9 The Force Lens on Motion

Formative assessment

What is a force?

How can we make an object move? We can push, pull, spin, bounce, throw, kick, and drop it, for example. Is it possible to make something move without these actions? No!  These actions are all forces. A force needs to act on an object that is not moving in order to make it move.

A force is a push or a pull. Forces always have a magnitude (size) and a direction. They are best represented using vectors, or arrows.


Newton’s First Law

Why do things not move? 

We may not be aware of it, but every object, every motion, even things that are not moving are all acted on by forces at all times. Consider a book on a table. It may seem to not have any forces, but when you hold it book in the palm of your hand, you feel the force of the weight of the book pushing down on your hand. In order to hold the book up, your hand pushes up on the book. If you put the book back on the table, the book pushes down while the table pushes up.

(note: N stands for Newtons, or the unit of force)

The table pushes up on the book with the same amount of force as the book pushing down on the table, which leads us to a more sophisticated definition of force:

A force is a push or pull upon an object resulting from the object’s interaction with another object.

A force does not just magically appear by itself on an object; there needs to be another object acting on it in some way.


Why do things move?

So what will make something move? In order to make our book move, we need to apply another force to it that is strong enough to make it move. Experiment with the simulation, “net force,” or the tug or war, below. What do you notice?



If you have equal forces applied to both ends of the rope, it does not move! The forces are balanced. In order to move, one side must have a bigger pull, or force, than the other. The forces must be unbalanced. When the forces on an object are unbalanced, things move. In other words, objects at rest remain at rest, while objects in motion stay in motion unless acted on by an unbalanced external force. This is Newton’s First Law:

Newton’s First Law

Objects at rest remain at rest, while objects in motion stay in motion unless acted on by an unbalanced external force


Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.

Car crashes vividly illustrate Newton’s first law. Watch the video below. Why do we need to wear our seat belts in cars?


In a car crash, the car stops suddenly but we keep going since we are an object in motion, staying in motion. We need the seat belt to stop us so we do not go flying through the windshield.


Why do things stop moving?

If objects in motion stay in motion, why do they stop eventually? Why will a block eventually stop sliding after you give it a hard push? As the block slides, the floor rubs against it, creating a force that acts against the direction of the block’s movement. This force is called friction.

Friction is:
A force that holds back the motion of a sliding object
Resistance to motion of one object moving relative to another


Try the friction part of the simulation. Notice what happens when you increase the friction slider bar.

Do objects have to be moving in order for there to be friction? No! You can have kinetic friction, which is generated when things are moving, or static friction, which is present even when objects are staying still. For example, a box on a ramp may not slide because static friction is preventing it from sliding down the ramp.
What affects how objects move? Inertia!

Try playing around with the motion part of the simulation. What do you notice?


You probably noticed that heavier objects require a bigger push, or force, to move. Heavier objects have more inertiaObjects with greater inertia require more force to change their motion: to start moving, stop moving, or change direction. For example, think of the Titanic trying to avoid crashing into an iceberg; it had too much inertia to change direction quickly enough and hit the iceberg, thus sinking.

We also see inertia and Newton’s first law in action when we perform a magic trick: pulling a tablecloth off a table that has dishes on it, and not breaking the dishes. Can you figure out why the dishes stay on the table?


Notice that the video offered a few tips to make the trick work. It suggested that you weigh down any light objects, thus increasing their inertia. It also suggests pulling the tablecloth quickly and straight down. Why do you need to pull quickly and down?

Newton’s Second Law

What factors affect force and how objects move?

We already see that mass affects how things move. Force has two other factors we need to be aware of: speed and direction. In the tablecloth trick, we had to pull the tablecloth quickly and down. We had to apply a fast force in a certain direction. If we moved the tablecloth slowly, the dishes would just come along with it. However, when we moved it quickly, the tablecloth accelerated, or sped up very quickly, and the dishes just couldn’t keep up. This leads us to Newton’s Second Law.


Newton’s Second Law

The greater the mass of an object, the more force it will take to accelerate the object.

The relationship between the force acting on an object and its resulting acceleration is given by the equation:

F = ma

For example, if we apply the same amount of force to a shopping cart loaded with groceries versus a empty cart, the loaded cart will move more slowly than the empty cart. If we want the two carts to move at the same speed, we need to use more force to move the loaded cart.


Newton’s Third Law

So far, we have been thinking of forces as single forces acting on an object. However, is that entirely accurate?When a book rests on a table, you have two forces: the force of the book on the table and the force of the table on the book.

Consider a woman throwing a ball. What forces are present?


