Chapter 12: Forces & Motion

12.1 (pg’s 356-362)Forces

  • Every object continues in a state of rest, or of motion, unless force is applied tochange things. This is a fancy way of saying that things tend to keep doing whatthey are already doing.
  • Force: can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.
  • A force is what we call a push or a pull, or any action that has the ability tochange motion. This definition does not, however, mean that forces always changemotion! If you push down on a table, it probably will not move.

Force is an action that has the ability to change motion.

  • Force is measured in newtons (N). 1 N is the force that cause a 1-kg mass to accelerate at a rate of 1 meter per second each second (1N = 1kg x m/s2 )
  • There are two units of force that are commonly used: pounds and newtons. Scientists prefer to use newtons. The newton is a smaller unit than the pound.There are 4.48 newtons in one pound. A person weighing 100 pounds wouldweigh 448 newtons.

Units of Force
Unit / Equivalents
1 newton / 0.228 pounds
1 pound / 4.48 newtons
  • The metric unit of force, the newton, relates force and motion. One newton equals1 kilogram multiplied by 1 meter per second squared. This means that a force ofone newton causes a 1-kilogram mass to have an acceleration of 1 m/sec2.
  • Intalking about force, “newton” is easier to say than “1 kilogram · m/sec2.”
  • Combining Forces:
  • Net Force: the overall force acting on an object after all the forces are combined.
  • Forces in the same direction add together
  • Forces in opposite directions subtract from one another.

Balanced and unbalanced forces

  • Balanced Forces: when the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.
  • Unbalanced Forces: an unbalanced force acts on an object, the object accelerates.

Friction and motion

  • Friction: a force that opposes the motion of objects that touch as they move past each other.

Friction is a term that is used to describe forces that result from relative motionbetween objects (like the wheel and axle of a car). Frictional forces always workagainst the motion that produces them.

Friction comes from two surfaces moving against each other. Surfaces of objectsthat appear smooth and shiny actually have microscopic hills and valleys. As thesurfaces slide across each other the hills and valleys interfere causing friction.

We use the word friction to describe any force that is caused by motion and thatacts to slow motion down. Some examples of friction include:

  • Static Friction: force that acts on objects that are not moving.
  • Sliding Friction: force that opposes the direction of motion of an object as it slides over a surface.
  • Rolling Friction: force that act on rolling objects.
  • Fluid Friction: force that opposes the motion of an object through a fluid.

Friction is a force that . That means the force of friction isopposite whatever force is causing motion. For a car rolling downhill, gravitysupplies a force pulling down the hill. Friction opposes motion, so it pushes thecar up the hill while gravity is pulling the car down the hill.

The difference between force and mass

Force and mass have different units. Force units are pounds or newtons. Massunits are grams or kilograms. To get the right answer when using formulas thatinclude force or mass, you need to use the correct units!

Force is a push or pulling action that can change motion. Mass is the amount of“stuff” or matter in an object. Mass is a basic property of objects. Mass resists theaction of forces by making objects harder to accelerate.

The weight of a person can be described in pounds or newtons. On Earth, a childweighs 30 pounds or about 134 newtons. In other words, the force acting on thechild, due to the influence of Earth’s gravity, is 134 kilograms · m/sec2.

A child that weighs 30 pounds on Earth has a mass of about 14 kilograms becauseon Earth 2.2 pounds equals 1 kilogram. Because mass is an amount of matter,mass is independent of the force of gravity. Therefore, the mass of a person is thesame everywhere in the universe. However, the weight of a person on Earth isdifferent from what it would be on the moon or another planet because the force ofgravity is different at these other places.

Mass and weight are commonly used to describe the quantity of something. Forexample, a kilogram of bananas weighs 2.2 pounds. You can describe the quantityof bananas as having a mass of 1 kilogram, or a weight of 2.2 pounds. Using twodifferent kinds of measurement to describe the same quantity of bananas does notmean pounds and kilograms are the same thing.

We often use different units to describe a quantity. For bananas, you can use a unitof mass (kilograms) or a unit of force (pounds). Likewise, buying one gallon ofmilk is the same as buying 8.4 pounds of milk. Pounds and gallons both describethe same quantity but one unit is a measure of volume (gallons) and one is ameasure of force (pounds).

Weight, Gravity, and Friction

Gravity

What is the force that causes an object like a car to accelerate down a ramp?You probably know gravity is involved. Gravity is a force that pulls every masstoward every other mass. Since Earth is the biggest mass around, gravity pullseverything toward the center of Earth.

  • Ask someone the meaning of the worddown and they point toward the center of Earth.
  • Down is the direction of the forceof gravity.

The force of gravity depends on how much mass you have. If you have more mass,gravity pulls on you with more force. That is why we can use force to measuremass. When you use a digital balance to measure the mass of a sample, you arereally measuring the force of gravity acting on your sample.

