Ideas about forces and energy

Ideas about forces and energy

Pencil and paperOnline interactive
Overview
Using this Resource
Connecting to the Curriculum
Marking Student Responses
Working with Students
Further Resources
This task is about explaining what affects how far toy cars roll.
John, Margaret, and Deepak like to play with toy cars. They let the cars roll down a slope and then along the floor. One car goes further along the floor than the other.
ramps and toy cars
Image source: NZCER

Question 1Change answer

  1. John thinks that the carpet might affect how far each car goes. Why might the surface of the floor affect the distance the car goes?

Question 1Change answer

  1. Margaret thinks that one car always goes further than another. Why might the type of car affect how far the car goes?

Question 1Change answer

  1. Deepak thinks the car goes further if it rolls down a steeper track. Why might the steepness of the track affect how far the car goes?

Question 1Change answer

  1. Some cars go further than others, but all of them stop in the end. Explain why they stop.
Task administration: 

The task was trialled by students individually completing the written worksheet. However, students could work in groups or as a class, debating their answers. Carrying out the task in this way is likely to provide additional evidence of students' thinking. This helps to promote the nature of science idea that there may be a number of explanations for why one car may go further than another. It could also help students to build on and respond critically to each others' ideas.

Levels:
3, 4, 5
Curriculum info: 
Science, Investigating in science, Physical World
Keywords: 
physical concepts, forces, energy, friction, capabilities, use evidence, explanations, group assessment
Description of task: 
Explain what affects the distance travelled by toy cars, and why they eventually stop. Assessment focus: science explanations using ideas about forces and energy.
Curriculum Links: 
Science capabilities
The capabilities focus is brought about by the conversations you have and the questions you ask 
Capability: Gather and interpret data
This resource provides opportunities to discuss the different variables that can affect investigations.
 
 
 

 
Science capabilities: 
Gather and interpret data
Answers/responses: 
    Y6 (03/2010) Y8 (03/2010) Y10 (03/2010)
a) Links properties of the floors' surfaces to slowing down the vehicles, e.g., a rough surface rubs against the car tyres more than a smooth surface and slows the car down.
difficult
difficult
difficult
Partial answer 
Identifies properties about the surface of the flooring only, e.g., The carpet is bumpier than the wood.
or
Identifies that the carpet will slow the car down more than the wood surface, or will go faster on the wood surface, but doesn't explain why, e.g., Cars go further on smooth surfaces.		 very easy very easy very easy
b) Identifies a variable of the toy cars (shape, size, weight, number of tyres, etc.) and links to science concepts such as aerodynamics, increased friction, or momentum, e.g., The taller the car the more resistance from the air.
very difficult
very difficult
difficult
Partial answer
Identifies a variable of the cars only, e.g., One car might be light and the other may be heavy.		 easy easy very easy
c) Explanation about cause and effect on the ramp and the floor, e.g.: There is more force on the car so it accelerates and travels further along the floor. (They may use words such as push, gravity, less friction, angle to describe the idea of force.)
or
The higher the car is the more energy the car will gain, the faster it will come off the ramp and the further it will travel. (They may use words such as potential, gravitational potential, kinetic, momentum, speed, acceleration, or velocity to describe energy.)
very difficult
very difficult
very difficult
Partial answer 
Explanation about cause and effect on the ramp only, e.g., The higher up the car is the more force will be pushing it down making it go faster.		 very easy very easy very easy
d) Any 1 of:

  • Forces (including using words such as friction, wind resistance, gravity, surface texture,  or grip force) slows it down.
  • Kinetic energy/ power/ momentum runs out/ is lost.
 very difficult very difficult moderate
Based on a representative sample of 112 Year 6, 143 Year 8 and 299 Year 10 students

NOTE: This task was also trialled with Year 4 students. It was subsequently adapted to better meet Level 2 requirements of the curriculum. For Year 4 data, go to Ideas about rolling.

Teaching and learning: 

Background information

This resource focuses on students explaining their ideas. A key feature of science explanations is the focus on improving current explanations as we gather more evidence. The task asks students to identify what they currently believe, and why, and this opens up the space to debate, challenge, check, adapt, and build on their explanations. The motion of objects is an every day experience for students, but the science of motion is complex. Playing with toy cars is an engaging context to start building explanations about motion, based on their observations.

