Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• explore the connection between resultant force and change in motion
• draw out children's ideas about forces, persuading them to be explicit
• connect scientific ideas of motion to the ideas that children already have

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• move from a description of forces acting on an object to a prediction of the motion of that object

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Quantifying forces by looking at motion

Thinking about the teaching

In the SPT: Forces topic we suggested that you look at the physical interactions of the objects with the surroundings as a way of identifying the forces acting. Once identified, you can begin to think about the relative values of the forces. Ascertain the type of motion and from that deduce the relative value of the forces acting:

If the object is speeding up then…

… the driving force(s) is/are greater than the retarding force(s).

If the object is slowing down then…

… the driving force(s) is /are less than the retarding force(s).

If the object has a constant speed then…

… the driving force(s) is/are equal to the retarding force(s).

Or, in terms of resultant forces:

Object speeding up  →  resultant force is not zero (driving force is greater than retarding force)

Object slowing down  →  resultant force is not zero (driving force is less than retarding force)

Object at constant speed  →  resultant force is zero (driving force is equal to retarding force)

Teacher Tip: Don't use these relationships to identify the forces that are acting. Instead use them to begin to quantify those forces.

A resultant force is required to change speed

Wrong Track: I hit the snooker ball with the cue and it speeds up right across the table.

Right Lines: The snooker ball speeds up from rest for as long as the cue is exerting a force on it. Once the ball loses contact with the cue it rolls across the table slowing down all the way (because of friction).

Motion changes for as long as the resultant force acts

Thinking about the teaching

The key point to get over here is that the motion changes for just as long as a resultant force is acting. Very often objects gain their motion through a force acting over a short time (for example in kicking a ball; flicking a paper pellet; firing a catapult). In all such cases, it is worth spending some time in emphasising just when it is that the force is acting and the motion is changing.

Running out of force

Wrong Track: When I stop pushing the trolley it gradually runs out of force and slows down.

Right Lines: When the driving force on the trolley stops, the only force acting is friction and that force slows down the trolley.

Carrying forces

Thinking about the learning

The very common misconception here is that if you push a trolley to set it moving, the trolley carries that driving force with it and as the force runs out the trolley slows down. The key point here is that the driving force is being applied to the trolley only for as long as you are pushing it. Once you stop pushing the trolley, the only force acting along the direction of motion is friction and that friction force acts to slow down the trolley.

Thinking about the teaching

In your teaching, it is important to be very clear about the forces acting on objects as they speed up and slow down and to tackle head-on those situations where pupils suggest forces which simply don't exist. Here is another such example.

Supposing you throw a ball vertically up into the air. What motion will it have? From the moment it leaves your hand, the ball slows down and actually comes momentarily to a halt as it reaches its top point and then falls back to your hand.

When asked to identify the forces acting on the ball as it is moving upwards, many pupils suggest that there is gravity and air resistance acting downwards and a driving force acting upwards.

The crucial point here is that the driving force acts on the ball only for as long as your hand is in contact with it, throwing it upwards. Once the ball leaves your hand the only forces acting are the downward ones.

So, the ball slows down as it moves upwards due to the resultant force acting on it. Many pupils find it difficult to accept that the ball is moving upwards and the only forces acting on it are in the opposite direction, downwards.

Driving force exists only for as long as you are pushing

Here is another classic: a series of snapshots in the trajectory of a thrown ball. See if you can sketch in the forces acting on the ball for the four steps.

The forces acting on a thrown ball

Here are the forces acting on the ball for the four steps.

There's a good chance you could improve your teaching if you were to:

Try these

• working mostly with contact forces
• emphasising the connection between resultant force and changing motion
• focusing on the forces acting on a single object before trying to predict its motion
• using only the resultant force to predict the motion

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• suggesting that force is the kind of thing that might run out
• discussing multiple objects at once
• speaking as if component forces are trying to make the object do something

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.