Forces and Motion

Prentice HallPhysical Science: Concepts in Action

Chapter 12

May the Force be with youForces exist all around us

There are 4 types of forces: gravitational, electromagnetic and strong and weak nuclear forces

Which do you think would be the strongest?Gravity is actually the

weakest but has a far reaching effect and nuclear forces (attractions) are the strongest with an effect only on things nearby

What is Force?

Force: a push or pull that acts on an object by changing its speed or direction

Can cause a resting object to moveCan accelerate a moving object

How is force measured?

Spring scaleStretch of the spring depends on the

mass of the object acting on itEx: vegetable scale at the grocery store

Unit of Force Newton (N)1 kg to accelerate 1 m/s2

(named after Sir Isaac Newton)

21

smkg

N

How is force represented?

Use arrowsDirectionStrength

Length represents strength or magnitude

The scale with more apples, greater mass, has a longer arrow. The arrow is pointed downward due to mass is below the balance pulling downwards.

Combining Forces

Have you ever tried to push a broken down car alone?Did you ask for help? Why?

Forces in the same direction are added together

Force in the opposite direction are subtracted

Net Force the overall force acting on an object after all the forces are combined

Balanced vs. Unbalanced Forces

BalancedCombine to produce a net force of zeroNo change in the object’s motion

Ex: tug of war, arm wrestling

UnbalancedNet force equals the size of the larger

force minus the size of the smaller forceNet force does not equal zeroCauses an object to accelerate

Representing ForcesForces can add

together or subtract from one another A- 2 forces acting in the

same direction add B- forces in opposite

directions subtract from one another

C – forces that are equal in size and opposite in direction result in no net force

Friction

All moving objects are subject to friction

Friction is a force that opposes the motion of objects that touch as they move past each otherActs at the surface where objects are in

contact4 types of friction:

Static frictionSliding friction Rolling friction Fluid friction

4 Types of FrictionStatic friction

Force that acts on objects that are not movingAlways acts in the direction

opposite to that of the applied forceEx: taking a step

Sliding frictionForce that opposes the direction of

motion of an object as it slides over a surfaceLess force is needed to keep an

object moving than to start it moving

Ex: pushing a potted plant

4 Types of Friction Rolling friction

The friction force that acts on rolling objects

Causes change in shape at the point of rolling contactEx: furniture

Fluid friction The force that opposes the motion of

an object through fluid Increases the speed of the object

moving through the fluidEx: cake batter

Fluids (gas and liquids) Fluid friction acting on an object

moving through the air is known as air resistance

Gravity

Gravity is a force that acts between two masses

Attractive force that pulls objects together

Earth’s gravity exerts a force of attraction on every other object that is near Earth

Acts downwards towards the center of the earth

Because the boulder is at rest, what do you Know about the net force acting on it?

Gravity and Falling ObjectsGravity causes objects to

accelerate downward

Air resistance (fluid friction) acts in the direction opposite to the motion and reduces acceleration

The flying squirrel to the right takes advantage of air resistance to slow its fall by increasing it’s surface area.

Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity. (in other words, top speed)

Projectile Motion

Projectile Motion: motion of a falling object after given an initial forward velocityAir resistance and gravity are the only forcesThe combination of an initial velocity and the downward

vertical force of gravity causes the ball to follow a curved path

The 2 balls fall with the same acceleration and hit the

ground at the same time

12:2 Newton’s 1st Law of Motion

Sir Isaac Newton studied motion and gave us the basic laws of motion

The 1st Law of Motion states that the motion of an object does not change as long as the net force acting on the object is zero.Sometimes also called the Law of Inertia

Inertia Tendency of an object to resist change in its motion

In other words… An object at rest stays at rest, an object in motion stays in motion at the same direction and speed (until something acts on it)

Newton’s 1st Law of Motion

Crash Dummy: Example of inertia

Newton’s 2nd Law of Motion How do unbalanced forces affect the motion of an

object? It causes the velocity to change (accelerate)Ex: you apply a net force to a ball when you throw it

The harder you throw, the more the ball accelerates.The acceleration is directly proportional to the net force acing

on it.Newton’s Second Law of Motion The acceleration of an

object is equal to the net force acting on it divided by the objects mass

Newton’s 2nd Law of Motion Mass is a measure of inertia of an object and depends

on the amount of matter the object contains If you double the mass of an object, it cuts the acceleration

in halfThe acceleration of an object is always in the same direction

as the net forceNewton’s 2nd Law applies when a net force acts in the

opposite direction of object’s motionForce produces deceleration and reduces speedEx. Seat belts

Force decelerates the passenger in order to prevent injury

Units for Acceleration are equivalent (mean the same thing)

N/kg=m/s2

Newton’s 2nd Law of Motion

Weight and Mass Are weight and mass the same?

