Normal Or Abnormal

Normal or Abnormal? – Demonstration Lab

Purpose: How does the “support” or normal 5E: Explain force work? Concepts addressed: - “discover” normal forces as forces Materials: that exist when an object is in contact A wood block with a surface Soft and stiff springs - normal force is perpendicular to the A sheet of foam board or piece of surface thin wood Matter model (optional) Demo – a large plastic mirror (without frame) and a pen laser

Directions: 1. Ask students to conduct a thought experiment: Imagine you are lifting an object as in the picture. What forces do you think act on the object? What do you think the strength of the force you exert is compared to the force of gravity on the object you are lifting?

Here’s another one: When you stand on the floor the force of gravity pulls down on you. When you are sitting the force of gravity pulls down on you. How much force is your own body exerting on the chair you are sitting in now? If your body is exerting that much force on the chair, why are you not falling to the floor?

2. Pass out springs, about 5 cm long, that can be compressed between students’ fingers. Have them examine the springs, then push down on them as if they were resting an object on the spring. Ask students what they see and feel when they push down on the spring.

Next, ask them what they think would happen if they set a wood block on the spring. What forces act on the spring? What would happen to the spring? What forces would the wood block feel? Would the block feel the spring pushing back on it too?

Next, ask, if you placed the block on firmer material, like the piece of foam board or thin wood, what would happen to the board? What force would the board feel? What forces do you think the block feels now?

Now move the block to a much firmer surface, such as a table. What force does the table feel? What happens to the table? Does the table push back the same way the spring or the thin board did? This is a bit hard to tell… Now ask, what they think would happen if a person stood on the table? How much force do they think the table is exerting on the person to hold the person up? At this point, you can conduct a demonstration. Place a plastic mirror on the table. Have a pen laser (which is attached to something other than the table) reflect off the mirror at grazing incidence. Have a student climb on top of the mirror, and have the class observe the reflected spot. The slight bowing of the table can be picked up by the light beam, which will deflect. It is important to have a table without cross braces on the bottom!

Questions: Do normal forces always point up? How about if I lean on a wall? How about normal forces on a tilted ramp?

3. Have students add their conclusions from this activity to the “What is a force?” chart started in the Exerting Forces activity.

Notes: Explanation: Students will feel that as the spring is compressed by force of their fingers, the spring pushes back on their fingers. If a wood block rests on the spring and compresses it, the spring pushes back on the wood block. The molecules that make up the object are connected to each other by bonds that are also springy – they deform a bit (maybe just a microscopic amount) when a force is applied, but they do push back – and this “pushing back” is the origin of the “support force.” The support force is often called the normal force – not normal / abnormal, but normal meaning perpendicular. The normal force is always perpendicular to the surface at the point where the object touches the surface. On a flat, horizontal surface, the normal force is equal to the weight of the object. It has to be! For example, in the picture above, if the weight of the apple were larger than the normal force, the apple would fall through the book. If the normal force were larger than the weight, the apple would fly off the book. Students encounter several different forces in the previous activity (Exerting Forces). In the course of this unit they will repeatedly encounter four of these many forces: the gravitational force (weight), the normal force, elastic forces and friction forces. For this reason, these four forces are addressed through specific activities. It is important for students to realize that these (and other) forces are produced by specific agents and each produces specific effects on their receivers. The forces also have specific strengths (a factor not commonly appreciated by students). Some of these strengths can be engineered (for example, the strength of the elastic force can be changed by changing the stiffness of the spring or the thickness of the rubber band), but other strengths cannot be easily changed (such as gravity – unless one can travel to another planet). 3.1. Practice: Force Challenge

Directions: For each situation below, choose one object (receiver) in the picture, and answer the following questions:  What is one force (other than gravity) that acts on the object?  What is the agent causing the force?  What object is receiving the force? (the “receiver”). Some of the examples have the receiver already indicated.  What effect is the force having on the object?  Mark a place where the force acts, and direction in which it acts (Note: There may be several forces, but we will analyze just one for now.)

A.  B. 

Force: Force:

Agent: Agent:

Receiver: Receiver:

Effect/s: Effect/s:

C.  D. 

Force: Force:

Agent: Agent:

Receiver: Receiver: tire

Effect/s: Effect/s:

E.  F.  Force: Force:

Agent: Agent:

Receiver: lower branch Receiver: Thesaurus book

Effect/s: Effect/s:

G.  H. 

Force: Force:

Agent: Agent:

Receiver: bicycle seat Receiver:

Effect/s: Effect/s:

I.  J. 

Force: Force:

Agent: Agent:

Receiver:skies Receiver:ball

Effect/s: Effect/s:

K.  L.  Force: Force:

Agent: Agent:

Receiver: Receiver: ball

Effect/s: Effect/s:

M.  N. 

Force: Force: Agent: Agent: Receiver: Receiver: girl Effect/s: Effect/s:

O. P. 

Force: Force: Agent: Agent: Receiver: chair Receiver: toolbox Effect/s: Effect/s:

Q.  R.  Force: Force: Agent: Agent: Receiver: Receiver: doorknob Effect/s: Effect/s:

S.  T. 

Force: Force:

Agent: Agent:

Receiver: Receiver:

Effect/s: Effect/s:

U.  V. 

Force: Force:

Agent: Agent:

Receiver:person Receiver:

Effect/s: Effect/s:

W.  X.  Force: Force: Agent: Agent: Receiver: Receiver:basket Effect/s: Effect/s:

Y.  Z. 

Force:

Agent: Force:

Receiver: Agent:

Effect/s: Receiver:

Effect/s: