Forces Chapter 10

Section 10-1 The Nature of Force What is a force? - a push or a pull

Unbalanced Forces Unbalanced forces - - can cause an object to start moving stop moving, or change direction. - acting on an object will change the object’s motion.

Unbalanced forces in the same direction Notice the arrows

Unbalanced forces in the opposite direction

Balanced Forces Balanced forces- - acting on an object will NOT change the object’s motion.

Balanced forces in the opposite direction

NO motion

Newton’sNewton’s First Law of Motion States that: an object at rest will remain at rest & an object in motion at a constant velocity will continue moving at constant velocity unless acted upon by an unbalanced force. INERTIA - the tendency of an object to resist change of its motion.

Inertia

INERTIA What will happen here? How does it demonstrate inertia?

INERTIA How does this picture demonstrate inertia?

Mass - the amount of matter an object contains The amount of INERTIA an object has depends on its mass. Which jar would be harder to move? Greater mass = greater inertia Mass then can be defined as a measure of the inertia of an object.

Section Force, Mass, & AccelerationAcceleration Newton’s Second Law of Motion states that: the net force on an object is equal to the product of its acceleration and its mass. Force = mass x acceleration

Newtons 1 N = the force required to accelerate 1 kg of mass at 1 m/s/s 1 N = 1kg x 1m/s/s Newtons are the unit that measure force.

Newton’s Second Law of Motion Mass x acceleration = Force N’s 2nd law shows how mass, force, & acceleration are related.

Newton’s Second Law of Motion What would happen to the force required to move the wagon if the people got out?

Section Friction & Gravity Friction - the force that OPPOSES motion. The force that one surface exerts on another when the two rub against each other. Friction explains why objects will roll or slide to a stop eventually. (Opposes motion- remember)

Friction - The strength of friction depends on two factors: - types of surfaces - how hard surfaces push together

Friction These skiers can travel very fast because of very little friction between their skis & the snow. What if they were being pulled over rocks?

Friction Is friction a bad thing?? Friction enables us to: - WALK - LIGHT A MATCH - STOP OUR CARS - WRITE - KEEP ALL KINDS OF THINGS FROM SLIDING AROUND!

3 Types of Friction SLIDING FRICTION - when solid surfaces slide over each other.

3 Types of Friction ROLLING FRICTION - when an object rolls over a surface.

3 Types of Friction Ball bearings are often used in producing ROLLING FRICTION - because much less force is needed to overcome rolling friction than sliding friction.

3 Types of Friction FLUID FRICTION - occurs when an object moves through a fluid. (A liquid or a gas)

FLUID FRICTION Why?

3 Types of Friction The force needed to overcome fluid friction is usually less than that needed to overcome sliding friction. We use fluids called lubricants to reduce contact of surfaces, reducing friction in machine parts and other things.

GRAVITY Gravity is a force like friction but this force induces motion. Sir Isaac Newton recognized gravity as the force that pulls objects straight down towards the center of the earth.

Newton’s Universal Law of Gravitation States that: all objects in the universe attract each other by the force of gravity. The size of the force depends on 2 factors: the masses of the objects and the distance between them.

Gravity acts on all objects in the universe. It is what keeps our moon orbiting around the earth.

In fact, gravity holds our solar system together as well as the whole universe!

GRAVITY Free fall - when the only force acting upon an object is gravity. - an object accelerates as it falls - near the surface of the earth, objects accelerate at 9.8 m/s/s

GRAVITY - object’s velocity increases as it falls so add 9.8 m/s for every second it falls (after 10 seconds, it is falling at 98m/s)

GRAVITY - Projectile Motion - happens when an object is thrown or projected horizontally.

You have 2 balls, to be released at the exact same time. One will just be dropped, the other will be thrown horizontally. (A projectile object) So which will hit the ground first?

The balls both hit the ground at the same time because gravity acts upon them with the same force.

What is this picture showing? Hint - a very important concept that Galileo proved in the leaning Tower of Pisa.

So the picture shows that objects fall at the same rate here on earth - even if they have different masses.

Which one will hit the water first?

Right - the elephant! (I know you’re arguing that objects fall at the same rate on earth. So they should hit at the same time.) BUT there’s a little something that will slow one of them down...

Gravity & Air Resistance Objects fall through the air here on earth. Even though it is rather thin it can & does resist an object’s movement through it.

The leaf will fall slower than the acorn because of the air pushing on it has a greater effect & slows its fall.

If the leaf and the acorn were dropped in a vacuum tube, they would fall at exactly the same rate.

Explain these pictures.

