Force
Forces and Motion

Resultant force sets acceleration - Teaching approaches

Classroom Activity for 14-16

A Teaching Approach is both a source of advice and an activity that respects both the physics narrative and the teaching and learning issues for a topic.

The following set of resources is not an exhaustive selection, rather it seeks to exemplify. In general there are already many activities available online; you'll want to select from these wisely, and to assemble and evolve your own repertoire that is matched to the needs of your class and the equipment/resources to hand. We hope that the collection here will enable you to think about your own selection process, considering both the physics narrative and the topic-specific teaching and learning issues.

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Guiding with forces

Force
Forces and Motion

Guiding with forces

Classroom Activity for 14-16

What the Activity is for

Students guide a ball along a path by exerting forces on it using puffs of air through a straw. The key idea is that the puffs can be re-described as forces acting on the ball, and that these forces (puffs) change the existing motion by adding to it. The discussion needs to bring this out.

This is a tabletop version: you could choose a larger version of the experiment, depending on the space and apparatus available. Any large ball and mallets or nylon-headed hammers could be substituted for the small balls and straws.

There are variants of this that are effectively computer simulations. We'd suggest preferring the physical, for a more immediate connection to the lived-in world. The point is that forces change motions, they don't set motions.

What to Prepare

  • a small ball (About 1 centimetre in diameter.)
  • a straw to blow down (Perhaps with a cardboard arrow added, to show the direction in which the force is exerted by the stream of air.)
  • a drawn path (To propel the ball along: A3 paper is suitable.)

Safety note: Students should not share straws.

What Happens During this Activity

Students take turns to guide the ball along the path, using only short puffs through the straw.

The summary discussion might very usefully bring out the rather obvious link between the puffs and the changes in motion, and that the puffs alter the existing motion, rather than setting a new motion. Somewhat lightheartedly, you might write:

new motion = old motion + change due to puff

You can vary the difficulty of the paths and the mass of the ball: both introduce new things to discuss, particularly if some paths contain sharp angles, rather than smooth curves, and some balls are steel, rather than glass or plastic.

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An inertia meter

Inertia
Forces and Motion

An inertia meter

Classroom Activity for 14-16

What the Activity is for

Increasing inertial mass reduces acceleration.

This apparatus gives students the chance to separate out the effects of mass from those of the pull of gravity, and that's rather hard to do on this planet. So we regard it as an essential experience. It represents a similar situation to that faced by astronauts in free fall for long periods (say in the international space station) who have to determine their mass in the absence of any perceptible weight.

What to Prepare

  • an inertia balance

What Happens During this Activity

Students add mass to the inertia balance, and note the increasing period. Then there is a chain of reasoning to be developed, in order to understand the connection between adding mass and the longer period of oscillation:

  • Same force and larger mass  →  lower average acceleration.
  • Lower average acceleration  →  lower average speed.
  • Lower average speed and same distance to cover  →  longer time to travel back and forth.

You might point out that calibrated versions of this device are used to measure the mass of astronauts and show a photograph of the chair.

Scaling down such a chair to work in the laboratory requires some serious engineering to reduce the frictional forces by an acceptable amount, tempting though it is to try.

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Formative questions

Force
Forces and Motion

Formative questions

Classroom Activity for 14-16

What the Activity is for

Select some questions that can be used to explore students' understandings of these rather difficult ideas. Because the shift from the idea that force causes motion to force causes a change in motion is hard, it's worth selecting questions that promote discussion of this very particular point.

There are many questions available on this topic, and in a variety of formats, from discussion about instances, through predict, observe, explain, to multiple choice in two and three step formats (first step – answer; second step – reason; third step – confidence). Many have been developed from research projects, and extensively trialled.

The questions should be selected to help learning, so for their formative value, rather than to make summative judgements.

What to Prepare

  • printed copies of the questions

What Happens During this Activity

Students might be set some preparatory work, but the essence is to get a discussion going, and so allow space to fully explore the ideas. You'll need to create an environment where it's acceptable to make mistakes and engage in exploratory talk in order to ensure that they have got a grip on the ideas, and that you have the chance to find out where they are still uncertain.

