Force and the Law of Motion

Changes in Motion Force is an action exerted on an object in order to change the object’s state of rest or motion Force is a vector: It depends on magnitude and direction.

Forces Cause: A stationary object to move A moving object to stop A moving object to change direction

Sir Isaac Newton Contributed major knowledge to the modern understanding of force and motion. A unit of Force is a Newton 1 N = 1 kg * 1m/s2

Two Types of Forces Contact Forces Field Forces Physically Interacting objects Field Forces Non-physical interaction between objects

Contact Forces Physical contact between objects. Examples Pull on a spring, it stretches. Push a cart, it moves. Catch a ball, it stops.

Field Forces Non-physical contact between objects Gravitation Force Attraction through electrical charges

Diagram In physics, Free-Body Diagrams are used for problems. Only considers forces acting on the object.

Example of Free-Body Diagram Image shows: Force of gravity on the car and wall(arrows down) Normal Force on the car and wall (arrows up) Force the car applies to the wall. Forces the wall applies to the car.

Example of Free-Body Diagram Want to only consider forces on the car: Force of gravity on the car (arrow down) Normal Force on the car (arrow up) Forces the wall applies to the car. (arrow to the left)

Newton’s First Law of Motion

Newton’s First Law (law of inertia) An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

Forces may be balanced or unbalanced Balanced forces – all forces acting on an object are equal There is NO ACCELERATION Unbalanced forces – one or more forces acting on an object are stronger than others There is MOTION A NET FORCE

Objects at Rest Objects at rest tend to stay at rest unless acted upon by a force. [push or pull] Newton described this tendency as inertia. Inertia can be described as the tendency of an object to keep doing whatever’s it’s doing.

Mass & Inertia Mass is the amount of matter in an object. The more MASS an object has, the more INERTIA the object has. Bigger objects are harder to start & stop

What about objects that are already in motion? Newton stated that objects in motion tend to stay in motion until acted upon by a force (or hits it.)

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck. Slide from www.science-class.net

Newton’s Second Law An object acted upon by an unbalanced force will accelerate in the direction of the force. Force = Mass x Acceleration SI Units of Force 1 Newton (N) = 1kg * m/s2

Newton’s Second Law Force equals mass times acceleration. F = ma

Weight WEIGHT is a measure of the force of GRAVITY on the mass of an object ACCELERATION of GRAVITY(Earth) = 9.81 m/s2 Weight (force) = mass x gravity (Earth) Your weight on Earth is the gravitational force between you and Earth.

Gravity and Weight Weight is a force, like the push of your hand is a force, and is measured in newtons. Mass is the amount of matter in an object, and doesn’t depend on location. Weight will vary with location, but mass will remain constant.

Newton’s Third Law of Motion

Remember: Newton’s Second Law: An object acted upon by an unbalanced force will accelerate in the direction of the force. Force = Mass x Acceleration SI Units of Force: Newtons (1N = 1kg * m/s2) Weight is the force due to gravity on the object

3rd Law of Motion Describes the motion that happens when one object exerts a force on another object. Forces always act in equal but opposite pairs

3rd Law of Motion Short Definition For every action, there is an equal but opposite reaction.

Example This means that when you push on a wall, the wall pushes back on you with a force equal in strength to the force you exerted.

Action – Reaction Pair The forces exerted by two objects on each other are called and action-reaction force pair. Either force can be considered the action force or the reaction force. Action and reaction force pairs don’t cancel because they act on different objects.

Think about it . . . What happens if you are standing on a skateboard and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

EXAMPLES… Fnail = F (going up) Suppose a hammer and nail: The force of the hammer on to the nail: Fhammer = -F (going down) The force of the nail on to the hammer: Fnail = F (going up) The forces are equal in magnitude but not direction.

Consider a Rocket This downward push is the action force. The reaction force is the upward push on the rocket engine by the gas molecules. This is the thrust that propels the rocket upward.

Newton’s Third Law A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus. Which will have the most force? The bug on the bus The bus on the bug

Newton’s Third Law The force would be the same. Force (bug)= m x A Force (bus)= M x a What would is the greater value?

Review Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.