## Examples Of Force Pairs When Doing Homework

## What is a force?

Everything on Earth is powered by **forces, pushes and pulls which act on our bodies and the things around us**. Forces make things move and stop moving.

Some of the forces we are subject to are **gravity** (which keeps us on the Earth's surface), the **centripetal force** (the force that makes things move in circles) and **friction** (the force which makes things stick or slide).

**Simple machines** work by turning small forces into larger ones, allowing us to perform tasks with more strength or speed. Examples of simple machines are **levers, gears, pulleys, wheels and screws**.

### What do children learn about forces in primary school?

**Children in primary-school learn that a force is a push or a pull. **They will look at different types of forces including gravity, air resistance, water resistance, surface resistance and magnetic forces.

**Gravity** is the pulling force acting between the Earth and a falling object, for example when you drop something. Gravity pulls objects to the ground. Gravity also holds our universe together, moving the planets in our solar system around the Sun.

**Friction** is a 'sticking' force – the resistance that a surface or object encounters when moving over another surface or object. Friction both stops and makes things move: it causes things to stick and rub against each other, and also causes slipping and sliding. Air resistance, water resistance and surface resistance are kinds of friction.

**Air resistance** is the force on an object moving through air, such as a plane moving through the sky. Air resistance affects how fast or slowly objects move through the air; some objects are more streamlined than others, which means the air pulls on them less and they travel faster. A parachute uses air resistance to slow down descent to the Earth.

**Water resistance** is the force on objects floating on or moving in water.

**Surface resistance** is the force on objects moving across a surface, auch as an ice-skater skating on ice. Grips on your shoes or car tires use friction to stop you slipping. Shiny surfaces have less fiction so they are slippier; rough surfaces have more friction so slow things down.

**Magnetic force** is an invisible force created by electrons. Magnetic force controls magnetism and electricity.

**Mechanisms or simple machines** are tools or equipment such as pulleys, gears and levers. Simple machines can be used to turn a small force into a bigger force; this means we can use these machines to accomplish things more easily. Examples of simple machines are levers (which give us extra pushing or pulling force and help us lift great weights), gears (different-sized cogs which work together and give a machine extra force or speed) and pulleys (wheels and ropes used together to lift heavy objects).

### When are children taught about forces in primary school?

In **Year 3** children will:

- Explore how objects move on different surfaces
- Investigate magnets

In **Year 5** children will:

- Learn what gravity is
- Explore air resistance and water resistance
- Investigate simple machines such as levers, pulleys, gears, wheels and screws and how they allow small forces to have a bigger impact
- Further develop their knowledge of how forces can slow or stop objects moving

### How are children taught about forces in the classroom?

Children will learn about forces through a range of experiments. They will plan and carry out tests in small groups, pairs or whole-class situations; examples include:

- How different objects move over a surface (investigating surface fiction)
- How a toy car travels over different surfaces (investigating surface fiction)
- The best shape and size for a parachute (investigating air resistance)
- Which objects fall to the ground fastest (investigating air resistance)
- Which shape of aeroplane travels most quickly (investigating air resistance)

Children will also observe and explore forces using equipment and examples of simple machines around them. For example:

- Exploring a range of gears, pulleys and levels, looking at how they work and the effect they have on moving objects.
- Exploring examples of fiction by looking at different surfaces in the local area or looking at the grip on different shoes.

### Books about forces for children

### Activities for at-home learning about forces:

- Investigate what objects float and sink in the kitchen sink or the bath and find out about Archimedes' principle.
- Investigate different shapes of paper aeroplanes and which are most effective; discuss why.
- Make a mini-parachute for a teddy. What shape or size is most effective?
- Explore gravity by dropping a selection of objects on the floor – do they always fall? Discuss why.
- Test how a toy car moves down a ramp made of different surfaces or covered in different materials.
- Find out about Leonardo Da Vinci's work with simple machines.

#### Objective:

Students will learn how to calculate torque. Students also learn how torque is used in simple machines, everyday life, and in sports.

#### Introduction Notes:

SCIENCE OF NFL FOOTBALL

Newton's 3rd Law, Force Pairs (Grades 5-8)

STEM Lesson Plan

Lesson plans produced by Lessonopoly (lessonopoly.org)

Video produced by NBC Learn in partnership with the NFL and the National Science Foundation

SPECIFIC OBJECTIVES:

Students will be able to: State and explain Newton’s 3rd Law. Forces always occur in pairs that are equal in size and opposite in direction. Give real life examples of Newton’s 3rd Law. Identify the force pairs in a variety of examples. Explain, in their own words using examples, that although force pairs are equal in magnitude, that the consequences of these forces are usually not equal. Use Newton’s 3rd Law to analyze the physics of an open field tackle in football.

REQUIRED MATERIALS

2 spring scales (or a class set). Optional: 2 bathroom scales, 2 rolling chairs or skateboards, 2 dynamics carts, a balloon

LESSON PLAN PROCEDURE

Define a “force” as a push or pull. Come to a class consensus on the definition of a force, “a push or pull,” and document the definition on Question 0 of the activity worksheet.

__Question 1: Establishing that forces occur in pairs. __

Explain and demonstrate that one method to measure a force is to use a spring scale. In this case, a scale measures how much it pulls on an object.

Attach 2 spring scales together and ask for one or two volunteers (you can be one of them if it will help demonstrate the point). Explain that you will know if a person is applying a force to the other if the spring scale registers the force.

