1.11-1.16 Forces, Scalars/Vectors, & Friction

Force

Push or pull on an object with mass causing it to accelerate (change its velocity). Force is measured in newtons (N)

Forces act on bodies and can have affects that cause these bodies to change their …

1. Speed; acceleration or deceleration


1. E.g: Thrust of engine’s magnitude can affect car’s speed if air resistance (drag) stays constant.
2. Shape; stretching, compression, or deformation


1. Hooking a mass onto the end of a spring can cause it to stretch.
3. Direction; change direction of motion due to force


1. Sun’s gravitational attraction can change the direction of a comet’s orbit/motion.

Types of forces

1. Contact forces
2. Non-contact forces
3. Weight
4. electrostatic
5. Thrust
6. Upthrust (buoyant force)
7. Friction
8. Air resistance (drag)
9. Compression
10. Tension
11. Normal reaction force

Contact forces

Forces experienced only when 2 bodies are in contact.

Non-contact forces

Forces that do not require 2 bodies to be in contact to be experienced.

Weight (gravitational force, force of gravity)

Force between any 2 bodies with mass due to gravitational attractions between them. When 2 objects with mass are close enough together, they exert gravitational forces on each other, which we measure as weight.

Electrostatic

Force of attraction or repulsion between 2 charged bodies.

Thrust

Force that propels an object in a particular direction, which is created by expelling mass at high velocity in the opposite direction to the desired movement. This expulsion creates an equal and opposite reaction force (thrust) which propels the body in the desired direction.

Upthrust (buoyant force)

Upward force exerted by a fluid (liquid or gas) on the body immersed in the fluid, which opposes the object’s weight.

Friction

Force that opposes/resists motion between 2 surfaces in contact with each other, it results in dissipation of thermal energy (heat). Friction emerges when surfaces rub against each other because of the way imperfections on the 2 surfaces at molecular level push against one another.

What do imperfections mean on surfaces?

Areas on surfaces that are not perfectly smooth.

Air resistance (drag)

Force of friction between air particles and objects moving in/through the air, which opposes the relative motion of an object as it passes through the air.

What causes air resistance?

Particles in the air move in the direction opposite to/from an object as it passes through the air, hence creating friction and creating resistance/opposition to the object’s movement. Air resistance may increase or decrease depending on the shape as well as speed pf the moving object.

Compression

Force that squeezes an object inwards.

Tension

Force that stretches an object; forces pull/stretch the body in opposite directions, causing it to stretch.

Normal reaction force

Force between any 2 objects in contact. This force is perpendicular to the surface of contact, and is equal in magnitude but opposite in direction to the weight of the object in contact with the surface. The normal reaction force prevents objects from passing/falling through the surface it’s in contact with.

Quantity

Characteristics or properties of an object that can be measured. All quantities are either scalar or vector.

Scalar quantity

Quantities that can be fully described by a magnitude (numerical size/value) alone.

Examples of scalar quantities

Distance (m)

Speed (m/s)

Time (s)

Mass (kg)

Temperature (K)

Pressure (Pa or N/㎡)

All forms of energy (J)

Work done (J)

Current (A)

Potential difference (V)

Resistance (Ω)

Volume (㎥)

Density (kg/㎥)

and many more …

Vector quantity

Quantity described by both a magnitude (numerical size/value) and a direction.

Examples of vector quantities

Displacement (m)

Velocity (m/s)

Acceleration (m/s^2)

Force (N)

Weight (N)

Moment (Nm)

All forces are vector quantities because they have both a magnitude/size and direction.

How do scalar and vector quantities relate with one another?

Sometimes vectors and scalars align in corresponding pairs because they measure the same thing and have the same units. E.g: mass and weight, speed and velocity, distance and displacement, etc.

Free-body diagrams

Diagrams showing all forces acting on a body by drawing arrows.

Length of arrow shows force’s magnitude: longer arrow → higher magnitude of force, shorter arrow → lower magnitude of force.

Direction arrow points in shows direction force acts in.

Pure free body diagrams depict objects as a point particle. However in IGCSE, the object is still drawn. Some forces are at an angle and are therefore described with respect to the horizontal or vertical.

Resultant force (a.k.a. net force)

Single force describing the overall result of all forces acting on a body, it’s obtained through vector addition.

Vector addition

Adding or combining 2 or more vectors to obtain a single vector with the same effect as all the original vectors acting together. Resultant forces are vectors so show the object’s final direction and magnitude of force.

What are the conventional signs for directions in vectors?

\+ for up or right

\- for down or left

Opposite directions can also be described as negatives of one another. E.g: When required to describe a leftward force, a right ward force would be negative and subtracted from the leftwards force.

What happens when the resultant force is 0?

Opposite forces all cancel out → Resultant force = 0 (i.e. Σ Forces = 0) → Balanced forces → No acceleration (no change in velocity) → Stationary objects stay stationary, moving objects move at a constant speed in the same direction.

What happens when the resultant force ≠ 0?

Opposite forces don’t all cancel out → Resultant force ≠ 0 (i.e. Σ Forces ≠ 0) → Unbalanced forces → Acceleration (change in velocity) → stationary objects move, moving objects speed up/slow down/change direction.

How to calculate resultant forces of forces that act along a line in the same direction?

Add the 2 or more forces together, state the number of newtons (including the units) and the direction of the force (as forces are vector quantities).

How to calculate resultant forces of forces that act along a line in opposite directions?

Subtract 1 force from the other. The resultant force will be the subtraction result (in the direction of the force that was NOT subtracted from the other or whichever direction specified from the question - in this case you need to change which force is subtracted from which). State the number of newtons (including the units) and the direction of the force (as forces are vector quantities).

How to calculate resultant forces of forces that are perpendicular to one another?

Move one of the forces over such that the end of 1 force is connected to the start of the other force. Use Pythagoras’ theorem to calculate the resultant force and state its direction.

Friction forces … the … of an …

oppose, motion, object