Newton s Laws: Force and Motion The First Law: Force and Inertia The Second Law: Force, Mass and Acceleration The Third Law: Action and Reaction
The First Law: Force and Inertia Investigation Key Question: How does the first law apply to objects at rest and in motion?
Force changes motion A force is a push or pull, or any action that is able to change motion.
The definition of force The simplest concept of force is a push or a pull. On a deeper level, force is the action that has the ability to create or change motion.
5.1 Force Force is an action that can change motion. A force is what we call a push or a pull, or any action that has the ability to change an object s motion. Forces can be used to increase the speed of an object, decrease the speed of an object, or change the direction in which an object is moving.
Newton s first law Newton s first law says that objects continue the motion they already have unless they are acted on by a net force. If the net force is zero, an object at rest will stay at rest. If an object is acted upon by unbalanced forces, its motion will change. Remember Newton s first law tells us that motion cannot change without a net force.
Net force Newton s first law is often written in terms of the net force: An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force. According to these vectors, in what direction is the net force?
Force changes motion Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.
Law of inertia Key Question: Why are heavier objects harder to start or stop moving?
Force, mass, and inertia Some objects resist changes in motion more than others. Inertia is the property of an object that resists changes in its motion. The greater an object s inertia, the greater the force needed to change its motion. A bowling ball has more inertia than a golf ball.
Force, mass, and inertia Inertia comes from mass. Objects with more mass have more inertia and are more resistant to changes in their motion. A 5-kilogram bowling ball is 100 times as massive as a 50 gram golf ball, so it has 100 times the inertia.
Inertia Inertia is a term used to measure the ability of an object to resist a change in its state of motion. An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop. The word inertia comes from the Latin word inertus, which can be translated to mean lazy.
The net force The motion of objects changes in response to the total force acting on the object, including gravity and any other forces that are present.
The net force in the horizontal direction The term net force is used to describe the total of all forces acting on an object. When used this way, the word net means total.
The net force in vertical direction Gravity exerts a force downward on the box. The floor exerts an equal and opposite force upward on the box.
The net force in vertical direction The net force on the box in the up-down direction is zero. When equal forces applied to the same object are in opposite directions they cancel.
Applications of Newton s First Law Two very important safety features of automobiles are designed with Newton s first law in mind: seat belts and air bags. Both supply a restraining force to counteract your inertia and to slow your body down.
Applications of Newton s First Law Prior to the invention of cup holders, drink containers left on the dash obeyed the first law of motion and made quite a mess. Can you think of other applications of Newton s first law?
The Second Law: Force, Mass, and Acceleration Key Question: What is the relationship between force, mass, and acceleration?
Newton s second law Newton s first law tells us that motion cannot change without a net force. According to Newton s second law, the amount of acceleration depends on both the force and the mass.
Newton's Second Law If you apply more force to an object, it accelerates at a higher rate.
Newton's Second Law If the same force is applied to an object with greater mass, the object accelerates at a slower rate because mass adds inertia.
Acceleration and force The second law says that acceleration is proportional to force. If force is increased or decreased, acceleration will be increased or decreased by the same factor.
Applying the second law Keep the following important ideas in mind: 1. The net force is what causes acceleration. 2. If there is no acceleration, the net force must be zero. 3. If there is acceleration, there must also be a net force. 4. The force unit of newtons is based on kilograms, meters, and seconds.
Newton's Second Law Acceleration (m/sec 2 ) a = F m Force (newtons, N) Mass (kg)
Three forms of the second law When using the second law, the force that appears is the net force. Consider all the forces that are acting and add them up to find the net force before calculating any accelerations.
Using units in calculations In terms of solving physics problems, use the following units when using force in newtons: mass in kilograms (kg) distance or position in meters (m) time in seconds (s) velocity in meters per second (m/s) acceleration in meters per second per second (m/s 2 )
A car has a mass of 1,000 kg. If a net force of 2,000 N is exerted on the car, what is its acceleration? 1. Looking for: the car s acceleration. Using Newton s second law 2. Given: car s mass (m= 1,000 kg) and the net force (F net = 2,000N). 3. Relationship: Use: a = F m 4. Solution: a = 2,000N = 2 kg m/s 2 = 2 m/s 2 1,000 kg kg
5.2 Finding force from acceleration Wherever there is acceleration there must also be force. Any change in the motion of an object results from acceleration. Therefore, any change in motion must be caused by force.
1. You asked for the force (F). 2. You are given the mass (m) and acceleration (a). 3. The second law applies: a = F m Calculating force An airplane needs to accelerate at 5 m/sec 2 to reach take-off speed before reaching the end of the runway. The mass of the airplane is 5,000 kilograms. How much force is needed from the engine? 4. Plug in the numbers. Remember: 1 N = 1 kg m/s 2.
Newton's Third Law Newton s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction. Action and reaction forces act on different objects, not on the same object.
Newton's Third Law Newton s third law states that for every action force there has to be a reaction force that is equal in strength and opposite in direction. Action and reaction forces act on different objects, not on the same object. The forces cannot cancel because they act on different objects.
The Third Law: Action and Reaction For every action there is an equal and opposite reaction. This statement is known as Newton s third law of motion. Newton s third law discusses pairs of objects and the interactions between them.
Forces occur in pairs The two forces in a pair are called action and reaction. Anytime you have one, you also have the other. If you know the strength of one you also know the strength of the other since both forces are always equal.
Forces occur in pairs The astronauts working on the space station have a serious problem when they need to move around in space: There is nothing to push on. One solution is to throw something opposite the direction you want to move.