (Fᶠ = Fₕ).

What is tension and what is its symbol?

Tension (Fₜ) is the pulling force transmitted along a rope, string, cable or chain. It comes from Latin "tensio" meaning "to stretch." Tension is zero if the rope is pushed.

For a hanging object at rest, how does tension relate to weight?

Tension equals weight (Fₜ = Fᵍ) because net force is zero. The rope pulls up with the same magnitude as gravity pulls down.

When an object is accelerating upwards, what is the relationship between tension and weight?

Tension is greater than weight. The net force equals Fₙₑₜ = Fₜ - Fᵍ, which can be rearranged to Fₜ = Fₙₑₜ + Fᵍ.

What does a frictionless pulley do to forces?

It redirects forces without changing their magnitude or losing energy. The tension in the cord remains the same on both sides of the pulley.

What is an applied force (Fₐ)?

A force applied to an object by a person or another object, can be a push or a pull. Example: pushing a desk or pulling a cart with a rope.

When pulling an object at an angle θ, what are the horizontal and vertical components of the applied force?

Horizontal: Fₕ = Fₐcos θ (moves object forward). Vertical: Fᵥ = Fₐsin θ (tends to lift object, reducing normal force).

When pushing an object at an angle θ, how do the force components differ from pulling?

Horizontal: Fₕ = Fₐcos θ (moves object forward). Vertical: Fᵥ = Fₐsin θ (pushes object into ground, increasing normal force). This is why pulling is often easier than pushing.

Why is it easier to pull a pram or lawnmower rather than push it?

When pulling, the vertical component lifts the object slightly, reducing normal force and friction. When pushing, the vertical component pushes down, increasing normal force and friction.

A 1.5 kg block is pulled horizontally along a frictionless surface with tension 4.5 N. If a cotton thread breaks at this tension, what is the maximum acceleration possible?

Using Fₙₑₜ = ma: a = Fₜ/m = 4.5/1.5 = 3.0 m s⁻². This is the maximum acceleration before the thread breaks