Chapter 4: Motion and Forces

Chapter 4: Motion and Forces

The Nature of Force and Motion (pages 116-121)

What Is a Force? (pages 116–117)
  • Definition of Force:
    In science, a force is a push or a pull.
  • Force Interaction:
    When one object pushes or pulls another object, the first object is exerting a force on the second object.
  • Descriptions of Forces:
    • Circled options in the study guide:
    • a. direction
    • c. strength
Unbalanced Forces and Motion (pages 117–118)
  • Combination of Forces:
    • When two forces act in the same direction, they combine together.
    • Addition of Forces:
      Adding a force acting in one direction to a force acting in another direction is equivalent to adding a (n) positive number and a (n) negative number.
  • Force Representation:
    • The width of the arrows in Figure 1 represents the strength of a force.
    • A wider arrow depicts a greater force.
  • Net Force:
    • The overall force on an object after all forces are summed is called the net force.
Effects of Unbalanced Forces
  • Effects of Unbalanced Forces: Unbalanced forces can cause an object to:
    • Start moving
    • Stop moving
    • Change direction
  • True or False:
    • Unbalanced forces acting on an object will change the object’s motion: True.
  • Characteristics of Net Force:
    • True statements include:
    • a. When two forces act in opposite directions, the net force is the difference between the two forces.
    • c. When two forces act in opposite directions, the net force is equal to the greater force.
    • d. When two forces act in the same direction, the net force is the sum of the two individual forces.
Balanced Forces and Motion (page 118)
  • Definition of Balanced Forces:
    • Equal forces acting on one object in opposite directions are referred to as balanced forces.
  • Motion and Balanced Forces:
    • Statement: Balanced forces acting on an object will change the object’s motion: False.
  • Net Force with Balanced Forces:
    • When equal forces exerted in opposite directions are added, the net force is zero.

Newton’s First Law of Motion (pages 120–121)

  • Inertia Definition:
    Inertia is the tendency of an object to resist change in its motion.
  • Newton’s First Law of Motion:
    An object at rest will remain at rest, and an object that is moving at constant velocity will continue moving at constant velocity unless acted upon by an unbalanced force.
  • Also Known As:
    Newton’s first law of motion is also called the law of inertia.
  • Example of Inertia:
    It explains why you continue to move forward if you are in a car that suddenly stops.
  • Mass Definition:
    Mass is the amount of matter in an object.
  • SI Unit of Mass:
    The SI unit of mass is the kilogram (kg).
  • Inertia Dependency:
    The amount of inertia an object has depends on its mass.
  • Mass and Inertia Definition:
    Mass can be defined as a measure of the inertia of an object.

Force, Mass, and Acceleration (pages 124-126)

Newton’s Second Law of Motion (pages 124–125)
  • Definition of Newton’s Second Law:
    The net force acting on an object is equal to the product of its acceleration and its mass.
  • Equation for Force, Mass, and Acceleration:
    extForce=extMassimesextAccelerationext{Force} = ext{Mass} imes ext{Acceleration}
  • Units of Force: Circle the letters of two ways to write units of force:
    • b. N (newton)
    • c. kg • m/s²
  • Equation for Finding Acceleration:
    Using Newton’s second law, ext{Acceleration} = rac{ ext{Force}}{ ext{Mass}}.
Changes in Force and Mass (page 126)
  • Effect of Increasing Force on Acceleration:
    An increase of force increases acceleration.
  • Ways to Increase Acceleration: You can increase the acceleration of an object by:
    • Increasing the force
    • Decreasing the mass of the object.
  • Effect of Increasing Mass on Acceleration:
    An increase of mass decreases the acceleration when forces remain the same.
  • True or False:
    One way to increase the force used to pull a wagon is to decrease the mass in the wagon: False.

Friction and Gravity (pages 127-133)

Friction (pages 128–129)
  • Friction Definition:
    Friction is the force that one surface exerts on another when the two rub against each other.
  • Direction of Friction:
    Friction acts in a direction opposite to the object’s direction of motion.
  • Factors Influencing Friction: The strength of the force of friction depends on:
    • The types of surfaces involved
    • How hard the surfaces push together
  • Friction in Walking:
    Friction helps a person walk by acting between the soles of shoes and the floor. Without friction, shoes would just slide across the floor.
Types of Friction:
Kind of FrictionOccurs When
Fluid frictionAn object moves through a fluid
Sliding frictionSolid surfaces slide over each other
Rolling frictionAn object rolls over a surface
  • Comparative Force of Friction:
    • Which kind of friction requires more force to overcome: rolling friction or sliding friction? The answer will vary.
  • Effect of Oil on Friction:
    Oil between machine parts reduces friction by preventing direct contact, resulting in fluid friction being present instead of sliding friction.
Gravity (pages 130–132)
  • Definition of Gravity:
    The force that pulls objects toward Earth is called gravity.
  • Free Fall Definition:
    An object is said to be in free fall when the only force acting on it is gravity.
  • Acceleration Due to Gravity Near Earth's Surface:
    Near the surface of Earth, the acceleration due to gravity is approximately 9.8 m/s².
  • Projectiles:
    An object that is thrown is called a projectile.
  • True or False:
    An object that is dropped will hit the ground before an object that is thrown horizontally: False.
  • Air Resistance:
    Objects falling through air experience fluid friction called air resistance.
  • Greater Surface Area Effects:
    The greater the surface area of an object, the greater the air resistance.
  • Terminal Velocity:
    The greatest velocity a falling object reaches is called terminal velocity.
  • Weight Definition:
    Weight is the force of gravity on an object at the surface of a planet.
  • Comparison of Weight and Mass:
    Weight measures the force of gravity on an object while mass measures the amount of matter in an object.
  • Weight Measurement Unit:
    Weight is usually measured in newtons (N).
Universal Gravitation (pages 132–133)
  • Universal Law of Gravitation:
    The force that makes an apple fall to the ground is the same force that keeps Earth orbiting the sun, stating that the force of gravity acts between all objects in the universe.
  • Factors Affecting Attractiveness: The force of attraction between two objects varies with:
    • Mass of the objects
    • Distance between the objects

