Forces, Motion,& Energy

Chapter 1 Section 1Measuring Motion

What is motion?

• Motion-changing position over time.

• Frame of Reference: whenever something is moving, it is compared to something that is not moving.

• Most common is the Earth’s surface.

How do you measure motion?

• Speed

• Velocity

• Acceleration

What is Speed?

• Speed-distance covered per unit time– Calculated: speed=distance/time– Measured in Km/h & m/s (also use ft/s and

mi/h)– Earth’s orbit is 30 Km/s– Constant vs. Average

What is Velocity?

• Velocity: speed in a given direction– Used by weather forecasters, air traffic

controllers, people traveling– Can be combined: traveling with the

wind vs against the wind.– Calculated the same as speed but

direction is added.

What is Acceleration?

• Acceleration: rate at which velocity changes.– Positive acceleration: speed up– Negative acceleration: slow down– Change directions– Calculated:

• Measured in Km/h2 or m/s2

• Acceleration= Final Velocity-Initial Velocity time

Chapter 1 Section 2What is a force?

What is a force?

• Defined as a push or a pull.

• Factors that affect all forces include size and direction.

• SI unit for force is a Newton= the force necessary to move a 1Kg mass at 1m/s/s

What is a net force?

• A net force is the sum of all forces acting on an object. Balanced forces produce no change in motion.

• If the net force is not zero, there will be motion in the same direction that the greater force is moving. The forces are considered unbalanced.

• For example: Tug-of-war, vehicle collisions, and water travel

Chapter 1 Section 3Friction: A Force That Opposes

Motion

What is Friction?

• Defined as a force that opposes motion.

• Friction occurs as two surfaces touch.

• Factors that affect the amount of friction include:– The smoothness of the surface.– The force pushing the surfaces together.

Types of Friction

• Sliding kinetic friction: occurs as two surfaces slide across each other.

• Rolling friction: occurs between the surface of wheels and the object.

• Fluid friction: occurs between a liquid and the surface of the object.

• Static friction: holds an object in place.

Helpful or Harmful??

• Friction is helpful : allows us to walk, stop our cars, and to ice skate.

• Friction is harmful: causes wear on surfaces-car parts heat up, erosion occurs, tears holes in clothes.

Chapter 1 Section 4Gravity: A Force of Attraction

Newton & the Study of Gravity

Why do objects fall toward the Earth? What keeps the planets moving in the sky?

In 1665, British scientist Sir Isaac Newton made the connection between these two questions when he saw an apple falling from a tree.

What did Newton know?

• That unbalanced forces are needed to change the motion of objects. He concluded that an unbalanced force on the apple made the apple fall.

• He also reasoned that an unbalanced force on the moon kept the moon moving around the Earth.

What is the Law of Universal Gravitation?

• Objects in the Universe attract each other by the force of gravity.

Newton’s Law of Gravitation

• Newton compared the acceleration of a falling apple to the acceleration of the moon and concluded that the same force was responsible.

• Gravity-force of attraction between two objects

• Gravitational force depends on the mass of the object and the distance between two objects.

Chapter 2 Section 1Gravity and Motion

Falling Objects

• Newton stated that all objects fall at the same rate of speed no matter their mass.

• The acceleration due to the force of gravity is 9.8m/s2

Air Resistance

• Air resistance opposes the downward fall of objects.

• The amount of resistance depends on size, shape, and speed of the object.

• Terminal velocity-when an object falls at a constant velocity.

• Free fall occurs when there is no air resistance.– A vacuum has objects falling b/c there is no

air resistance.

• Orbiting & Centripetal ForceThe unbalanced force that causes objects to

move in a circular path.

Types of motion

• Projectile Motion-the curved path an object follows when it is thrown or propelled near the surface of earth.– Horizontal Motion

• Motion that is parallel to the ground.• Ex: Throwing a ball (hand exerts force on ball

– Vertical Motion• Motion that is perpendicular to the ground.• Ex: After you throw a ball, gravity pulls it downward

and gives the ball vertical motion.

Chapter 2 Section 2Newton’s Laws of Motion

Newton’s 3 Laws of Motion

• Who was Newton? Sir Isaac Newton developed three

laws that describe all states of motion 1665-1666– Lived 1642-1727– mathematician & physicist– Developed a scientific approach

to explain nature.Laws

1st Law: What did Newton base it on?

