5-DFAC-04

Outcomes

• Pupils should be able to sketch a velocity-time graph for a person jumping from an aircraft. They should appreciate the sections where the person is accelerating and not accelerating and what this means in terms of the forces acting on them.
• Pupils should understand that acceleration comes from an unbalanced force.
• Pupils should appreciate that speed, mass, road condition and reaction time are the factors that affect vehicle stopping distance.

Specification References

• 1.16 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity.
• 1.17 describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time.

Starter

• Using a motion sensor to record the data of a block being pushed away. How can we work out the initial speed (top speed) using the distance time graph? the velocity time graph? How can the velocity time graph be used to work out the distance travelled by the block? Measure its distance. Do they match up?

Main Body of Lesson

Velocity vs. stopping distance

• Proceed in having students push the block with an interruter through a light gate. This a wonderfully quick way to do this experiment. Record a range of speeds and stopping distances.
• What do they notice? Have them plot a graph of speed vs. stopping distance. How are they related? Have them quote values from the graph. What happens to the distance when the speed is doubled? What can they plot to get a straight line. If there is time have them plot distance against speed squared.
• The factors affecting the stopping distance need to be discussed. They must understand that the stopping distance is made up of both the thinking distance and braking distance. The first is affected by the driver (i.e drinking, drugs, tired etc.) the second is affected by the performance of the car or the state of the road. (i.e. icy conditions, poor brakes, poor tire tread, etc.)
• I have made a questions sheet on this. It is quite long and takes the students through everything that they need to know. It also ties in velocity-time graphs.

Plenary

Terminal velocity

• Sketch a velocity-time graph for a falling object. A person jumping out of a plane.
• Make sure to include force diagrams of a the person at each stage of the jump. This will show when the resultant force is greatest (at the beginning) and when it is balanced (at terminal velocity both without the chute open and with it open). This also points out that the weight vector does not change but the air resistance vector changes in magnitude according to the speed or if the parachute is open.

Homework

• Question sheet on Stopping Distance - use of speed = distance/time, using a velocity time graph to work out distance covered with different thinking times and including factors affecting stopping distances.
• Questions on Terminal velocity
• These two question sheet can be found in science shared

Additional Information

Textbook References

• None

Website References

• None

Skills Addressed

• Obtaining

Safety/Hazards

Risk Assessment

Notes

• None

Forces & Matter Outline

Forces & Matter