Understanding Forces and their effects: Safety in public transport to Moments and levers: OxfordAQA IGCSE CORE Physics (Short Course) Explained

From braking cars to balancing seesaws

Right, let us get straight into it. This guide covers OxfordAQA IGCSE CORE Physics (Short Course) Forces and their effects: Safety in public transport to Moments and levers, four topics that all come back to one question: how do forces stop things, hold things up, or tip things over? If you have landed on this page after typing forces and their effects: safety in public transport to moments and levers OxfordAQA IGCSE into a search bar, you are almost certainly revising braking distances, terminal velocity, centre of mass or moments, so here is everything you need, in the order it is easiest to learn.

Safety in public transport

When a vehicle travels at a steady speed in a straight line, the resistive forces acting on it (air resistance and friction) are exactly balanced by the driving force from the engine. That balance is why "steady speed" and "zero resultant force" mean the same thing on this specification: no net force, no change in velocity.

The greater the speed of a vehicle, the greater the braking force needed to stop it within a given distance, and the greater that braking force, the greater the deceleration produced. Push decelerations too high and brakes can overheat, or the driver can lose control of the vehicle entirely. For a given braking force, the higher the speed a vehicle starts at, the longer the stopping distance will be, which is the core reasoning behind speed limits near schools and in residential areas.

Stopping distance itself splits into two parts you must be able to name separately:

• Thinking distance: the distance travelled during the driver's reaction time, before the brakes are even applied.
• Braking distance: the distance travelled once the brakes are applied, under the braking force, until the vehicle stops.

Stopping distance is simply the sum of the two. A driver's reaction time, and therefore thinking distance, can be increased by tiredness, distraction, drugs and alcohol. Braking distance, on the other hand, is affected by adverse road and weather conditions, such as wet or icy surfaces, and by the poor condition of the vehicle itself, which on this specification is limited to worn brakes or worn tyres. Exam questions love to test whether you can correctly assign a factor to thinking distance or braking distance, so learn the two lists separately rather than as one blurred idea of "things that make stopping harder".

Forces and terminal velocity

Terminal velocity is what happens when a falling object's weight is exactly balanced by the resistive force of air resistance acting against its motion. As an object accelerates from rest, air resistance increases with speed, gradually reducing the resultant force and therefore the acceleration, until the resultant force reaches zero and the object falls at a constant maximum speed, its terminal velocity. A skydiver reaches terminal velocity for exactly this reason, and opening a parachute changes that balance abruptly by increasing air resistance, which briefly makes the resultant force act upward and decelerate the skydiver to a new, much lower terminal velocity.

The key skill examiners test here is describing the changing forces at each stage of a fall, not just stating the final outcome. Practise narrating the whole story: weight greater than air resistance, accelerating; air resistance increasing as speed increases; weight equal to air resistance, terminal velocity reached; and, if a parachute opens, air resistance suddenly exceeding weight, decelerating.

Centre of mass

The centre of mass of an object is the single point at which its entire mass may be thought to be concentrated, for the purposes of analysing its motion or stability. You should be able to describe a method for finding the centre of mass of a thin, irregularly shaped lamina, which typically involves suspending the shape freely from several different points and marking a vertical line down from each suspension point using a plumb line; the centre of mass lies where those lines cross.

If an object is freely suspended, it comes to rest with its centre of mass directly below the point of suspension. For a symmetrical object, the centre of mass lies along its axis of symmetry, which is why you can often locate it by inspection for regular shapes such as a rectangle or a circle rather than needing the practical method. The position of the centre of mass also affects how stable an object is: lowering the centre of mass, or widening the base an object stands on, both improve stability, which is why racing cars are built low and wide rather than tall and narrow.

Moments and levers

A moment is the turning effect of a force around a pivot, and it depends on both the size of the force and the perpendicular distance from the pivot to the line of action of that force. This is the physics behind every lever, spanner, seesaw and door handle you have ever used: pushing further from the hinge of a door takes less force to achieve the same turning effect than pushing close to the hinge, because the perpendicular distance is larger.

For an object in equilibrium, meaning it is not turning, the total clockwise moment about a pivot equals the total anticlockwise moment. This is the principle of moments, and it lets you solve for an unknown force or an unknown distance when a system is balanced, such as two people of different weights sitting at different distances from the centre of a seesaw.

Worked example

A spanner applies a force of 40 N at a perpendicular distance of 0.25 m from a bolt. The moment is force multiplied by perpendicular distance: 40 × 0.25 = 10 N m. If a second spanner applied the same 40 N force but at 0.5 m from the bolt, the moment would double to 20 N m, showing why a longer spanner makes a stiff bolt easier to turn without needing any extra strength.

Thinking distance vs braking distance at a glance

Notice that speed is the one factor on both sides of the table: a faster driver both travels further during their fixed reaction time and needs a longer distance to lose that extra speed once braking, so raising speed increases stopping distance twice over rather than once.

Moments turn up far beyond spanners and seesaws once you start looking for them. A door handle is placed away from the hinge for the same reason a long spanner works better on a stiff bolt: a larger perpendicular distance from the pivot means less force is needed for the same turning effect. Wheelbarrows, bottle openers and even the human forearm acting at the elbow joint are all examples your teacher may use in class, and OxfordAQA can set a question around any everyday lever, not only the classic seesaw diagram, so practise identifying the pivot and the perpendicular distance in an unfamiliar picture rather than only in familiar textbook diagrams.

Common mistakes worth eliminating now

• Mixing up thinking distance and braking distance when listing the factors that affect each one, especially alcohol and tiredness, which affect thinking distance, not braking distance.
• Describing terminal velocity as the point where "there is no air resistance", when the correct description is that air resistance has increased until it exactly equals weight.
• Forgetting that a moment calculation needs the perpendicular distance to the pivot, not simply any distance measured along the object.
• Assuming a symmetrical object always has its centre of mass at its geometric centre point rather than along its full axis of symmetry, which matters for elongated or oddly proportioned symmetrical shapes.

When a question describes a vehicle, a falling object, or a seesaw, resist the urge to jump straight to a formula. Sketch the forces first, label them, and only then decide which equation applies. Most lost marks in this section come from applying the right equation to the wrong force, not from getting the arithmetic wrong.

Self-check questions

1. List three factors that increase thinking distance and three that increase braking distance, keeping the two lists separate.
2. Describe, stage by stage, how the forces on a skydiver change from the moment they jump to the moment they reach terminal velocity.
3. Explain why lowering an object's centre of mass makes it more stable, using the idea of overturning around a pivot point.
4. A 60 N force acts at 0.4 m from a pivot. Calculate the moment produced, showing the equation before you substitute the numbers.

These oxfordaqa igcse core physics (short course) revision notes and oxfordaqa igcse core physics (short course) notes, and the accompanying oxfordaqa igcse core physics (short course) practice questions, are designed to be worked through actively rather than just read. Once you are confident here, move on to the energy section, since several of these ideas, particularly the balance between driving force and resistive force, reappear when you study energy transfers and dissipation. If a search for igcse 9223 forces and their effects: safety in public transport to moments and levers is what brought you here, this page is the oxfordaqa igcse core physics (short course) explained version worth bookmarking for your next revision session.