• Inertia: is the tendency of an object to remain in motion or stay at rest and is related to the mass of the object.

• Newton determined that if friction was not present the object in motion would continue to move forever.

What does the 1st Law state?

• An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by and unbalanced force.

Laws

2nd Law: What is it based on?

• Newton explained how acceleration and force are related.

• Force-a push or a pull.

• Acceleration-rate at which velocity changes

• Force is measured in Newtons (N)

• 1 Newton=the force needed to accelerate a 1Kg mass at 1m/s2

What does the 2nd Law state?

• Force=Mass x Acceleration

F=ma

• A greater force is required to accelerate an object with a greater inertia.

Laws

What is the 3rd law based on?

• Opposing forces.

• If forces are not balanced motion will occur.

• Momentum of bodies before and after collisions.

• Friction-force that opposes motion between two surfaces– Types of friction

What does the 3rd Law state?

• For every action there is an equal and opposite reaction.

Laws

Chapter 2 Section 3Momentum

Law of Conservation of Momentum

• Any time objects collide, the total amount of momentum stays the same.

• Objects sticking together-they move as one. The mass is = to the masses of the 2 objects added together.

• Objects bouncing off each other-momentum is transferred from one object to the other. The transfer makes them move in different directions.

Chapter 4 Section 1Work & Power

What is scientific Work?

• Work-result of using force to move an object over a distance.

• Joule- the amount of work done when a force of 1N exerted for 1m.

• The direction of motion must be in the same direction that the force is applied.

How is work measured?

• Calculated: work= force x distance– w=f x d– Measured in N-m or Joules (J)

X

What is Power?

• Power-The rate at which work is done.

• Power tells us how fast the work is being done.

• Large quantities of power are measured in kilowatts.

How is power measured?

• Measured in Joules/second or watts

• Calculated: power = work Time

Chapter 4 Section 2What is a machine?

What is a machine?

• Machine-a devise that helps to do work by multiplying force or distance.

• Six types of simple machines– Inclined plane– Wedge– Screw– Lever– Pulley– Wheel & axle

How do Machines make work easier?

• Two types of work– Work input-work that goes into the machine

• Input (effort) force is applied to the machine through a distance

– Work output-work that the machine does• Output force is used to overcome the resistance

(load) force.• It is the combined force from you and the machine.

• Machines either multiply the force or distance, but never both.

How helpful is a machine?

• Efficiency is expressed as a percent.

• The less work a machine has to do to overcome friction, the more efficient the machine is. Mechanical efficiency is a comparison of a machine’s work output with the work input.

mechanical efficiency work outputwork input

100

• Mechanical Advantage (MA)-The factor by which a machine multiplies the effort force.

• MA can be calculated by using either the input/output forces or comparing the distances.

• Force and distance are inversely related.

Is there such thing as an ideal machine?

• An ideal machine would be a machine that had 100% mechanical efficiency.

• Ideal machines are impossible to build, because every machine has moving parts. Moving parts always use some of the work input to overcome friction.

Chapter 4 Section 3Types of Machines

Simple Machines

• Inclined plane

• Wedges

• Screw

• Lever

• Pulley

• Wheel & Axle

What is an inclined plane?

• A simple machine consisting of a plane set at an angle to a horizontal surface, forming a ramp.

• Has no moving parts, a flat slanted surface.

What makes the best inclined plane?

• The lower the angle of the slant the greater the distance traveled.

• MA(inclined plane)=length of ramp/height of ramp

What is a wedge?

• A specialized inclined plane that moves.

• As the force is applied the wedge moves the object.

• Most are two inclined planes back to back.– Such as a knife, ax or chisel.

What makes the best wedge?

• The longer and thinner the wedge, the less effort force is required to overcome the resistance.

• Can be found by dividing the length of the wedge by its greatest thickness.

What is a screw?

• A specialized incline plane that is wrapped around a central bar or cylinder and forms a spiral.

• The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical

advantage is.

What is a lever?

• A lever is a simple machine that consists of a rigid bar that pivots around a fixed point called a fulcrum.

• Divided into three classes depending upon the location of the fulcrum and the forces.

What is a 1st Class Lever?

