1.5.1States of Matter
Braking Force — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 30 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Forces While Braking
A car keeps its velocity while the driving and resistive forces balance, and braking creates an unbalanced force.
Constant velocity
- Constant velocity The driving force equals the total resistive forces, so the resultant force is zero and the car keeps its velocity.
Braking force
- Braking force A frictional force between the brake pads and the wheels, which does work on the wheels and slows them down.
Work done by braking
- Work done by braking Energy transfers from the car's kinetic store to the thermal store of the brakes, which heat up as a result.
Deceleration
- Deceleration The car slows down because the braking force creates a resultant force opposite to the direction of motion.
Part 2 of 5Energy Transfer When Braking
Braking is a transfer of energy, not the destruction of energy.
1
The driver presses the brake pedal, pushing brake pads against the discs or drums attached to the wheels.
2
Friction between the pads and the discs does work, transferring energy away from the car's kinetic store.
3
The kinetic energy of the car decreases, while the thermal energy of the brakes increases.
4
The energy stored in the hot brakes is eventually transferred to the surroundings by heating.
Part 3 of 5Braking at High Speed
Braking at High Speed
Kinetic energy
- Kinetic energy Kinetic energy is proportional to speed squared, so doubling the speed gives four times the energy to be removed.
Braking force
- Braking force A higher speed needs a larger braking force to stop within the same distance.
Deceleration
- Deceleration A larger braking force gives a larger deceleration, which can overheat the brakes and warp the discs.
Road surface
- Road surface Wet or icy roads reduce the friction between the tyres and the road, so less braking force is available.
Part 4 of 5Worked Example
Question
- A car of mass 1000 kg travelling at 20 m/s brakes to rest.
- Calculate the kinetic energy that must be transferred away from the car by the brakes.
- Write the equation Ek = ½ × m × v².
- SubstituteEk = ½ × 1000 × 20².
- 20² = 400, so Ek = ½ × 1000 × 400 = 200 000 J.
- Write the answer to two significant figures: 2.0 × 10⁵ J.
Answer The kinetic energy to be transferred is 2.0 × 10⁵ J.
Part 5 of 5Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- Braking force The frictional force between brake pads and wheels that slows a vehicle.
- Kinetic store The energy store of an object that is moving.
- Thermal store The energy store of an object that is hot.
- Deceleration A reduction in speed, which is an acceleration in the opposite direction to the motion.
- Work done The energy transferred when a force moves an object through a distance in the direction of the force.
Common Mistakes
- Attributing the heating of the brakes to friction between the tyres and the road;
- the heating comes from friction between the brake pads and the wheel.
- Describing the braking force as a non-contact force; it is a contact friction force.
- Stating that kinetic energy is destroyed;
- it is transferred to the thermal store and then to the surroundings.
- Assuming that doubling the speed only doubles the kinetic energy; it depends on the speed squared.
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