This focus idea is explored through: Show
Contrasting student and scientific viewsStudent everyday experiencesIt is very difficult to tell with the naked eye if an object is accelerating (for example, a ball flying through the air) and so students do not tend to think of motion in terms of whether it is accelerated or not. Accelerated motion is also a difficult concept for students because it occurs when an object changes either speed or direction or both. Students are familiar with objects accelerating from rest or braking, for example a car speeding up or to a lesser extent slowing down at the traffic lights. However, it is much more difficult to determine if an object already moving is changing its speed unless the change is dramatic. Students seldom identify a car turning a corner at constant speed as accelerating because their common understanding requires the object to be changing speed for it to be accelerating. Even senior students regularly confuse ‘acceleration’ and ‘speed’; for example they may think that if speed is increasing then acceleration is also increasing. Research: Champagne, Klopfer & Anderson (1980), Trowbridge & McDermott (1981), Loughran, Berry & Mulhall (2006) Students often have a set of ‘intuitive rules’ that seem to explain everyday examples of motion (see the teaching ideas on forces and motion at the lower levels). These rules appear to work when students ignore friction and air resistance; friction and air resistance are generally not seen by students as involving forces. These ideas are also discussed in the focus idea Friction is a Force. Research: Mitchell (2007) A common view that persists at this level is that a moving object must have a force acting on them in the direction of their motion. Some students also strongly believe that this force is being used up if the object is slowing down. This may be partly a terminology problem: what students label as a force in these situations is similar to what scientists call ‘momentum’. Research: Champagne, Klopfer & Anderson (1980), Gunstone & Watts (1985), Gunstone, Mulhall & McKittrick (2007), Osborne & Freyberg (1985) Students often struggle to grasp the concept of net force, and often think it is an extra force in addition to the actual forces on an object. Research: Gunstone, Mulhall & McKittrick (2007) Scientific viewThe net force is the combined effect (the sum) of the real forces acting on the object. Net force is a valuable construct that has no separate existence of its own, unlike the real forces acting on the object, i.e. it is not an additional force. Research: Gunstone, Mulhall & McKittrick (2007) While the net force on an object is zero, its speed and direction of motion remain unchanged (and stationary objects remain stationary). See Newton’s first law of motion. When there is a net force on an object, it causes the object to accelerate in the direction of the net force; this is not the same as the direction of the motion unless the object is going in a straight line. The magnitude of the net force on the object is the product of its mass and its acceleration (Newton’s second law of motion). Critical teaching ideas
Research: Loughran, Berry & Mulhall (2006), Gunstone, Mulhall & McKittrick (2007) Explore the relationships between ideas about force and acceleration in the Concept Development Maps – Laws of Motion The ideas about forces and motion below contribute to student understanding of this topic. These are each covered in greater detail in the sequence of teaching ideas introduced at the lower levels:
It is important for students to have a sound qualitative understanding of the ideas of Newton’s second law before mathematics is introduced. Students need exposure to situations that require verbal explanations about the forces involved before they are exposed to quantitative work involving formulae. A simple mathematical relationship exists between the mass of an object (m), the net force on the object (f) and its acceleration (a). The acceleration of an object is directly proportional to the net force and indirectly proportional to the object’s mass (a = f/m). Teaching activitiesOpening up discussion via a shared experienceStudents should use the expression ‘force of A on B’ to identify the agent and receiver of a force in various situations, and use an arrow to represent the force’s direction. They can then be asked to identify all the actual forces and the net force on objects in a wide range of motion contexts. Some examples include: a ball rolling along a table; a ball that was rolling on a table but is now stopped; a ball thrown up in the air that is moving upwards (or downwards); a skateboard rider moving down a gently sloping path at a constant speed. In particular, the role of friction needs to be explored because it has such a huge influence on the observed motion of everyday objects. Ideas about friction are explored in the focus idea Friction is a force. Open up discussion via a shared experiencePOE (Predict-Observe-Explain) can be used to develop powerful understandings about balanced and unbalanced forces. Using a bicycle wheel mounted as a pulley, with a bucket of sand hanging from either side, students can respond to each question below. They should observe what happens and then explain their observation:
Research: Loughran, Berry & Mulhall (2006) Challenge existing ideasP.O.E Place a puck on air table and ask students to predict how they would keep it moving at a steady speed by pushing on it with a ruler. Most students are surprised that it is impossible to achieve a steady speed with this action and are intrigued as any amount of push with the ruler makes the puck keep 'running away from them'. Promote reflection on and clarification of existing ideasRoad safety is a topic where students can explore the issues associated with mass and speed of vehicles involved in car accidents and how these may influence the injuries inflicted on passengers. Encourage students to think of the advantages of introducing light weight vehicles and the disadvantages of being involved in collisions with much heavier trucks. The Digilearn objects below provide some simulations students can experiment with. Promote reflection on how student ideas have changedStudents can look for examples in films and cartoons where Newton’s laws are not obeyed. The popular ‘Coyote and the Road Runner’ cartoons use frequent scenes where students’ alternative conceptions are displayed. Identify situations and discuss these in class. Collect evidence/data for analysisData loggers can be used to record and graph students’ movement and the motion of other objects. Utilise software programs that can analyse video (or digitally recorded) motion. Further resourcesScience related interactive learning objects can be found on the FUSE Teacher Resources page. To access the interactive learning object below, teachers must login to FUSE and search by Learning Resource ID:
How does weight affect acceleration?Acceleration of Falling Objects
Heavier things have a greater gravitational force AND heavier things have a lower acceleration. It turns out that these two effects exactly cancel to make falling objects have the same acceleration regardless of mass.
Does greater mass have greater acceleration?The greater the mass of an object, the less it will accelerate when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration for the same amount of force.
Why would a heavier object require a greater force to accelerate?The question is about the impact or momentum change. The impact of an object depends on mass and velocity. Since in this case, the heavier object has more mass, their impact or change in momentum requires more unbalanced force.
Which object will have the greater acceleration?the object with greater mass will experience a greater acceleration and the object with less mass will experience a smaller acceleration.
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