Terminal Velocity If a falling object can fall far enough, it will eventually achieve terminal velocity. This is as fast as an object can fall through a fluid.

Air resistance will stop the object’s acceleration. Terminal velocity for the average human body through air is about 150 mph. What about water? Would an object sink to the bottom of the ocean?

Weight Weight is a measure of the force of gravity on an object, and mass is a measure of the amount of matter in that object. Weight = mass x acceleration due to gravity So weight is measured in newtons since it is a measurement of force.

Our moon is only a sixth of the earth’s size. So you would weigh less there because there is less GRAVITY to act upon your mass.

How much would you weigh on Jupiter? HINT: it’s 2.5 times larger than earth. These small black spots are from the Shoemak er-Levy comet impact.

REMEMBER: Gravity is affected by 2 factors: - SIZE of objects - DISTANCE between objects

Section Action & Reaction Newton’s 3rd Law of Motion states that: that for every action there is an equal and opposite reaction.

Newton’s 3rd Law of Motion Newton realized that forces are not one sided but are paired. For every action there is a reaction.

Newton’s 3rd Law of Motion This squid will squeeze water through its siphon tube (the action). This jet of water will cause it to move in the opposite direction in reaction.

Newton’s 3rd Law of Motion If one skater pushes against the other, what will happen? What if it’s a BIG push, little push?

Newton’s 3rd Law of Motion (Suzy-Q can’t help but obey Newton’s 3rd Law of Motion. She should have paid closer attention in science class!) Explain what’s happened.

Newton’s 3rd Law of Motion How does Newton’s 3rd law apply here?

Do Action-Reaction Forces Cancel each other out? Remember: the net force of equal & opposite, “balanced” forces is 0. (No motion) The volley ball players force against the ball cancels out. The ball won’t move.

Do Action-Reaction Forces Cancel each other out? Newton’s 3rd Law refers to forces on 2 objects. This players arms exert an upward force on the ball (action.) The returns a reaction force. One force is on the ball, one is on the player. Forces can only be added together if they are acting on the same object.

Momentum Newton called the the “quantity of motion” The momentum of an object is the product of its mass and its velocity. Momentum = Mass X Velocity

Momentum What kind of crash would it be if this car was going slower? What would be changed mass or velocity?

Momentum Imagine a bullet someone throws versus one shot from a gun. How about if a dog were walking and bumped into you? What if an elephant was walking the same speed and bumped into you. A VW and a Mack truck are rolling on the street. Which could you jump in front of and stop? Why?

Momentum What is the momentum of the yellow car? What is the momentum of the red car? What is the total momentum of the system?

Momentum If the two cars collide and stick together, what will the direction of their resulting motion be?

Momentum The yellow car has a greater momentum (because even though the cars have the same mass the yellow has a greater velocity) and so will push the red car back.

Yellow car’s momentum = 14 m/s x 10 kg 140 kg-m/s or 140 N Red car’s momentum = 10 m/s x 10 kg 100 kg-m/s or 100N Total system’s momentum = 240 kg/m/s

Momentum Law of Conservation of Momentum states that: the total momentum of any group of objects remains the same unless outside forces act on the objects.

Section Orbiting satellites A satellite is any object that travels around another object in space. The moon is the earth’s satellite. The earth is the sun’s satellite.

A rocket can rise into the air because the gases it expels with a downward force exert an equal but opposite force on the rocket. Which of the laws of motion is this?

The Space Shuttle becomes a satellite once it is circling (orbiting) earth. Eventually the shuttle’s path will tilt until it is parallel but miles above the earth’s surface.

Satellite Motion Newton wondered what would happen if you stood on a high mountain & were able to throw a stone as fast as you wanted. The faster you threw it, the further away it would land. At a certain speed, the stone’s path would match the curve of the earth. Although the earth’s gravity would continually pull the stone towards it, the ground would curve away at the same rate. So the stone would circle around the earth.

Satellite Motion In order for a ball to be “thrown” into & stay in orbit, it must achieve a speed of about 7,900 m/s - or 20 x’s as fast as a pitcher can throw a ball.

Circular Motion Any force that causes an object to move in a circle is called centripetal force. Centripetal means center seeking. Remember: Acceleration is any change of velocity or direction.

A satellite falls around earth instead of into it. The speed of a satellite is very important. It must be fast enough to maintain it altitude. If it slows down what will happen to it?

Satellite Location Some satellites are in low orbits, like the space shuttle. It takes about 90 minutes to orbit the earth.

Higher orbits (36,000 km) have satellites, that travel slower. Ex. - communications satellites high above the earth’s equator, travel at the same rate that the earth rotates so the satellite remains in a fixed position relative to earth.

The End