PS-5.7 Explain the motion of objects on the basis of Newton’s three laws of motion: inertia; the relationship among force, mass, and acceleration; and action and reaction forces.
1st Law: Inertia, Law of Inertia, net force, newton
2nd Law: applied force, friction, air resistance
3rd Law: action force, reaction force
The effect of forces on the motion of objects is described by Newton's Three Laws of Motion.
A force can be called either a push or a pull. Push or pull on a wagon with a force greater than 0N and its motion changes. This means it will do one of three things. It will speed up, or it will slow down, or it will change directions. The flip side of this is that you can always tell if a force greater than 0N is acting on an object because it will either be speeding up, slowing down, or changing directions.
NEWTON'S FIRST LAW OF MOTION:
An object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity, unless acted upon by an unbalanced force.
Newton's First Law states several important things. The first thing it says is that if there is no force acting on an object its motion will not change. Here are several examples where no force is acting on a cart. You should be able to analyze each and understand why there is NO change in motion in each situation.
A. No change in motion
B. No change in motion
C. No change in motion
In all three situations above there is no change in velocity ( motion) of the wagon.
In situation (A) the two forces acting on the wagon, the pushing force and the friction of the wheels, are in equal in force and opposite in direction, so they cancel each other out producing a net force of 0N. Forces that are opposite in direction but equal in force are called balanced forces. Because the Net force is zero Newtons, the wagon's velocity remains constant.
In situation (B) there are simply no forces being applied to the wagon, so its motion will not change. The velocity will remain at a constant 5m/s. It is important to note here that the wagon will remain moving to the right at a constant (unchanging) velocity of 5m/s FOREVER, unless another force is applied to change its motion!
In situation (C) the velocity of the wagon is 0m/s and it will remain unchanged because there is no force being applied to the wagon.
Newton's First Law of Motion is sometimes referred to as the law of Inertia. Inertia is a measure of an objects resistance to a change in motion. Newton's First Law shows that all objects resist changes in motion. If you are riding in a car that comes to a sudden stop, your body resists the change in motion and you have a tendency to continue moving forward until some unbalanced force, hopefully from the seatbelt, changes your motion to slow you down.
If you don't know what inertia is, then you should try this simple experiment, and you will quickly gain an understanding of inertia.
1. Stand up and hold a 500g ( .500kg) mass at arms length in front of you.
2. Swing the mass back and forth as many times as you can in 15seconds.
3. Now set the 500g mass down and pick up a 1000g ( 1.00kg) mass and hold it at arms length.
4. Swing the mass back and forth as many times as you can in 15seconds.
In the above experiment you were probably able to swing the 500g mass back and forth more times than you could swing the 1000g mass. This is not because the 1000g object weighs more, but rather because it has more matter ( Protons and Neutrons) in it. In other words, the 1000g object has twice as much mass (matter) than the 500g object does, and therefore has twice the resistance to any changes in motion. When you swing the mass back and forth in front of you, you are changing the objects direction. You have to constantly speed it up and then slow it down in order to get ready for the next swing.
In summary, inertia is a resistance to a change in motion. This means that objects resist speeding up, slowing down, and changing in direction! Inertia is proportional to an objects mass. The greater an objects mass, the more inertia it has.
NEWTON'S SECOND LAW OF MOTION:
A force acting on an object will cause the object to accelerate in the direction of the force, the acceleration being proportional to the force and inversely proportional to the mass of the object.
Though Newton's Second Law sounds complicated, it really is quite simple. This law states three things, often referred to as three precepts ( a precept is principle or rule. ).
The first precept states that a force acting on a mass will cause the mass to accelerate in the direction of the force. This means you can always tell the direction of a force. If a car is accelerating to the left, then the force is also acting to the left. If a rocket is accelerating upward then the force is acting upward. Here are several situations. Analyze each and determine the direction of the force.
A. B. C.
In situation "A" the force is acting to the right. You know this because the acceleration is to the right.
In situation "B" the force is acting to the left because the acceleration is to the left.
In situation "C" ( same situation as "B") the force is acting towards the left because the acceleration is negative. The force is acting in the direction of the negative acceleration.
Look at the diagram labeled "A". If the driver takes his foot off of the gas and puts the car in neutral the car will slow to a stop. Note that the acceleration and the forces of friction and air resistance are acting in the same direction. Friction of any kind, whether from air resistance or mechanical friction opposes motion. If the driver of the race car applies the brakes then the breaking force acts opposite the direction of motion.
The second precept states that acceleration is proportional to the applied force. This is a very simple concept, it simply means that the bigger the applied force the greater the acceleration.
In the diagram above three separate masses. Each has the same mass, but a different applied force. If the acceleration of mass "A" is 3m/s/s then the acceleration of mass "B" is twice that of mass "A and the acceleration of mass "C" is 3 times that of mass "A". If the mass remains constant, then the acceleration will increase proportionally with an increase in applied force.
The graph below shows the relationship between applied force and an objects acceleration. Note that the acceleration is proportional to the force. The slope of the line on the graph is constant for all points. The slope of the graph tells you the amount of acceleration for each unit of applied force. This object gains an acceleration of 1.5m/s2 for each Newton of applied force.
The third precept of Newton's Second Law states that the acceleration is inversely proportional to the mass of the object. In other words, keeping a constant applied force, acceleration is greater for less massive objects than more massive objects. Look at the objects below. Which of the 4 objects shown will have the greatest acceleration?
If you said object "B" you were correct. Each of the four masses has a 20N applied force causing acceleration to the right. Because object "B" has less mass than the others it will have the greatest acceleration of the four. Object "C" will have the lowest acceleration because it has the greatest mass.
Newton's second law is defined mathematically by the equation F=ma. Where F is force, m is mass, and a is acceleration. Here is one way to visualize the relationship between Force, Mass, and Acceleration in Newton's second law.
F=ma This shows that a large force acting on a mass will result in a large acceleration. In other words, acceleration is proportional to force!
F=ma This shows that a force acting on a larger mass will cause a smaller acceleration. In other words, acceleration is inversely proportional to mass!
Newton's Third Law of motion states: Forces act in pairs, equal in magnitude and opposite in direction.
Often referred to as the Law of Action and Reaction, this law states that forces never act alone. Every action force produces and equal opposite reaction force. Look at the two examples in diagram "Q" below. Lay the book on the table and the weight of the book pushes down on the table. The table also pushes up on the book to balance the downward push (weight) of the book. Push on a book to slide it across the table and the book pushes back on your hand with and equal force. These matching forces are called action - reaction pairs.
Here is another example of Newton's Third Law.
Newton's Third law also explains how a rocket works. Hot gases from the burning fuel are forced backwards out the rocket nozzle. This is the action force. The hot gasses force the rocket forward. This is the reaction force.