The drag force on an object moving through a fluid increases with its speed and can be reduced by making the object more streamlined
Observable learning outcome:
Explain how using a parachute can make it safe to jump out of an aeroplane
Question type:
Explanation story
Key words:
Drag, weight, resultant force, speed
Mida ütlevad uurimused
When the speed of an object is being increased, students tend to focus on the applied force that appears to be needed to get it going, and keep it going. They often think that a moving object has force that keeps it moving, and which runs out when it comes to rest (Gunstone, R and Watts, 1985; Driver et al., 1994a). Osborne (1985) found that as students get older they increasingly hold the view that a force, pushing in the direction of motion, is needed to keep an object moving. In a study of 200 students he found 46% of 13 year olds believed this, increasing to 53% of 14 year olds and 66% of 15 year olds.
Instead of concentrating on the applied force students need to think about all the forces acting and how they combine to produce the resultant force. They need to identify when the resultant force acts, when it changes and when it ceases. This involves understanding drag and the direction it acts in in order to recognise how it contributes to the resultant force (Driver et al., 1994b).
For most students the idea that a bigger resultant force produce a bigger effect is intuitive, but it is important to emphasise that resultant force does not produce speed, but a change in speed (Driver et al., 1994b). In other words: any moving object will continue to accelerate whilst a resultant force is acting on it.
The effect of drag on falling objects can be very confusing because although drag (air resistance in air) can significantly affect the motion of an object falling in the real world, students often do not take it into account. In a study, Lee and Kwok (2009) found that when 11- to 12-year-olds (n=204) were asked to label the forces on a falling ball, 90% were able to label the gravitational force, but none of them included any drag force.
This question investigates students’ understanding of how changing the drag force with a parachute (making drag larger than the driving force) affects the motion of a moving object.
Kuidas seda töölehte kasutada
This task is intended for discussion in pairs or small groups. It is best done using sort cards to organise.
Students should read the statements and follow the instructions on the worksheet. Listening in to the conversations of each group will often give you insights into how your students are thinking. Each member of a group should be able to report back to the class.
Feedback from each group can be used, with careful teacher questioning, to bring out a clear description or explanation of the science.
Differentiation
The quality of the discussions can be improved with a careful selection of groups; or by allocating specific roles to students in the each group. For example, you may choose to select a student with strong prior knowledge as the scribe, and forbid them from contributing any of their own answers. They may question the others and only write down what they have been told. This strategy encourages contributions from more members of each group.
NB in any class, small group discussions typically improve over time and a persistence with this strategy is often very successful in the medium to long term.
Do it
Uuri allpool kirjeldatud olukorda. Vasta küsimustele.
Ohutu maandumine
Jüri hüppab lennukist alla.
Ta kasutab ohutuks maandumiseks langevarju.
Järgnevad väited räägivad langevarjuga lennukist välja hüppamisest. Pane need õigesse järjekorda, nii et tekiks lugu, kuidas langevari seda teeb.
Õiged vastused
There is a big resultant force downwards.
The speed of the jumper goes up quickly.
The jumper is going too fast to land safely.
The parachute is opened.
Drag is made much bigger.
There is a resultant force upwards.
The jumper slows down.
The jumper’s weight is the same size as the drag on the parachute.