Action and Reaction (pages 134-139)

Newton’s Third Law of Motion (pages 134–136)
  • Definition of Newton’s Third Law:
    If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.
  • Types of Forces Defined by Newton:
    • Action Force: The force exerted by the first object on a second object.
    • Reaction Force: The force exerted by the second object back on the first object.
  • Equal Action and Reaction Forces:
    • The action and reaction forces in any situation will always be equal and opposite.
  • Example: Squid Movement:
    A squid expels water out its back end, an action force. The water expelled pushes back, exerting a force on the squid, which results in reaction force that propels the squid through the water.
  • Why Do Equal Forces Not Cancel?
    Equal action and reaction forces do not cancel each other when one person hits a ball because they act on different objects (the ball and the person).
Momentum (page 137)
  • Definition of Momentum:
    The product of an object’s mass and velocity is its momentum.
  • Equation for Momentum:
    extMomentum=extMassimesextVelocityext{Momentum} = ext{Mass} imes ext{Velocity}
  • Unit of Measurement for Momentum:
    The unit for momentum is kilogram-meters per second (kg • m/s).
Conservation of Momentum (pages 138–139)
  • Law of Conservation of Momentum:
    The total momentum of the objects that interact does not change.
  • Example:
    Suppose a train car moving down a track at 10 m/s hits another train car at rest. The momentum is transferred from the first train car to the second, which moves forward at 10 m/s after the collision while the first car stops.

Forces in Fluids (pages 140-150)

What is Pressure? (pages 140–141)
  • Pressure Definition:
    Pressure is the force exerted on a surface divided by the total area over which the force is exerted.
  • Example of Snowshoes:
    Snowshoes distribute weight over a larger area, resulting in lower downward pressure.
  • True or False:
    Force and pressure are the same thing: False.
  • SI Unit of Pressure:
    The SI unit of pressure is the pascal (Pa).
Fluid Pressure (page 142)
  • Definition of Fluids:
    A substance that can flow easily is called a fluid.
  • Molecular Movement in Fluids:
    Molecules in fluids are always moving in all directions and colliding with each other and their surroundings.
  • Pressure in Fluids:
    The pressure exerted by a fluid is the total force exerted by the fluid divided by the area over which the force is exerted.
Pascal’s Principle (page 143)
  • Effect of Pressure Increase in a Confined Fluid:
    When force is applied, the pressure in that confined fluid is transmitted equally to all parts of the fluid.
Force Pumps and Blood Flow (page 144)
  • Function of a Force Pump:
    A force pump causes a fluid to move from one place to another by increasing the pressure in the fluid.
  • Heart as a Force Pump:
    The heart pumps blood similarly, first to the lungs and then throughout the body via increased pressure.
Bernoulli’s Principle (pages 148–150)
  • Bernoulli’s Principle Statement:
    The pressure exerted by a moving stream of fluid is less than the pressure of the surrounding fluid.
  • Effect of Fluid Speed on Pressure:
    Faster moving fluids exert less pressure than slower moving fluids.
  • Application of Lift in Aviation:
    Differences in pressure above and below the wing of an airplane result in lift, which is an upward force created by the pressure difference.
  • Air Pressure and Smoke:
    Wind crossing the top of a chimney lowers the pressure there, allowing higher pressure from below to push smoke upwards.
WordWise
  • Define key terms relevant to Chapter 4:
    1. Force: A push or pull.
    2. Net Force: The overall force after all forces are combined.
    3. Inertia: The resistance of an object to change in its motion.
    4. Newton: The force required to accelerate 1 kg at 1 m/s².
    5. Friction: The force one surface exerts on another.
    6. Sliding Friction: Occurs when solid surfaces slide over each other.
    7. Fluid: A substance that can flow easily.
    8. Gravity: The force pulling objects toward Earth.
    9. Free Fall: When an object is only acted upon by gravity.
    10. Air Resistance: A type of fluid friction acting on falling objects.
    11. Momentum: The product of mass and velocity.
    12. Pressure: Force exerted on an area.
MathWise
  • Example Calculations:
    1. Force calculation example: extForce=65extkgimes3extm/s2ext{Force} = 65 ext{ kg} imes 3 ext{ m/s}^2
    2. Weight calculation example: extWeight=7extkgimes9.8extm/s2=68.6extNext{Weight} = 7 ext{ kg} imes 9.8 ext{ m/s}^2 = 68.6 ext{ N}
    3. Momentum calculation example: extMomentum=5extkgimes6.5extm/sext{Momentum} = 5 ext{ kg} imes 6.5 ext{ m/s}
    4. Pressure calculation example: ext{Pressure} = rac{ ext{Force}}{ ext{Area}}