• The fulcrum is located in the middle between the effort force and resistance force.

• Examples: seesaw, pliers, crowbar

What is a 2nd Class Lever?• The load of a second-class lever is between

the fulcrum and the input force.

• Examples: Wheelbarrow, Door, Nutcracker

What is a 3rd class lever?

• The input force in a third-class lever is between the fulcrum and the load. Examples: fishing pole, shovel, hammer

How does a lever help us do work?

• In a lever where the effort force and resistance are located on the bar influences the amount of force needed.

• MA= distance of the effort armdistance of the resistance arm

What is a Pulley

• A pulley is a rope, belt, or chain that wraps around a grooved wheel.

• Influences work by changing the direction of the force or changing the amount of the force.

Types of Pulleys

• Fixed pulley: the pulley is attached to a structure

• Does NOT multiply force• Changes the direction of the effort force

• Movable pulley: the pulley moves as the resistance moves because it is attached to the object.– Multiplies the effort force– Cannot change the direction of the effort force– Load is supported by the ropes.

• Combination pulleys: combine both a fixed pulley and a movable pulley.– Block & tackle– Multiplies effort force– Increases the MA

• MA= count the number of strings that support the load.

What is a Wheel & Axle?• A wheel & axle is made of two circular

objects of different sizes. – The larger one is the wheel– The smaller one is the axle

• Multiplies the force.

• Examples: gears, screwdrivers, knobs

• MA= radius of the wheelRadius of the axle

Compound Machines

• Machines that are made of 2 or more simple machines.

• The mechanical efficiency of most compound machines is low, because compound machines have more moving parts than simple machines do. Thus, there is more friction to overcome.

Chapter 5 Section 1What is energy?

What is energy?

• Energy -the capacity to do work

• Energy can be gained because work is being done.– Example: if work is done to lift a rock, the rock

has energy.

What are the two kinds of energy?

• Kinetic energy -energy due to motion.– If an object is moving, it is capable of doing

work. • Work can be considered a transfer of energy.• An increase in either the mass or velocity means

the object’s kinetic energy has increased. In other words KE depends upon the mass & velocity of the object.

• KE=1/2 mv2

• Potential Energy-stored energy

• PE is energy of position– Gravitational PE is due to the height an object

is raised• GPE=mgh• The higher the object the more PE the object has.

– Chemical PE is due to the orientation of atoms within a molecule

– Elastic PE is due to the stretching/ compression of an object

Mechanical Energy• Mechanical energy is the total energy of

motion and position of an object. Both kinetic energy and potential energy are kinds of mechanical energy.

mechanical energy potential energy kinetic energy

What are the different forms of energy?

• Thermal (Heat)

• Chemical

• Electrical

• Sound

• Light

• Nuclear

What is thermal energy?

• Energy due to the internal motion of atoms.

• The faster the particles move the more heat energy there is.

• Usually results from friction.

• Thermal energy causes temperature and phase changes.

What is chemical energy?

• A form of potential energy that is stored in fuels, food, and other chemicals and that is released by chemical reactions.

• The energy is stored in the chemical bonds of molecules when the bonds are broken energy is released.

What is electrical energy?

• Energy due to the motion of electrons.

• Electrical energy is used to run electrical devices.(electricity)

What is sound energy?

• Considered a form of kinetic energy.

• Caused by an objects vibrations.

What is light energy?

• Energy associated with light.

• This energy travels in waves and has some electrical and some magnetic properties.

• Examples: visible light, ultraviolet radiation, microwaves, infrared radiation

What is nuclear energy?

• The potential energy formed in the nucleus of an atom. – Fission: splits the nucleus to release energy

• Nuclear reactors

– Fusion: Joins the nucleus together releasing tremendous amounts of energy.

• Example: the sun

Chapter 5 Section 2Energy Conversions

What is an energy conversion?

• A change from one form of energy to another.

• Kinetic energy is converted to potential energy when an object is moved against gravity.

• Elastic potential energy is another example of potential energy.– EX: energy is stored by stretching a rubber

band. It is released when the rubber band goes back to its original shape.

• Your body uses the food you eat to convert chemical energy into kinetic energy.

• Plants convert light energy into chemical energy.

• Machines can transfer energy and can convert energy into a more useful form.