P2.1.3 Forces and braking

P2 Physics

KS4 ADDITIONAL SCIENCEMr D Powell

Mr Powell 2012Index

Connection

• Connect your learning to the content of the lesson

• Share the process by which the learning will actually take place

• Explore the outcomes of the learning, emphasising why this will be beneficial for the learner

Demonstration

• Use formative feedback – Assessment for Learning

• Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex

• Offer different ways for the students to demonstrate their understanding

• Allow the students to “show off” their learning

Activation

• Construct problem-solving challenges for the students

• Use a multi-sensory approach – VAK• Promote a language of learning to

enable the students to talk about their progress or obstacles to it

• Learning as an active process, so the students aren’t passive receptors

Consolidation

• Structure active reflection on the lesson content and the process of learning

• Seek transfer between “subjects”• Review the learning from this lesson and

preview the learning for the next• Promote ways in which the students will

remember• A “news broadcast” approach to learning

Mr Powell 2012Index

P2.1.3 Forces and braking

a) When a vehicle travels at a steady speed the resistive forces balance the driving force.

b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

f) A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

d) A driver’s reaction time can be affected by tiredness, drugs and alcohol.

Mr Powell 2012Index

a) When a vehicle travels at a steady speed the resistive forces balance the driving force. A

For any car travelling at constant velocity, the resultant force on it is zero.

• This is because the motive force of its engine is balanced by the resistive forces (i.e. friction and air resistance) on it.

• A car driver uses the accelerator pedal (also called the gas pedal) to vary the

• motive force of the engine.

• The resistive forces change to meet to the rise or fall.

Mr Powell 2012Index

b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance. A

The braking force needed to stop a vehicle in a certain distance depends on:

• the velocity of the vehicle when the brakes are first applied• the mass of the vehicle.

We can see this using the equation F = ma in which the braking force is the resultant force.

• The greater the velocity, the greater the deceleration needed to stop it in a certain distance. So the braking force must be greater than at low velocity.

• The greater the mass, the greater the braking force needed for a given deceleration.

Mr Powell 2012Index

c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

A

Travel this whilst thinking about applying the brakes

Now this when you actually have your foot on the brakes

Mr Powell 2012Index

d) A driver’s reaction time can be affected by tiredness, drugs and alcohol. A

Poorly maintained vehicles, for example with worn brakes or tyres, take longer to stop because the brakes and tyres are less effective.

Tiredness, alcohol and drugs all increase reaction times. So they increase the thinking distance (because thinking distance = speed x reaction time). Therefore, the stopping distance is greater.

The faster a vehicle is travelling, the further it travels before it stops. This is because the thinking distance and the braking distance both increase with increased speed.

In adverse road conditions, for example on wet or icy roads, drivers have to brake with less force to avoid skidding. Stopping distances are therefore greater in poor road conditions.

f) A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

Mr Powell 2012Index

e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

A

Kinetic Energy = ½ mv2

As you slow the KE is transferred into thermal by friction between the discs and pads

Mr Powell 2012Index

Summary Questions... D

Consolidate – How can you explain these things...P2.1.3 Forces and braking Part I

a) When a vehicle travels at a steady speed the resistive forces balance the driving force.

b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

Consolidate – How can you explain these things...P2.1.3 Forces and braking Part II

e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

f) A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

d) A driver’s reaction time can be affected by tiredness, drugs and alcohol.