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Building your own world

Force
Forces and Motion

Building your own world

Classroom Activity for 14-16

What the Activity is for

A modelling package is used to create a world with which students interact. Adding rules to this world corresponds to making empirical discoveries made in the lived-in world. You can exploit this to make explicit aspects of how physics works, including unpacking the idea of a law of nature – here a constraint on the values of interacting quantities.

What to Prepare

  • a modelling program (A sample model is provided, or choose your own tool)
  • a means of projecting the screen to share it with the class

What Happens During this Activity

First build a simple imagined world, where there are three quantities: force, mass and acceleration. Add sliders to control these variables. You might also like to add an x–y scatter graph to plot a pair of these variables, and perhaps other representations as you choose. Set the model running, and use the sliders to vary the quantities. Notice that there is no connection between the variables: no pattern on the graph. This does not mimic the world we live in.

Now add the relationship: acceleration = forcemass

Start the model running again. Suddenly you can't have any combinations of values of variables that you like – the quantities are constrained. There is a rule in the world. Here, you've inserted that rule – in history Newton found the rule. It's Newton's second law.

Here is a simplified model that you can use:

InsertCMR{FmImaginedWordlRulesSupportCMRS}

Another approach is to start with the physical, using sensors to measure the force and acceleration on an object, whilst representing those quantities on a large screen. We'd suggest measuring them independently first, then both at once. Again you'll see that the force and acceleration are constrained (a live plot of force against acceleration is a good way to show this). You'll need some quite sophisticated equipment for this, and will need to practise.

The two approaches both produce live plots of force against acceleration: one from the model and one from the experiment. That these behave in the same way, exhibiting the same behaviour, is a really good opportunity to explore the idea of functional mimicry as an aim of modelling in physics.

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Feeling forces

Force
Forces and Motion

Feeling forces

Classroom Activity for 14-16

What the Activity is for

Experiencing constant and varying forces.

This series of explorations paves the way for the quantification of forces. Questions such as these begin to mean something to students as they can be related to their own experiences.

Value: How big is this force in newtons?

Vary: How does this force change as I change this?

What to Prepare

  • a feeling force board
  • a length of thin shock cord
  • a foam ball
  • a pencil and some plain paper
  • some safety glasses

Safety note: As the stretched shock cord can spring back rather suddenly when it snaps, safety glasses should be worn.

What Happens During this Activity

The forces board can profitably be firmly mounted at the side of the classroom, and used opportunistically, with individuals or groups invited to go up and experience exerting 1 newton or 10 newton.

Each student should have the chance to stretch the band and to squeeze the ball. They should then sketch out the shape of the force-extension graph. The axes should be left blank at this stage. For this reason we'd suggest not using graph (or even lined) paper.

That forces acting may change systematically depending on the systematic changes in the environment is the first notable outcome.

Small groups should then compare their sketches.

If there is disagreement, so much the better.

From these differences can evolve a useful discussion of how such arguments are settled, so long as you resist the temptation to give the answer. (The reason for using the shock cord and foam ball is that it heads off suggestions that the answer can be looked up – say in a textbook or online).

Teacher Tip: Do allow time for a consensus to emerge, and for comparison with the sketch graphs.

Settling differences of opinion by making measurements

If you are going to go down this route, you'd better be prepared for further investigation. Collect together apparatus for the appropriate number of groups:

  • some graph paper
  • hanger masses matched to the shock cord chosen
  • retort stand, boss and clamp
  • a G-clamp
  • a metre ruler

This is a time for careful, skilful experimentation. The results need to be reliable and checked with other groups. You might plan for that by arranging for the plots to be displayed on a wall, or to some standard scales, to allow easy comparison.

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Car safety features

Force
Forces and Motion

Car safety features

Classroom Activity for 14-16

What the Activity is for

These short experiments, using simple apparatus, can be used to explore understanding of the inertia of objects, providing physical models of situations which are connected to the everyday lives of many students. One way of encouraging interaction, engagement, and exploring understanding is to ask one student to explain the process using the apparatus and any diagrams they need.

