Chapter 10: Forces

The Nature of Force


  • In science, the word force has a simple and specific meaning.
  • Force is a push or a pull
  • When one object pushes or pulls another object, you say that the first object exerts a force on the second object.
  • Force is a vector quantity.
  • Described by magnitude and direction.
  • An arrow represents the direction and strength of a force and the longer the arrow, the greater the force.
  • The SI unit for force is a Newton.

Combining Forces

  • Often, more than a single force acts on an object at one time.
  • Net Force is the combination of all forces acting on an object.

Balanced Forces

  • Balanced Forces are equal forces acting on one object in opposite directions.

  • When equal forces are exerted in opposite directions there is no net force.
  • Balanced forces acting on an object do not change the object's velocity.

Friction, Gravity and Elastic Forces


  • Friction is a force that two surfaces exert on each other when they rub against each other.
  • Friction acts in the direction opposite to motion.
  • Without friction or other unbalanced forces, an object will not stop until it hits another object.

Causes of Friction

  • Smooth surface has less friction than rough surface.
  • Strength of force of friction depends on types of surfaces involved and how hard the surfaces push together. (i.e. rubbing hands together)

4 Types of Friction
  • Static Friction - Friction that acts on objects that are not moving
  • Example: moving a piece of furniture across room.
  • Must use extra force to start friction of stationary objects.
  • Sliding Friction - Friction where two solid surfaces slide over each other. When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding friction.
  • Rolling friction - Friction where an object rolls across a surface.
  • Rolling friction is less than sliding friction.
  • Fluid friction - Friction where a solid object moves through a fluid.
  • Less than sliding friction.
  • Parts of machines bathed in oil.

  • Gravity is a force that pulls objects toward each other.
  • Newton realized gravity acts everywhere in universe.
  • Law of Universal Gravitation states that the force of gravity acts between all objects in the universe.
  • Any two objects in the universe, without exception, attract each other.

Mass and Distance
  • Force of gravity between object increases with greater mass and decreases with greater distance.
  • The further from Earth it is, the less gravitational force.
  • Mass is the measure of the amount of matter in an object. (Kg)

Gravity & Weight
  • Weight is gravitational force exerted on a person or object at the surface of a planet.
  • Stepping on scale (weight) shows the gravitational force Earth exerting on you.
  • Objects with greater mass have greater weight.
  • Weight = Mass x Acceleration due to gravity
  • Weight – Newtons (N).
  • Mass – Kilograms (KG).
  • Acceleration due to gravity = 9.8 meters per second per second (m/s²) (at Earth's surface)
  • Weight varies with strength of gravitational force – mass doesn't.
  • Strength of gravitational force exerted on an object or person by the moon is 1/6 of the force exerted by Earth.

Gravity and Motion
  • Free fall – when the only force acting on an object is gravity.
  • In free fall, the force of gravity alone causes an object to accelerate in the downward direction.
  • All objects in free fall accelerate at the same rate.

Air Resistance
  • Air resistance is a type of fluid friction.
  • Air resistance is an upward force exerted on falling objects.
  • Objects with greater surface area experience more air resistance as they fall. (Flat piece of paper vs. crumpled piece of paper)

Projectile Motion
  • A projectile is an object that is thrown.
  • A ball thrown horizontally falls at the same rate as ball dropped.

Elastic Forces
  • Matter is considered elastic if it returns to its original shape after is squeezed or stretched.
  • Compression is an elastic force that squeezes or pushes matter together.
  • Tension an elastic force that stretches or pulls matter.

Newton's Laws

Isaac Newton
  • In the early 1600s Galileo said, that when an object is in motion, a force is needed to change the motion of an object.
  • In the late 1600s, Isaac Newton built upon Galileo's idea and proposed three basic laws of motion.

First Law of Motion
  • An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.
  • An unbalanced force will cause an object to speed up, slow down or change direction.

  • Inertia is the tendency of an object to resist a change in motion.
  • Newton's first law is also called the law of inertia.
  • The greater the mass of an object - the greater its inertia - the greater the force required to change its motion.

Second Law of Motion
  • Acceleration depends on the net force acting on the object and on the object's mass.
  • Acceleration = Net Force / Mass (Acceleration is the rate at which velocity changes)

Third Law of Motion
  • 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.

  • Action and reaction forces do not cancel out because they are acting on different objects.

  • Momentum is a quantity of motion.
  • Characteristic of a moving object that depends on both the mass and velocity of the object.
  • Momentum = Mass x Velocity Measured in kilogram-meters per second (kg-m/s)

Conservation of Momentum
  • In science, conservation refers to the conditions before and after an event.
  • The Law of Conservation of Momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change.
  • The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects.

Rockets and Satellites

How Do Rockets Lift Off?
  • Rockets and space shuttles lift into space using Newton’s third law of motion.
  • They burn fuel and push the exhaust gases downward at a high velocity as they lift off. In turn, the gases push upward on the rocket.

  • A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket.
  • As long as this upward force, called thrust, is greater than the downward pull of gravity, there is an unbalanced force in the upward direction that causes a change in the rocket's velocity. As a result, the rocket accelerates upward into space.

What is a Satellite?
  • Satellite any object that orbits another object in space. (e.g. Moon or Artificial Satellite)

Circular Motion
  • Artificial satellites travel around Earth in an almost circular path.
  • An object traveling in a circle is constantly changing direction, so it is accelerating.
  • If an object is accelerating, an unbalanced force must be acting on it.
  • Centripetal Force is any force that causes an object to move in a circular path. (Center seeking, and acts perpendicular to the direction the object is moving)

Satellite Motion
  • Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it.
  • The speed with which an object must be thrown in order to orbit Earth is 7,900 meters per second.

Satellite Location
  • Some travel in low orbits. (Mapping, or Observation) These satellites complete a rotation around the Earth in a couple of hours.
  • Others in higher orbits travel more slowly. (Communications) These travel 36,000 kilometers above Earth's surface.

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