Now do a series of scenarios to make it clear to the class that one person (spring scale) cannot apply a force to a second person (spring scale) unless the second also applies a force to the first. You can do this by asking one of them to pull (you can specify a particular force within the range of their spring scale), but asking the second person to NOT pull. You can repeat this and let them try again, or have the participants reverse roles. In all cases, they will find that one can only register a force if the other one does also.

You can reinforce this idea with two bathroom scales that can be pushed together. Scale 1 cannot push scale 2 if scale 2 does not provide resistance (push back), and vice versa.

Ask the students to list other examples where forces occur in pairs.

NOW ask for the students to try and think of an example where forces do NOT occur in pairs. That is, can any of the students think of an example where an object applies a force to a second, and the second does not apply a force back? Help them to see that in each case there are always force pairs (later in this discussion you will cover the concept that although forces are equal, the results/consequences of these forces are not equal, but you don’t need to cover that in detail at this point).

On Question 1 of their activity sheets, have them record that “forces always occur in pairs.”

__Question 2: These force pairs occur simultaneously__

__ __

Perhaps one of the biggest misconceptions regarding the 3rd Law is that, it implies an action and then a reaction. It should be made clear to the students that the two forces occur simultaneously. Generally the students will easily come to a consensus if posed the question described below:

Ask the students to discuss with a neighbor or in small groups the following question: Consider a force pair of your choice and determine whether the two forces occur simultaneously or if one happens and then the other happens. Have the students share their answers either on small whiteboards or orally. In each case, if necessary, assist the students to understand that the force pairs always occur simultaneously.

Have the students record this conclusion in Question 2: “These force pairs occur simultaneously or at the same time.”

__Question 3: These pairs are equal in size__

__ __

Either as a class demo or in small groups, ask students to compare the magnitude of the force pairs by reading the approximate values on the spring scales (if necessary, define “magnitude” as “size”). The students should conclude that the relative magnitude of the forces is the same. Note: Spring scales tend to be a bit inaccurate, so encourage the students to look at general trends rather than exact scale readings. Also, if available, perhaps the best demonstration of this can be done with 2 electronic force sensors hooked together with graphical readings and one of the readings inverted.

Have the students record the class consensus in Question 3: “Forces occur in pairs that have equal magnitude.”

__Question 4: These pairs are opposite in direction__

__ __

Through discussion, the students should be able to identify that the direction of the force pairs is equal and opposite. You can simply ask them to check with a neighbor to determine the relationship between the directions of each force in the pair. Have the students record the answer under Question 4: “Forces occur in pairs that have opposite directions.”

__Question 5: Putting it all together, formalizing the 3rd Law__

__ __

Demonstration: Blow up a balloon and let it sputter around. Ask the students to explain this using the concepts addressed in Questions 1 through 4. Explain that rockets propel themselves by applying a force to the exhaust gas which in turn applies a force to the rocket.

Now that you have covered the concepts of the 3rd Law, have the students record a formal description for Question 5: “Forces occur simultaneously in pairs that are equal in magnitude and opposite in direction.”

Note: This description of the 3rd Law tends to avoid creating the many misconceptions that are generated by the layperson definition, “For every action there is an equal and opposite reaction.”

__Question 6: Alternative description of the 3rd Law__

__ __

Have the students complete Question 6 with a very useful and functional description of the 3rd Law: “If object A applies a force to object B, then object B simultaneously applies a force to object A that is equal in magnitude and opposite in direction.”

Explain to the students that they can replace “object A” and “object B” with any force pair objects. Give examples, and have the students give examples.

Note: This definition is very useful in helping students apply the 3rd Law to situations. If they can identify the force pair, this definition will help them remember that the forces are simultaneous and equal.

__Question 7: Applying the 3rd Law__

__ __

Referring back to the NBC Learn video, have students answer Question 7.

__Question 8 and 9__: Although the magnitudes of forces are the same, the consequences and results of these forces are often NOT the same.

Use Question 8 to discuss with students that although the force pairs are equal in magnitude, the consequences of these forces are usually NOT the same.

Through discussion and demonstration, you can show that equal forces produce different results. Any of the demonstrations below can get this point across:

Two people of significantly different sizes sitting on rolling chairs or skateboards push off each other, and the smaller one will have a greater acceleration. Rather than pushing, they can also pull on a rope.

If available, you can use 2 small demonstration carts with different masses. You can compress something elastic between them and release. You can also connect them to a chain of rubber bands and notice that the cart with less mass will experience greater acceleration.

To clarify the point that a given force can produce very different results, as in the windshield and bug question, you can drop a brick on a rock and drop the same book on a paper cup lying on its side.

Ask the students to give other examples in which the force pairs create very different results. You can help them by giving a few of your own:

A raw egg is dropped and hits the ground.

A car runs into a thick concrete barrier.

A bullet fires from a gun (the recoil velocity of the gun is much less).

After the above demonstrations and discussions, have the students answer Question 9.

CLOSURE (REFLECT ANTICIPATORY SET)

Exit question: Ask the students to answer the following question: A brother and a sister are in a skirmish. When their mother comes to find out what is going on, the brother says that the sister pushed him first. The sister claims that the brother push.

PLAN FOR INDEPENDENT PRACTICE

For homework the students can list several examples of force pairs in their daily lives. They can identify the forces and describe whether or not the forces produce equal results.

ASSESSMENT BASED ON OBJECTIVES

Quiz: Newton’s 3rd Law.

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