# Dynamics

- use of F = ma when mass is constant
- one-and two-dimensional motion under constant force
- independent effect of perpendicular components with uniform acceleration
- projectile motion

Although most textbooks will list the relationship as F=ma, it is much easier for students to appreciate it as a=F/m. In this form the acceleration is clearly the consequence of a (resultant) force acting on a mass; much clearer.

The ideas in this topic will lead towards momentum, which most students will be familiar with from GCSE. The momentum and impulse equations are arguably some of the hardest they will deal with before A level so it is probably worth spending some time consolidating their understanding of simple forces on a body before moving on. (Newton's work was first expressed in terms of momentum, not acceleration.)

You will be building on their previous work into forces as vectors. In most cases a constant acceleration is due to the weight of an object (ie because of gravity) or a constant force such as friction. For most students drawing a free-body diagram with labelled forces is a good starting point. This narrative approach makes it less likely that the calculations will go awry unnoticed.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

## Links and Resources

### Investigating Newton's Second Law of Motion

This guidance is particularly good for teachers (or technicians) unfamiliar with the classic experiment. Students may have seen this at GCSE but it is a good way to ensure their familiarity with dataloggers, as well as giving data which is usually a good fit to a straight line. This may be a useful precursor to other datalogger experiments such as 'g by freefall'.

If more student activities are desired, TAP Episode 211 is worth a look.

### F=ma in a Helicopter

This is a straightforward demonstration which provides some surprising insights for students about the effect of force, not on a helicopter but on an object hanging below it. Guidance is included which suggests some useful questions and prompts for your class. Very little equipment is needed but it may be an interesting exercise to ask students how they could change the method to collect quantitative data.

### Forces in 1 Dimension

In most departments, students will encounter one-dimensional forces with an air track (or equivalent), or a trolley on a ramp. The previous link to the IoP material gives good information about ways to manage this kind of learning. As ever, simulations such as this can be used as a 'trailer' or for students to consolidate their learning with reliable examples, at school or home.

The strength of this alternative is particularly clear with the PhET projectile motion simulation, where students can use objects such as cars and cannons. Like all the models, it's worth checking out the teacher materials which include worksheets, activities and insights into student thinking.

### Mechanics 1

Many of the approaches we teach will also be covered by students studying maths - this is why most departments routinely record who is and isn't doing A level maths. If it's possible to discuss timing and terminology with colleagues you can be clear with students about the occasional need for different language.

This resource may be a useful starting point; it is a textbook intended for a mathematical approach to Newtonian mechanics. Some sections may be used as they are, such as the summary problems from page 113 of the PDF. If you use extracts with students, ensure they are clear about what they need for their exams and what (such as derivations) may be superfluous. In particular, remind students that calculus (differentiation and integration) will not be required in A level physics.

### Perpendicular Components of Motion

One of a sequence of lessons on the Physics Classroom site, this page is a useful narrative for students to use independently. It might be useful follow-up material, or extracts could be used to prompt discussion in the classroom; the diagrams are clearly labelled. Links are given to supporting material on vector addition, which may be particularly useful for those who are less confident with the mathematical demands. They could also use the explanation and calculator for vector addition at Hyper Physics, if preferred.

The mathematical examples and problems set are in metric units, something which is not always true of American sources. There are associated testing materials (using Shockwave and similar protocols) and you can register for a teacher account, but this is not needed for most of the activities.

### Episode 207: Projectile Motion

Students will resist the idea that vertical and horizontal forces act independently on a body. Part of this goes back to the misconception that a thrown object has a 'forwards' force acting while it is moving; you may find it useful to ask them to draw the forces acting on a paper plane in flight, compared with those acting on a powered model plane.

The activities in this resource include several demonstrations and practicals to reinforce the idea that a parabola is used to describe the motion of a thrown or fired object. Explanations of the classic monkey and hunter example below are also given.

### Monkey and Hunter

A classic experiment with an outcome that often surprises students. Setting it up in the lab can be problematic, as the apparatus often sees little use, but this video gives suggestions about useful tips before and during the demonstration. As with the guinea and feather, simulations do not have the same impact, but a version of the video is available that can be used with students if needed.

The slow-motion scene at the finish is particularly useful and if possible, this is something you should aim to recreate in your setting. A high frame-rate and some simple annotation will give students something that illustrates the independance of vertical and horizontal motion nicely.

### Creating a 'Rockets in Motion' Project

This project summary explains how colleagues in several school departments, including physics, worked together to use projectiles to teach several linked STEM concepts. This would be an excellent start to an after-school STEM club, with students of different ages considering different aspects of the rocket motion.

Subject(s) | Science |
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Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

Rating | |

URL | https://www.stem.org.uk/lxpar |