The key ideas that are covered in the task are shown in the table below.

Nature of science idea Science knowledge
Understanding about science
  • Producing explanations is central to the discipline of science.
  • Science explanations are based on an understanding of the science concepts involved.
Physical concepts
  • Forces (pushes and pulls) affect how far objects move.
  • Frictional forces increase when surfaces are more resistant to each other (rub together more). They reduce with less resistance, e.g., a smoother surface.
  • Energy can change from one form to another.

This task also has links to Investigating in science. Although students are not asked to identify variables, the different things that students may identify are in fact variables that they might control during a fair test.

Diagnostic and formative information: 
Nature of Science: explanations
Diagnostic and formative information
Questions a) and b)
For question a) about a third of students from all three levels gave full explanations, identifying a property of the floor's surface and explaining how this affects the distance travelled.

  • Because the carpet's bumpy it slows it down, if it was on lino, because it's smooth it will keep on going. [Year 6]
  • The carpet will create friction which will bring the car to a stop sooner. The wood is smooth so will have less friction. [Year 10]

More than half of the students at all three levels gave partial answers. Although these students were able to identify what property of the flooring affected the car they did not explain how this affected the distance the car travelled. The response rates for question b) were similar except only about a fifth of Year 6 students gave a full explanation.
Next steps
Predict, Observe, Explain (POE) is a structured strategy to encourage students to explain their thinking. The strategy can be used in other contexts when you are working on explanations. Encourage students to critique and build on others' explanations, providing evidence or reasons for their ideas. Possible teaching points to focus on include:

  • Do students consider other information as it becomes available and revise their explanation?
  • Do students ask questions for clarification?

Students also need to be able to use language that links ideas if they are to explain their thinking.
See Linking words and Text connectives in Thinking about how language works.

Concept cartoons for assessment are also useful for promoting classroom discussions. A concept cartoon, Rolling cars, is based on some common responses from the trial and can be used to build on existing explanations.
 

Science concepts: forces and energy
Question c)
Students used their understanding of either forces or energy to answer this question.  About two thirds of Year 6 and Year 10 and a third of Year 8 students used the concept of energy to explain about steepness and how this affects the distance the toy car travels. Most of these students discussed the car gaining speed/going faster, although some (mostly Year 10) included scientific terminology, e.g., momentum, in their explanations.

  • Because the steeper the track the faster the car goes. [Year 6] (partial answer that only discusses what happens on the ramp, not the impact on the distance)
  • If the slope is higher the car will go faster and therefore farther. [Year 8]
  • Because going down a steep track lets the car gain speed and it can then travel further. [Year 10]
  • The steeper the track the more momentum the car gathers, making it go further. [Year 10]

Year 8 students were more likely than Year 6 or Year 10 students to use the concept of force to explain how the steepness of the slope affects the distance the toy car travels. Most students who identified force in their explanation identified a push force. Some Year 8 and Year 10 students specifically mentioned gravity in their explanation.

  • Because the higher you are the more push you will get and the faster at the start. [Year 8]
    (partial answer that only discusses what happens on the ramp, not the impact on the distance)
  • The steeper the track the more gravitational pull which makes it go faster. [Year 8]
    (partial answer that only discusses what happens on the ramp, not the impact on the distance)
  • Gravity will pull the car down with more force causing it to go further. [Year 10]

Everyday observations
About two thirds of Year 6 and about a third of Year 8 students used everyday observations to answer Question c) rather than applying science knowledge to explain their answer.

  • Because when it is tilted it will force the car to go faster. [Year 6]
  • Because if it was flat it won't move; you have to keep pushing it, but steepness it can roll down. [Year 6]
  • Because, going from higher, speeds the car up and it can't stop itself. [Year 8]

By Year 10, however, more students were attempting to apply science concepts to their explanations, with only about a tenth using everyday observations.

