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Introduction
1. Word Knowledge
2. Mathematics Knowledge
3. Paragraph Comprehension
4. Arithmetic Reasoning
5. General Science
6. Mechanical Comprehension
6.1 Forces and Motion
6.2 Work, Energy and Power
6.3 Simple Machines
6.4 Compound Machines
7. Electronics Information
8. Shop Information
9. Auto Information
10. Assembling Objects
Wrapping up
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6.1 Forces and Motion
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6. Mechanical Comprehension
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Forces and Motion

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To begin our conversation about forces and machines, we must first discuss what matter is.

Matter

Definitions
Matter
A physical substance that has mass and takes up space (volume).
Mass
the measure of the amount of matter in an object
Weight
the specific force of gravity acting on an object

Weight is a force acting on the mass of an object. Weight changes based on gravity, but mass never does. So if you went to the moon, your mass would stay the same, but you would weigh less because the moon has a lower gravity than Earth does.

Let’s talk more about other forces.

Forces

Definitions
Force
a push or a pull on an object
Gravity
an attractive force between masses,
Friction
a force that opposes the motion of objects that touch.
Compression
a pushing force that squeezes an object
Tension
pulling force
Net Force
the combined result of all forces acting on an object, found by adding the forces together.
Sidenote
Types of Friction

There are four types of friction:

Static (non-moving): static friction keeps a book from sliding off a slightly tilted table

Sliding: sliding friction slows a box pushed across the floor,

Rolling: Bicycle tires experience rolling friction when they move

Fluid (including air resistance): fluid friction is what you feel as air resistance when you stick your hand out a car window

Forces can be balanced or unbalanced. Unbalanced forces result in movement, whereas balanced forces are in equilibrium. Force is governed by Newton’s Laws.

Equilibrium

Definitions
Equilibrium
Equal forces are opposing, resulting in a net force of zero. An example is a balanced tug-of-war.
Center of Gravity
The point on an object where its weight is evenly balanced in every direction.

An object stays upright as long as its center of gravity is supported. Tip it too far, and the center of gravity moves outside its base, and the object falls over. This is why a loaded moving truck is more likely to tip on a turn than an empty one: shifting the weight higher moves the center of gravity further from the base.

Newton’s Laws

Newton’s 1st Law: Inertia

An object at rest stays at rest, and an object in motion keeps moving the same way, unless a force acts on it. This resistance to change is called inertia.

Newton’s 2nd Law: F=ma

Acceleration is equal to the net force divided by mass. An unbalanced force results in a change of motion. In other words, the greater the force, the greater the acceleration of the object. The greater the mass of the object, the harder it is to accelerate.

Let’s look at an example.

A 10 kilogram box is pushed with 20 newtons of force. What is the acceleration?

F=ma

20=10∗a

2=a

The box accelerates at 2m/s2.

Newton’s 3rd Law: Action= Reaction

Whenever one object exerts a force on a second, the second object exerts an equal and opposite force on the first. These forces act on different objects and do not cancel out.

For example, if you jump up, you are pushing down on the ground (action) with the same force the ground is pushing up at you (reaction), causing you to launch into the air.

Motion

To describe motion, we need a frame of reference. It tells us which direction something is moving and the amount.

For instance, when you are riding in a car, you appear to be in motion to someone standing on the sidewalk. Their frame of reference is the street. For you, however, the person is passing by the window while you sit still. Your frame of reference is the car. Same situation, different viewpoints or frames of reference.

Distance vs. displacement

Distance is the total length of a path.

Displacement is the straight-line distance and direction from an object’s starting point to its ending point.

For example, if you walk 100 meters down a hallway and back, your distance is 200 meters, but your displacement is zero because you ended up where you started

Speed and velocity

Speed is the distance an object travels per unit of time. On a distance-time graph, the slope represents speed.

Velocity is speed in a given direction.

A car traveling 60 miles per hour north has both a speed and a direction, so that describes velocity, not just speed.

Momentum

The product of mass and velocity. If a grocery cart and a marble are moving at the same speed, the cart has more momentum because it has more mass.

Matter, Mass, and Weight

  • Matter: substance with mass and volume
  • Mass: amount of matter (constant); Weight: gravity’s force on mass (varies by location, e.g., moon)

Forces

  • Force: push or pull; types: gravity (attraction), friction (opposes motion), compression (squeeze), tension (pull)
  • Friction types: static, sliding, rolling, fluid/air resistance
  • Net force: sum of all forces; balanced forces = no motion change, unbalanced = movement

Equilibrium and Center of Gravity

  • Equilibrium: opposing forces equal, net force = zero (e.g., tug-of-war)
  • Center of gravity: point of balanced weight distribution
  • Object tips when center of gravity shifts outside its base (e.g., loaded trucks tip easier)

Newton’s Three Laws

  • 1st Law (Inertia): objects resist change in motion unless acted on by force
  • 2nd Law: F=ma — more force increases acceleration, more mass resists it
    • Example: 20=10×a, so a=2m/s2
  • 3rd Law: every action has an equal, opposite reaction on a different object (forces don’t cancel)

Motion Concepts

  • Frame of reference: viewpoint determining perceived motion/direction (e.g., car vs. sidewalk)
  • Distance (total path) vs. displacement (straight-line start-to-end change)
    • Example: walk 100m and back = 200m distance, 0 displacement
  • Speed (distance/time; slope of distance-time graph) vs. velocity (speed with direction)
  • Momentum: mass × velocity; greater mass at same speed = greater momentum

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Forces and Motion

To begin our conversation about forces and machines, we must first discuss what matter is.

