# GCSE Questions: Forces

Q25. The table below shows thinking, braking and stopping distances for the same car travelling at different speeds.

 Speed (m/s) Thinking distance (m) Braking distance (m) Stopping distance (m) 8 6 6 12 16 12 24 36 32 24 96 120

(a) Describe how the thinking distance changes when the speed doubles.

Thinking distance doubles when speed doubles. (For example it goes from 6m to 12 m when speed changes from 8 m/s to 16 m/s). This indicates they are directly proportional to each other.

[1 mark]

(b) Calculate the reaction time (in seconds) of the person driving the car.

(Reaction) time = (Thinking) distance ÷ Speed

6 ÷ 8 or 12 ÷ 16 or 24 ÷ 32

= 0.75 s

[3 marks]

(c) Explain why the stopping distances are different for each speed shown in the above table.

This type of question is marked in two ways:

- the examiner looks for relevant points

- but also for a logical sequence...

It is not just a 'tick fest' - but a marker does look to see how many relevant points you have made, then considers how you have strung them together to put your answer into a 'level'

 Level 3: Explains quantitatively why the stopping distances are different for each speed in the table in terms of braking distance and thinking distance. There is a well-developed line of reasoning which is clear and logically structured. The information presented is relevant and substantiated. 5 - 6 marks Level 2: Explains qualitatively why the stopping distances are different for each speed in the table in terms of braking distance or thinking distance increasing with speed from the table There is a line of reasoning presented with some structure. The information presented is relevant and supported by some evidence. 3 - 4 marks Level 1: States basic ideas about thinking distance /braking distance/stopping distance OR Identifies variation of thinking distance/braking distance/stopping distance with speed There is an attempt at a logical structure with a line of reasoning. The information is in the most part relevant. 1 - 2 marks Level 0: No relevant information 0 marks

Examples of physics points that should be made in the response:

Demonstrates knowledge and understanding of thinking, braking and stopping distance:

- Thinking distance is the distance the car travels while the driver reacts

- Braking distance is the distance travelled while the brakes are applied

- Stopping distance is thinking distance + braking distance

Applies knowledge and understanding of thinking, braking and stopping distance in relation to the details in the table

- Increasing the speed, increases the thinking distance

- Increasing the speed, increases the braking distance

- Increasing the speed, increases the stopping distance

Analyses information to make judgements and draw detailed conclusions from table

- Thinking distance is directly proportional to the speed

- When speed doubles, thinking distance doubles

- Braking distance is proportional to speed2

- When speed doubles, braking distance quadruples

[6 marks]

[10 Marks TOTAL]