As the soccer player’s hand pushes against the ball, the ball pushes back against her hand equally as hard. The two forces are equal and oppositely directed. Forces ALWAYS come in pairs, each acting on the two objects that are interacting and pointed in the opposite direction. This is Newton’s third law.

Newton’s Third Law

If Object A pushes on Object B, then Object B simultaneously pushes back on Object A with the same size force in the opposite direction.


For example, when a diver jumps off a diving board, she pushes down on the board with her feet. The board pushes upwards against her feet just as hard and she is launched into the air.



Rockets are launched into space with Newton’s Third Law. Watch the following video (you can scroll ahead to 1:20 for the actual launch). What is the action and its equal and opposite reaction that launches the rocket into space?


As the rocket fuel burns, it is pushed down out of the rocket as gas, which produces its opposite force that pushes upwards on the rocket, sending it into space.


Can you make an object move without touching it?

Yes you can!

Gravity, magnets, and electrostatic forces (static electricity) are all forces acting between objects at a distance. Even if you’re not touching the Earth, it still pulls you downward. Two magnets don’t need to touch in order to attract or repel. Charged particles and objects can likewise attract or repel each other at a distance.




Performance Expectations

K-PS2-1. Plan and conduct an investigation to compare the effects of different strengths or different directions of pushes and pulls on the motion of an object.  [Clarification Statement: Examples of pushes or pulls could include a string attached to an object being pulled, a person pushing an object, a person stopping a rolling ball, and two objects colliding and pushing on each other.] [Assessment Boundary: Assessment is limited to different relative strengths or different directions, but not both at the same time. Assessment does not include non-contact pushes or pulls such as those produced by magnets.]
K-PS2-2. Analyze data to determine if a design solution works as intended to change the speed or direction of an object with a push or a pull.* [Clarification Statement: Examples of problems requiring a solution could include having a marble or other object move a certain distance, follow a particular path, and knock down other objects. Examples of solutions could include tools such as a ramp to increase the speed of the object and a structure that would cause an object such as a marble or ball to turn.] [Assessment Boundary: Assessment does not include friction as a mechanism for change in speed.]
3-PS2-1. Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object. [Clarification Statement: Examples could include an unbalanced force on one side of a ball can make it start moving; and, balanced forces pushing on a box from both sides will not produce any motion at all.] [Assessment Boundary: Assessment is limited to one variable at a time: number, size, or direction of forces. Assessment does not include quantitative force size, only qualitative and relative. Assessment is limited to gravity being addressed as a force that pulls objects down.]
3-PS2-2. Make observations and/or measurements of an object’s motion to provide evidence that a pattern can be used to predict future motion. [Clarification Statement: Examples of motion with a predictable pattern could include a child swinging in a swing, a ball rolling back and forth in a bowl, and two children on a see-saw.] [Assessment Boundary: Assessment does not include technical terms such as period and frequency.]
MS-PS2-1. Apply Newton’s Third Law to design a solution to a problem involving the motion of two colliding objects.* [Clarification Statement: Examples of practical problems could include the impact of collisions between two cars, between a car and stationary objects, and between a meteor and a space vehicle.] [Assessment Boundary: Assessment is limited to vertical or horizontal interactions in one dimension.]


MS-PS2-2. Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object. [Clarification Statement: Emphasis is on balanced (Newton’s First Law) and unbalanced forces in a system, qualitative comparisons of forces, mass and changes in motion (Newton’s Second Law), frame of reference, and specification of units.] [Assessment Boundary: Assessment is limited to forces and changes in motion in one-dimension in an inertial reference frame and to change in one variable at a time. Assessment does not include the use of trigonometry.]




PS2.A: Forces and Motion

PS2.B: Types of Interactions

PS3.C: Relationship Between Energy and Forces

ETS1.A: Defining Engineering Problems


Third Grade

PS2.A: Forces and Motion

  • Each force acts on one particular object and has both strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion. (Boundary: Qualitative and conceptual, but not quantitative addition of forces are used at this level.) (3-PS2-1)
  • The patterns of an object’s motion in various situations can be observed and measured; when that past motion exhibits a regular pattern, future motion can be predicted from it. (Boundary: Technical terms, such as magnitude, velocity, momentum, and vector quantity, are not introduced at this level, but the concept that some quantities need both size and direction to be described is developed.) (3-PS2-2)


Middle School

PS2.A: Forces and Motion


PS2.B: Types of Interactions




Crosscutting Concepts

Cause and Effect



Cause and Effect

Cause and Effect

Systems and System Models

Stability and Change

Lesson Ideas:

Too many to list: anything to do with motion: ramps, cars,




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