Matter

Definitions
Matter
A physical substance that has mass and takes up space (volume).
Mass
the measure of the amount of matter in an object
Weight
the specific force of gravity acting on an object

Weight is a force acting on the mass of an object. Weight changes based on gravity, but mass never does. So if you went to the moon, your mass would stay the same, but you would weigh less because the moon has a lower gravity than Earth does.

Let’s talk more about other forces.

Forces

Definitions
Force
a push or a pull on an object
Gravity
an attractive force between masses,
Friction
a force that opposes the motion of objects that touch.
Compression
a pushing force that squeezes an object
Tension
pulling force
Net Force
the combined result of all forces acting on an object, found by adding the forces together.
Sidenote
Types of Friction

There are four types of friction:

Static (non-moving): static friction keeps a book from sliding off a slightly tilted table

Sliding: sliding friction slows a box pushed across the floor,

Rolling: Bicycle tires experience rolling friction when they move

Fluid (including air resistance): fluid friction is what you feel as air resistance when you stick your hand out a car window

Forces can be balanced or unbalanced. Unbalanced forces result in movement, whereas balanced forces are in equilibrium. Force is governed by Newton’s Laws.

Equilibrium

Definitions
Equilibrium
Equal forces are opposing, resulting in a net force of zero. An example is a balanced tug-of-war.
Center of Gravity
The point on an object where its weight is evenly balanced in every direction.

An object stays upright as long as its center of gravity is supported. Tip it too far, and the center of gravity moves outside its base, and the object falls over. This is why a loaded moving truck is more likely to tip on a turn than an empty one: shifting the weight higher moves the center of gravity further from the base.

Newton’s Laws

Newton’s 1st Law: Inertia

An object at rest stays at rest, and an object in motion keeps moving the same way, unless a force acts on it. This resistance to change is called inertia.

Newton’s 2nd Law: F=ma

Acceleration is equal to the net force divided by mass. An unbalanced force results in a change of motion. In other words, the greater the force, the greater the acceleration of the object. The greater the mass of the object, the harder it is to accelerate.

Let’s look at an example.

A 10 kilogram box is pushed with 20 newtons of force. What is the acceleration?

F=ma

20=10∗a

2=a

The box accelerates at 2m/s2.

Newton’s 3rd Law: Action= Reaction

Whenever one object exerts a force on a second, the second object exerts an equal and opposite force on the first. These forces act on different objects and do not cancel out.

For example, if you jump up, you are pushing down on the ground (action) with the same force the ground is pushing up at you (reaction), causing you to launch into the air.

Motion

To describe motion, we need a frame of reference. It tells us which direction something is moving and the amount.

For instance, when you are riding in a car, you appear to be in motion to someone standing on the sidewalk. Their frame of reference is the street. For you, however, the person is passing by the window while you sit still. Your frame of reference is the car. Same situation, different viewpoints or frames of reference.

Distance vs. displacement

Distance is the total length of a path.

Displacement is the straight-line distance and direction from an object’s starting point to its ending point.

For example, if you walk 100 meters down a hallway and back, your distance is 200 meters, but your displacement is zero because you ended up where you started

Speed and velocity

Speed is the distance an object travels per unit of time. On a distance-time graph, the slope represents speed.

Velocity is speed in a given direction.

A car traveling 60 miles per hour north has both a speed and a direction, so that describes velocity, not just speed.

Momentum

The product of mass and velocity. If a grocery cart and a marble are moving at the same speed, the cart has more momentum because it has more mass.

Key points

Matter, Mass, and Weight

  • Matter: substance with mass and volume
  • Mass: amount of matter (constant); Weight: gravity’s force on mass (varies by location, e.g., moon)

Forces

  • Force: push or pull; types: gravity (attraction), friction (opposes motion), compression (squeeze), tension (pull)
  • Friction types: static, sliding, rolling, fluid/air resistance
  • Net force: sum of all forces; balanced forces = no motion change, unbalanced = movement

Equilibrium and Center of Gravity

  • Equilibrium: opposing forces equal, net force = zero (e.g., tug-of-war)
  • Center of gravity: point of balanced weight distribution
  • Object tips when center of gravity shifts outside its base (e.g., loaded trucks tip easier)

Newton’s Three Laws

  • 1st Law (Inertia): objects resist change in motion unless acted on by force
  • 2nd Law: F=ma — more force increases acceleration, more mass resists it
    • Example: 20=10×a, so a=2m/s2
  • 3rd Law: every action has an equal, opposite reaction on a different object (forces don’t cancel)

Motion Concepts

  • Frame of reference: viewpoint determining perceived motion/direction (e.g., car vs. sidewalk)
  • Distance (total path) vs. displacement (straight-line start-to-end change)
    • Example: walk 100m and back = 200m distance, 0 displacement
  • Speed (distance/time; slope of distance-time graph) vs. velocity (speed with direction)
  • Momentum: mass × velocity; greater mass at same speed = greater momentum

More from Mechanical Comprehension

  • Work, Energy and Power
  • Simple Machines
  • Compound Machines