UCTL/Teaching about Force
Contents
Types of Forces
Using the pictures to get you started, list all the types of forces you can think of . . .
File:Physicsimage01.pngFile:Physicsimage02.pngFile:Physicsimage03.png
This exercise may not be vital now, but it is helpful to unsuspecting physics teachers in the future. AND it will help clear thinking among students.
Free Body Diagrams
| A free body diagram shows ONLY the object in question and the forces acting ON the object. |
Represent forces as arrows with the sizes in proportion
Forces have three things: 1. Size (magnitude) 2. Direction 3. Line of Action (Needed at Form 6 level) Examples:
Plane travelling at constant speed
Picture | Free Body Diagram |
| File:Physicsimage04.png |
Draw ONLY forces touching the object. Keep velocity, direction arrows etc separate. Don’t mix pictures and FBD when there are other touching objects.
More Free Body Diagrams:
Piano being pushed but not moving
Picture | Free Body Diagram |
| File:Physicsimage05.png |
Bullet after being fired
Picture | Free Body Diagram |
| File:Physicsimage06.png |
Shark
Picture | Free Body Diagram |
| File:Physicsimage07.gif |
Summary:
Tips to teaching these ideas about force.
- Work from a list of forces. These have key ideas associated with each one.
- Introduce these ideas informally first as the ideas are needed, then come back to formal treatment later.
- Give plenty of practice at drawing Free Body Diagrams.
- Carefully distinguish the picture from the FBD.
- Later show in simple cases the adding of the forces to give a resultant force .
Adding forces as arrows (The technical name is vector)
Introduce:
- 1-D first (‘tug of war’ example)
- Simple 2-D at right angles
- Any angle (Form Six)
Note: Later three separate representations are useful.
Eg. You are piloting a rocket. It is leaving the atmosphere of a planet. The gravitational force is 40,000 N and the rockets are firing with a thrust of 60,000 N. What is the net force[2] on the rocket?
Picture | Free Body Diagram | Resultant Force |
| File:Physicsimage09.gif |
The mysterious hit force on a cricket ball in flight.
The problem:
A cricket ball is hit for six. Draw a free body diagram of the ball at the top of it’s arc.
|
Common Misconceptions I
TRUE OR FALSE?
Common misconception: Motion implies a (net) force. Motion implies an active force
Consider: A ball is rolling across the ground.
Students (and some of us also!!) think there must be a push force keeping it moving.
The truth is:
Acceleration implies a net force.
It is momentum that keeps the ball going.
Momentum is NOT a force. This directly relates to Newton’s First Law:
“An object will stay in the same state of motion unless acted upon by an unbalanced force”[3]
For our purposes a statement like this will do:
If it is still, it will stay still. If it is moving it will stay moving[4].
Common Misconceptions II
Actions and reactions.
File:Physicsimage05.png A man pushes a piano across the room. The piano is moving.
P is the push force of the man on the piano R is the force of the piano on the man
Which is true?
- P > R
- P = R
- P < R
- R = zero since the piano is moving
- Cannot tell without more information
Consider the free body diagrams:
The Man | The Piano |
Jargon
This is an example of one formulation of Newton;s Third Law:
“For every force on an object there is an equal and opposite force – which acts on a different object”
Commonly abbreviated: forces occur in pairs Or referred to as “Action and Reaction” However, student misconceptions get picked up when other situations arise:
Consider Joe at the supermarket . . . File:Physicsimage10.pngWith a full load of groceries. Is the force on Joe from the trolley bigger or smaller than the force Joe exerts on the trolley?
Now Joe has put his shopping in the car and is returning the trolley.File:Physicsimage11.png
How do the forces compare now?
Students will tend to assume the big thing pushing the little thing – or the heavy thing pushing the light thing – is exerting the biggest force.
By the way, merely telling the students this does little good !##$!!
Some teaching ideas:
Tons of pushing and pulling with force meters
- ↑ Also known as Net Force or Total Force or Unbalanced Force. This quantity is vital in later work.
- ↑ In year 12 students will be asked: Find the acceleration of the rocket. This requires the use of the resultant force.
- ↑ This is only close to the original statement!! But close enough.
- ↑ Caution: this footnote is not for the fainthearted, but it is worth the effort to read! Commonsense (erroneous) beliefs tend to be metaphorical and vague with situation-dependent meanings. This is reflected in the use of language. Thus terms like "force", "energy", and "power" are often used interchangeably, as are the terms "velocity" and "acceleration". Even so, most commonsense thinkers distinguish two kinds of force, which we will refer to as impetus and active force. Witness the 1996 SC science fiasco. Impetus. The term "impetus" dates back to pre-Galilean times before the concept was discredited scientifically. Of course, students never use the word "impetus": they might use any number of terms, but "force" is perhaps the most common. Impetus is conceived to be an inanimate "motive power" or "intrinsic force" that keeps things moving. This, of course, contradicts Newton's First Law. Evidence that a student believes in some kind of impetus is therefore evidence that the First Law is not understood. Impetus (according to this erroneous view) can be gained or lost in a variety of ways that vary from student to student. Note the underlying "container metaphor" in the impetus concept: Every object is (like) a container that can store a supply of impetus, like a car stores gas, a kind of "go-power" to keep it moving. Active Force The (erroneous) commonsense concept of active force is closer than impetus to the Newtonian force concept. It is attributed only to certain "active agents" (usually living things), and it acts only by direct contact. Active agents are casual agents - they have the power to cause motion - to create impetus and transfer it to other objects, as when a boy throws a ball. Active force is the commonsense concept that corresponds most closely to Newton's Second Law. The commonsense notion closest to a "casual law" is expressed by the syllogism: 1. Every effect has a cause. 2. Motion is an effect. 3. Therefore, motion has a cause. This leads to the (erroneous) commonsense concept motion implies active force. The vague commonsense analog of the Second Law is that active forces produce motion. When velocity and acceleration are not discriminated as descriptors of motion, it is to be expected that the concept "velocity is proportional to force" (False) is not distinguished from "acceleration if proportional to force" (True). In the erroneous concepts in a student active agents have their limits: a limited capacity to produce motion and a tendency to wear. (Comments here heavily adapted from Hestenes, Wells and Swackhamer, The Physics Teacher March 1992)