Weight & Mass are Different

Weight The force of gravity acting on an objectProduct of the mass and acceleration

due to gravityUnit is Newtons (N)

Weight and Mass

Newton’s 2nd Law of Motion

•How would the acceleration of a chain of two carts compare with the acceleration of a single cart if the same force acted on both?•When the same force acts, the single cart accelerates 8x faster than the chain of eight carts•As mass increases, acceleration has to decrease in proportion to the increase in mass

Newton’s 2nd Law of Motion

•How would the force have to change in order to have the same acceleration for the eight carts as for one cart?•The force would have to be 8x greater

•How would another force directed to the left on the cart affect the cart’s acceleration?•The acceleration would depend on the net force. •The net force would be the force acting to the right minus the force acting to the left.

Newton’s 2nd Law of Motion

a=F/m

= 60.0 N/40.0 kg

= 1.50 m/s2

1.A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

• 2.What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?

a=F/m = 10000 N/5000 Kkg

= 2 m/s2

Newton’s 2nd Law of Motion 3.An automobile with a mass of 1200 kg accelerates at a

rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.) a=F/m F=ma = 1200 kg(3.0 m/s2) = 3600 N

• 4.A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton's second law for mass.)a=F/m m=F/a = 25 N/0.50 m/s2

= 50 k/=g

Newton’s 3rd Law of Motion & Momentum

Newton’s 3rd Law – when an object exerts a force on a second object, that object exerts an equal and opposite force on the first objectA force cannot exist alone. Forces always exist in

pairs.These are called action and reaction pairs (forces)

Action and Reaction Forces

Pressing your hand against a wall produces a pair of forcesYour hand exerts a force on the wall – the action forceThe wall exerts an equal and opposite force against your

hand – the reaction forceUsing a hammer to drive a nail into a piece of wood produces

force pairs

Hammer applies force to the nailAction forces drives nail in wall

Nail exerts equal and opposite forceThis force brings the motion of the

hammer to a stop

Action and Reaction Forces Do Not cancel

These forces that are produced by the two objects are action and reaction forces.Sometimes action and reaction forces result in

motion – sometimes they don’t.

Action and reaction forces do NOT cancel themselves out.

Only when equal and opposite forces act on the same object do they result in a net force of zero

MomentumMomentum is the product of an object’s mass

and its velocity

An object at rest has a momentum of zero.

Momentum = Mass x VelocityMeasured in kilogram-meters/second (kg m/s)p = mv (p is momentum)

An object has a large momentum if the product of it’s mass and velocity is large

Law of Conservation of Momentum

What happens to momentum when objects collide?Momentum is conserved – the total momentum does not

increase or decrease

If no net force acts on a system, then the total momentum of the system does not changeA ‘system’ is part of the world under investigation (ie two

cars colliding) The rest of the world around the system is called the

‘surroundings’

A closed system of objects means that there are no outside forces or objects acting on the objects in the system.

In a closed system, the loss of momentum by one object equals the gain of momentum by the second object.

Law of Conservation of Momentum

Universal Forces- Electromagnetic

Observations of planets, stars or other galaxies suggest that the four forces exist throughout the universe

Electromagnetic force is associated with charged particlesElectric and magnetic force are 2 different parts of

the electromagnetic forceElectric and magnetic are the only forces that both

attract and repel

Electric and Magnetic Forces

Electric forces act between charged objects such as electrons or protonsObjects with opposite charges attractObjects with like charges repelClothes often acquire electric charges in the dryer

Clothes with opposite charges tend to cling together

Magnetic forces act on some metals, poles of magnets and moving charges

Nuclear ForcesIf the nucleus of an atom has protons

(+) packed into a small space, why don’t they repel each other and break apart the nucleus?

Two forces, the strong-nuclear force and the weak-nuclear force act within the nucleus to hold it together.The strong NF overcomes the electric

force of repulsion between the protons in the nucleusActs only on neutrons and protons.

100x strong than repulsive forcesThe weak nuclear force is involved in

radioactive processes and is weaker than the strong NF

Gravitational ForceGravitational force an attractive force that acts

between any two massesWeakest universal forceMost effective over large distances

Newton’s law of universal gravitationSays that every object in the universe attracts every

other object

The greater the mass of an object, the greater the gravitational force.

The more distance between two objects, the less the gravitational force

Gravitational forces quicklab

Centripetal ForceHow is the moon kept in orbit around earth? It has

inertia and should move in a straight path until acted on by a forceEarth’s gravitational force continuously pulls the

object toward it

Centripetal force: center-directed force that constantly changes the direction of an object and causes it to move in a circle

Centripetal ForceIf the Earth causes the moon to stay in a circular

orbit, what force does the moon apply on the earth?Gravitational pull from moon causes two bulges in

earth’s oceansEarth rotates 2x a day below these bulges resulting

in two high and two low tides per day on Earth

Examples affected by centripetal force:Satellites

Examples (there are too many to list) of satellite uses include:

communicationtracking Earth’s weather and climate