  • If you are on thesurface of Earth, every kilogram of mass creates a gravitational force of 9.8newtons.
  • You may recognize this number—9.8 newtons is the same as 9.8 m/sec2,the acceleration of gravity.

If you were on Mars, your force/mass balance would have to be adjusted. The planet is much smaller than Earth and therefore Mars’s gravity is weaker. Every kilogram of mass on Mars results in a gravity force of only 3.8 newtons (figure 3.8). The larger the planet you are on, the greater the force of gravity.

  • On Jupiter, the largest planet, gravity has a force 2.6 times stronger than on the surface of Earth. If you weighed 110 pounds on Earth, you would weigh 286 pound on Jupiter!
  • Gravity: is a force that acts between any 2 masses.
  • Gravity is an attractive force that pulls object together.
  • Earth’s gravitational force exerts a force of attraction on every other object that is near Earth.
  • The force of gravity does not require objects to be in contact for it to act on them.
  • Earth’s gravity act downward toward the center of Earth.
  • An upward force or supporting force balances the forward force of gravity.

Mass and weight

Weight is what we call the force created by gravity on objects. The weight of anobject depends on its mass. Your mass is constant throughout the universe, butyour weight changes depending on what planet you happen to be on. For example,because the gravitational force on Mars is less than that on Earth, you weigh lesson Mars than on Earth, but your mass is the same at both locations!

If you know the mass of an object, you can calculate its weight using Newton’ssecond law. When you drop something on Earth, gravity causes it to accelerate at9.8 m/sec2. Because there is acceleration, you know there must be a force. Youalso know the force is exactly equal to mass times acceleration. The force we callweight is equal to an object’s mass times the acceleration of gravity (9.8 m/sec2).

  • Since weight is a force, we use the letter F to represent it.

Because we live and work on the surface of Earth, we tend to use weight and massinterchangeably. Heavy objects have lots of mass and light objects have littlemass. Boxes and people are “weighed” in both kilograms and pounds. This is OKfor everyday use, but you must remember the difference when doing physics.

Physics is about the true nature of how the universe works and mass is truly afundamental property of an object. Force often depends on outside influences, likegravity. You cannot interchange force and mass in a formula; doing so would belike substituting a fork for a spoon when you are trying to eat soup. In physics,force and mass are different quantities with different units.

  • Falling Objects: Gravity causes objects to accelerate downward, where as air resistance acts in the direction opposite to the motion and reduces acceleration.

12.2 (pg’s 363-371) Newton’s First & Second Laws of Motion:

Mass and inertia

Newton’s first law is also called the law of inertia. Inertia is defined as theproperty of an object to resist changing its state of motion. An object with a lot ofinertia takes a lot of force to start or stop. Big trucks have more inertia than smallcars, and bicycles have even less inertia.

The amount of inertia an object has depends on its mass. Mass is a measure of theinertia of an object. Mass is what we usually think of when we use words like“heavy” or “light.” A heavy object has a large mass while an object described as“light as a feather” has a small mass. We can also define mass as the amount ofmatter an object has.

Mass is measured in kilograms. The kilogram is one of the primary units of themetric system, like the meter and second. For reference, 1 kilogram has a weightof about 2.2 pounds on the Earth’s surface. That means gravity pulls on a mass of1 kilogram with a force of 2.2 pounds.

The mass of an object does not change, no matter where the object is, what planetit is on, or how it is moving.

  • 1st Law: the state of motion of an object does not change as long as the net force acting on the object is zero.
  • Sometimes called the Law of Inertia: the tendency of an object to resist a change in its motion.
  • An object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed.

Newton’s second law of motion

Newton’s second law relates the applied force on an object, the mass of the object,and acceleration.

  • 2nd Law: The acceleration of an object is equal to the force acting on it divided by the object’s mass.
  • Doubling the mass of an object cuts its acceleration in half.
  • The acceleraion of an object is always in the same direction as the net force.
  • Mass & Weight:
  • Mass is a measure of the inertia of an object, how much “stuff” something is made up of.
  • Weight is a measure of the force of gravity acting on an object.

12.3 (pgs. 372-377) Newton’s Third Law of Motion & Momentum

  • Newton’s 3rd Law: whenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. (action and reaction forces)
  • Momentum: is the product of an object’s mass and its velocity.
  • An object has a large momentum if the product of its mass and velocity is large.
  • Conservation of Momentum: if not net force acts on a system, then the total momentum of the system does not change.
  • Closed system: other objects and forces cannot enter or leave a system.
  • In a close system, the loss of momentum of one object equals the gain in momentum of another object, momentum is conserved.

12.4 (pgs. 378-382) Universal Forces:

  • Electromagnetic Forces: are associated with charged particles
  • Nuclear Forces: the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together
  • Gravitational Force: every object in the universe attracts every other object.
  • It takes a huge mass such as Earth’s to exert a large gravitational force.
  • Gravity is the weakest universal force, but it is the most effective over long distances.