What to Prepare

  • a model seat attached to trolley
  • a plastic toy person, seated
  • some sticky tape, to make a seatbelt
  • a marble to represent a head
  • some ducting, drilled to form mounting shoulders, attached to a trolley
  • some modelling clay, to form a headrest

What Happens During this Activity

These short experiments are perhaps best used as presentations, where one student explains to a group of peers the physics behind the process. The remainder of the group should form an attentive, critical audience. As there are four parts to the experiment, you might fruitfully arrange groups of four, and allocate different individuals to lead each part.

The four processes to compare are:

  • Car speeds up, with seated passenger, with and without seatbelt.
  • Car stops suddenly, with seated passenger, with and without seatbelt.
  • Car stops suddenly, with head on shoulders, with and without headrest.
  • Car is struck from behind, with head on shoulders, with and without headrest.

As an alternative, you could run the four as a set of linked predict-observe-explain activities, either as group activities, or as a demonstration. They could, of course, be carried out in any combination, and at any cunningly chosen intervals.

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Taming bus drivers

Force
Forces and Motion

Taming bus drivers

Classroom Activity for 14-16

What the Activity is for

This activity is all about explaining using sketches. The focus should be on changes in motion and inertia.

What to Prepare

  • a short passage from a recent news item
  • a pencil and some paper
  • a cylindrical plastic container containing a little water and a bowl to catch the drips
  • perhaps some prepared force, velocity and acceleration arrows

Here is a sample situation:

A bus company in China has launched a new safe driving campaign by suspending bowls of water over its drivers. To avoid getting wet, drivers must drive carefully.

What Happens During this Activity

Present students with a challenge:

Teacher: Can you explain what is happening in this situation, using the ideas of force, acceleration and mass?

Then interpret the challenge for the particular situation you've chosen. For example:

Teacher: What does the driver have to do so that they stay dry? How does it work?

You ought to encourage your students to draw a number of diagrams, perhaps at least two:

  • Where the bus driver takes the hint and drives carefully.
  • Where the bus driver doesn't take the hint.

For others you could provide more support – for example asking for a sketch of what happens to the water under different circumstances before asking for diagrams involving forces acting on isolated objects. You might even go so far as to really break it down for some:

  • What happens when the bus is travelling straight along the road at constant speed?
  • What happens when it suddenly stops?
  • What happens as it pulls away again?

For others you might want to push them by asking them to consider cornering as well as straight line motion.

It may be worth getting them to do two cartoon sequences as stories of a motion in parallel: one for the lived-in world and one for the physical world.

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Canal boats

Force
Forces and Motion

Canal boats

Classroom Activity for 14-16

What the Activity is for

This is a rather open-ended investigation, where students can use their evolving understanding of force and motion both to shape their expectations of the effects of their interventions in setting up the experiments and in interpreting the outcome.

The essential ingredient is an anti-linear airtrack, where there are many and varied forces at work on the moving object. How you motivate engagement with this is up to you. There is a suggestion below.

It is a rich environment, so you'll need to structure the challenges you set to match your students.

What to Prepare

  • a prepared length of semi-circular guttering, sealed at both ends
  • a blow-moulded boat, approximately 4 centimetre beam and 12 centimetre waterline length.
  • a pulley
  • 10 gram hanger masses
  • some thread
  • some 5 gram masses to load boat
  • a metre ruler
  • a stopwatch
  • a plastic litre jug to fill and empty the waterway

What Happens During this Activity

Students are invited to offer advice to a rapid-transit-freight investor to maximise the profits – assumed to be achieved by carrying as much as possible down a waterway as rapidly as possible.

Introduce the options. The owner can:

  • Buy a more expensive engine and so increase the driving force, reducing the trip time.
  • Pay higher fees so that the waterway owner keeps more water in the waterway, thus reducing the retarding force.
  • Carry more load each time.

This sets up a nicely open-ended investigation, where there are many interacting variables, and careful measurement will be needed to offer useful advice. You will probably need to match the laboratory model to the real situation, including making clear how the objects in the lab model the real objects.

With some encouragement, the output from the experimental phase is likely to be one or more graphs with appropriately controlled variables. These will be data for the next phase, which will try to relate these results to the original challenge. It's unlikely that any one group of students will achieve a comprehensive set of results, so this can also be a useful exercise in appreciating the limits of evidence.

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