Many students have the idea that the natural state is for things to be stationary. They write about the object stopping because it runs out of force/push or has no motor to propel it forward. It is difficult for them to understand the scientific explanation that a stationary object still has forces acting on it but they are all balanced. This reasoning is also evident for some students when thinking about gravity, e.g., it only acts on things when they are dropped or run down a slope.

  • Because when they go down the track gravity will be pulling them. (possibly implying when they are on the flat it isn't)

Younger students were more likely to say the toy car runs out of speed as a reason why some cars go further than others. Although this is correct it is the reason for the speed decreasing (speed is the result) that is being asked for, e.g., the kinetic energy has all been changed to other forms of energy, or the opposing forces are the same (balanced).

Question d)
About a third of Year 10 students (less than a fifth of Year 6 and Year 8 students) explained that an object slowed down because of forces acting on it or the object losing its moving energy.

  • The cars lose their momentum because of friction slowing them down and stopping them in the end.
  • Because of friction on the surface it's rolling on. The friction in the wheel, wind might be going against it and gravity is pulling down on it.

Students also used their everyday knowledge to provide non-science based explanations for why the toy car would not keep going along the floor.

  • The steeper the track the faster it'll go down but if it is too steep it can't level out when it gets to the bottom and it'll crash. [Year 6]
  • It will go faster than the other track but it will hit the ground harder and might flip the car. [Year 8]
  • Because it might get more speed but when it hits the carpet going down it could flip over or it could just stop where the ramp ends. [Year 8].

Next steps
Understanding about speed and energy
To help students understand the difference between speed and energy get students to investigate an elastic wind up toy. The toy slows down and then stops moving (speed) when the elastic band (energy source) is unwound. Ask the students which one depends on the other, e.g., which one “runs out”. Clockwork toys work on the same principle.

Understanding about forces
For the younger students it is appropriate to investigate and describe ideas about pushes and pulls. Understanding the complex science of balanced forces is more appropriate for older students (Year 10 and some Year 8 students).

To compare how much pull is required to get light and heavy vehicles moving, attach a rubber band to the front of each vehicle and pull. Measure and compare the stretch of the rubber band. Students could also compare the stretch of the rubber band when starting and once the car gets moving, to show that less pull is needed to keep the car moving.

To support Level 5 students to understand the concept that forces are equal when an object is stationary, students could use force meters to:

  • Measure how different amounts of force affect an object, e.g., how much force is needed to slow an object down and in which direction does the force need to be acting;  and
  • Decide how much force is needed to stop a moving object and from which direction.
Resources for supporting students to develop explanations:
  • Text connectives - Thinking about how language works
  • Bull, A., Hipkins, R., Joyce, C. & MacIntyre, B. (2007). The Water Cycle: A Science Journey. Teacher's Notes. Wellington: NZCER Press. This resource, one of the Kick Starts, has a small section on pages 18 and 20 about teaching the language of explanation.
  • Hipkins, R., Roberts, J. & Bolstad, R. (2007). Key Competencies: The Journey Begins. Wellington: NZCER Press. The two posters Words that help us think, that are part of this resource, were designed to support students to extend their ability to explain their ideas.
  • Ministry of Education (1999). Making Better Sense of the Physical World. Refer to the section, Force and Motion.
  • Ministry of Education (2003). Building Science Concepts Book 42, Marbles. Wellington: Learning Media. This book is aimed at Levels 3 and 4. The teachers' notes are useful for helping teachers with their own science understandings.
  • Ministry of Education (2010). Figure It Out. Forces. Mathematics in science contexts. Wellington: Learning Media. Zoom, zoom (pages 19-21) addresses the same context, and provides ideas for relevant investigations and support for recording results.

Other useful resources:

  • Rolling marbles III PW3688 scaffolds students to plan a simple fair test in the context of rolling marbles. This could be adapted to investigate rolling cars. 
  • Rolling marbles II PW4111 includes a useful strategy for measuring the distance marbles travel that enables students to see the pattern that eventuates. This could be adapted to investigate rolling cars.
  • Predict, Observe, Explain (POE) provides a full description of this strategy.
  • Concept cartoons provides a full description of this strategy.

For other ARB resources about explanations see Throwing balloons 2, and Melting ice.

Other ARB resources about forces and friction: