Station 1: What Is a Force ?

STATION 1: WHAT IS A “FORCE”?

Carson has been riding a scooter for almost as long as he can remember. As you can see, he’s really good at it. He can even do tricks in the air. It takes a lot of practice to be able to control a scooter like this. Carson automatically applies just the right forces to control his scooter.

Defining Force

Forceis defined as a push or pull acting on an object. There are several fundamental forces in theuniverse, including the force of gravity, electromagnetic force, and weak and strong nuclear forces. When it comes to the motion of everyday objects, however, the forces of interest are gravity,friction, and applied force. Applied force is force that a person or thing applies to an object.

Q1: What forces act on Carson’s scooter?

Force and Motion

Forces cause all motions. Everytime the motion of an object changes, it’s because a force has been applied to it. Force can cause a stationary object to start moving or a moving object to change its speedor direction or both. A change in the speed or direction of an object is called acceleration. Look at Carson’s brother Colton in theFigure below.. He’s getting his scooter started by pushing off with his foot. The force he applies to the ground with his foot starts the scooter moving in the opposite direction. The harder he pushes against the ground, the faster the scooter will go.

How much an object accelerates when a force is applied to it depends not only on the strength of the force but also on the object’s mass. For example, a heavier scooter would be harder to accelerate. Colton would have to push with more force to start it moving and move it faster.

STATION 2: FORCE AS A VECTOR

Force is a vector, or a measure that has both size and direction. For example, Colton pushes on the ground in the opposite direction that the scooter moves, so that’s the direction of the force he applies. He can give the scooter a strong push or a weak push. That’s the size of the force. Like othervectors, a force can be represented with an arrow. You can see some examples in theFigurebelow. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force.

Vector Basics

Avectoris a numericalvaluein a specificdirection, and is used in both math and physics. The force vector describes a specific amount offorceand its direction. You need both value and direction to have a vector. Both.Very important. Scientists refer to the two values as direction andmagnitude(size). The alternative to a vector is a scalar.Scalarshave values, but no direction is needed. Temperature, mass, andenergyare examples of scalars.
When you see vectors drawn in physics, they are drawn as arrows. The direction of the arrow is the direction of the vector, and the length of the arrow depends on the magnitude (size) of the vector.
We're hoping you know how to add and subtract. Scientists often use vectors to represent situations graphically. When they have many vectors working at once, they draw all the vectors on a piece of paper and put themend to end. When all of the vectors are on paper, they can take the starting and ending points to figure out the answer. The final line they draw (from the start point to the end point) is called theResultant vector. If you don't like to draw lines, you could always use geometry and trigonometry to solve the problems. It's up to you. Unlike normal adding of numbers, adding vectors can give you different results, depending on the direction of the vectors.

STATION 3: NET FORCE

Let’s use the example of a Tug-of-war to understand how forces work together. When the game starts, if two forces are equal, such as two people of equal weight and strength pulling on opposite ends of a rope, then noaccelerationtakes place. The forces are balanced. Balanced forces cause objects to keep the current type of motion or keep the samevelocity. If we add a second person to one side of the tug-of-war, then the force generated by the two people will be greater than that generated by the one person, and that force will cause an acceleration. We call thesetypes of forcesunbalanced.

When an object has many forces acting on it at one time, scientist can pretend that all the forces act as only one force. This is called the net force. The net force is the sum of all the forces acting on an object. If two forces act in the same direction on an object, then the forces are added together. If two forces act in opposite directions on an object, then the forces subtract. Balanced forces will always have a net force of zero netwons. The figurebelowshows a box with balanced forces acting on it. Unbalanced forces will never have a net force of zero newtons.

STATION 4: GRAVITY AS A FORCE

Gravity orgravitational forcesare forces of attraction. We're not talking about finding someone really cute and adorable. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work.
Every object in the universe that hasmassexerts a gravitational pull, orforce, on every other mass. The size of the pull depends on the masses of the objects. You exert a gravitational force on the people around you, but that force isn't very strong, since people aren't very massive. When you look at really large masses, like the Earth and Moon, the gravitational pull becomes very impressive. The gravitational force between the Earth and the molecules of gas in the atmosphere is strong enough to hold the atmosphere close to our surface. Smaller planets, that have less mass, may not be able to hold an atmosphere.

Planetary Gravity

Obviously, gravity is very important on Earth. The Sun's gravitational pull keeps our planetorbitingthe Sun. The motion of the Moon is affected by the gravity of the Sun AND the Earth. The Moon's gravity pulls on the Earth and makes the tides rise and fall every day. As the Moon passes over the ocean, there is aswellin the sea level. As the Earth rotates, the Moon passes over new parts of the Earth, causing the swell to move also. The tides are independent of the phase of the moon. The moon has the same amount of pull whether there is a full or new moon. It would still be in the same basic place.
We have to bring up an important idea now. The Earth always produces the sameaccelerationon every object. If you drop an acorn or a piano, they will gain velocity at the same rate. Although the gravitational force the Earth exerts on the objects is different, their masses are just as different, so the effect we observe (acceleration) is the same for each. The Earth's gravitational forceacceleratesobjects when they fall. It constantly pulls, and the objects constantly speed up.

They Always ask About Feathers

People always say, "What about feathers? They fall so slowly." Obviously, there is air all around us. When a feather falls, it falls slowly because the air is in its way. There is a lot ofair resistanceand that resistance makes the feather move slower. The forces at work are the same. If you dropped a feather in a container with no air (avacuum), it would drop as fast as a baseball.
What About the Moon?

But what keeps the Moon from falling down, if all of this gravity is so strong? Well, the answer is that the moon IS falling; all the time, but doesn't get any closer to us! Remember that if there wasn't a force acting, the Moon would be traveling in a straight line. Because there IS a force of attraction toward the Earth, the moon "falls" from a straight line into a curve (orbit) around the Earth and ends uprevolvingaround us. The Earth's gravity holds it in orbit, so it can't just go off in a straight line. Think about holding a ball on a string and spinning it in a circle. If you were to cut that string (no more gravity), the ball would fly off in a straight line in the direction it was going when you cut the string. That direction, by the way, is not directly away from your hand, buttangentto the circle. Tangent is a geometry term used to describe a direction that are related to the slope of a curve. Math stuff. The pull of the string inward (toward your hand) is like the Earth's gravitational pull (inward toward the center of the Earth).

STATION 5: FRICTION AS A FORCE

Friction is aforcethat holds back the movement of a sliding object. That's it. Friction is just that simple.You will find friction everywhere that objects come into contact with each other. The force acts in theoppositedirection to the way an object wants to slide. If a car needs to stop at a stop sign, it slows because of the friction between the brakes and the wheels. If you run down the sidewalk and stop quickly, you can stop because of the friction between your shoes and the cement.
What happens if you run down the sidewalk and you try to stop on a puddle? Friction is still there, but the liquid makes the surfaces smoother and the friction a lot less. Less friction means it is harder to stop. The low friction thing happens to cars when it rains. That's why there are often so many accidents. Even though the friction of the brakes is still there, the brakes may be wet, and the wheels are not in as much contact with the ground. Carshydroplanewhen they go too fast on puddles of water.

Friction and Gases

Friction only happens with solid objects, but you do getresistanceto motion in both liquids and gases. This doesn't involve sliding surfaces like friction does, but is instead the kind of resistance you get if you try to push your way through a crowd. It's a colliding situation, not a sliding one. If the gas is air, this is referred to asair resistance.
If you were in the space shuttle and re-entering the atmosphere, the bottom of the shuttle would be getting very hot. The collisions that occur between the molecules of the air being compressed by the shuttle, heat up the air AND the shuttle itself. The temperature on the top of the shuttle is also warm, but nowhere near the temperatures found on the bottom.

Friction and Liquids

Although liquids offer resistance to objects moving through them, they also smooth surfaces and reduce friction. Liquids tend to get thinner (lessviscous) as they are heated. Yes, that's like the viscosity of the oil you put in your car. Car engines have a lot of moving parts, and they rub on each other. The rubbing produces friction and the result is heat. When oil is added to a car engine, the oil sticks to surfaces, and helps to decrease the amount of friction and wear on the parts of the engine. An engine that runs hotter requires a more viscous oil in order for it to stick to the surfaces properly.

Measuring Friction

Measures of friction are based on the type of materials that are in contact. Concrete on concrete has a very highcoefficient of friction. That coefficient is a measure of how easily one object moves in relationship to another. When you have a high coefficient of friction, you have a lot of friction between the materials. Concrete on concrete has a very high coefficient, and Teflon on most things has a very low coefficient.Teflonis used on surfaces where we don't want things to stick; such as pots and pans.
Scientists have discovered that there is even less friction in your joints than in Teflon! It is one more example at how efficient living organisms can be.

STATION 6: BALANCED FORCES .

Balanced forces are two forces acting in opposite directions on an object, and equal in size. Anytime there is a balanced force on an object, the object stays still or continues to move at the same speed and in the same direction. It is important to note that an object can be in motion even if there are no forces acting on it.
Balanced forces can be demonstrated in Hanging, Floating and Standing/sitting objects
Hanging objects
Take a look at this hanging glass bulb shade. The weight of the bulb shade pulls down and the tension in the cable pulls up. The forces pulling down and pulling up can be said to be in balance.
Floating objects
Take a look at this log floating on a pool of water. It is floating because the weight of the log is balanced by the upthrust from the water. If more weight is tied to the log, the force pulling it down may be more and will cause it to sink.
Standing/Sitting on a surface
Consider a metal block resting on a surface of a table. Its' weight is balanced by the reaction force from the surface. The surface pushes up against the metal block, balancing out the weight (force) of the metal block.

STATION 7: UNBALANCED FORCES = NET FORCES

Aforceis a push or a pull that is capable of changing the velocity of a mass. Forces are measured in “Newtons” or “N”, in honor of Sir Isaac Newton. According to Mr. Newton, an object willonly accelerate if there is “netforce” acting upon it.Anet forceis the sum of all forces acting on an object. A net force is capable of accelerating a mass. For instance, if the wheels of a car push it forward with 5Newtonsand drag is 3Newtons, the net force is 2Newtons, forward. Motion to the right is positive. Motion to the left is negative.

As we have said before, anet forceis the sum of all forces acting on an object. Look at the picture of the red plane. In this example, the difference between drag and thrust is 15Newtons, to the left. This net force is capable of accelerating (slowing down) the plane.Net forces always accelerate masses.

If an object has a net force acting on it, it will accelerate. The object will speed up, slow down or change direction. Anunbalanced force(net force) acting on an object changes its speed and/or direction of motion. An unbalanced force is an unopposed force that causes a change in motion. A net force = unbalanced force.If however, the forces are balanced (in equilibrium) and there is no net force, the object will not accelerate and the velocity will remain constant.

Unbalanced Forces:Placing a box on the seesaw unbalances it. The weight of the box is the unbalanced or net force which causes the seesaw to accelerate downward until it hits the ground. /
/ Let’s assume that the wheels of a car apply 10 N of force. What is the net force if friction and drag are negligible?
The net force would equal 10Newtons, forward.The mass will accelerate.
/ What is the net force if the wheels of the car apply 10Newtonsbut a parachute applies 7Newtonsin the other direction?
The net force would equal 3Newtons, forward.The mass will accelerate.
/ A rocket applies an additional force of 10Newtonsto the 10Newtonsthat are applied by the wheels. What is the net force if the parachute continues to apply 7Newtonsin the other direction?
The net force would equal 13Newtons, forward. The mass will accelerate.

STATION 7: NEWTON’S FIRST LAW OF MOTION (LAW OF INERTIA)

Newton's Laws of Motion

There was this fellow in England namedSir Isaac Newton. A little bit stuffy, bad hair, but quite an intelligent guy. He worked on developingcalculusandphysicsat the same time. During his work, he came up with the three basic ideas that are applied to the physics of mostmotion(NOT modern physics). The ideas have been tested and verified so many times over the years, that scientists now call themNewton’s Three Laws of Motion.

First Law

The first law says that an object atrest tends to stay at rest, and an object inmotiontends to stay in motion, with the same direction andspeed. Motion (or lack of motion) cannot change without an unbalancedforceacting. If nothing is happening to you, and nothing does happen, you will never go anywhere. If you're going in a specific direction, unless something happens to you, you will always go in that direction. Forever.
You can see good examples of this idea when you see video footage ofastronauts. Have you ever noticed that their tools float? They can just place them in space and they stay in one place. There is no interfering force to cause this situation to change. The same is true when they throw objects for the camera. Those objects move in a straight line. If they threw something when doing a spacewalk, that object would continue moving in the same direction and with the same speed unless interfered with; for example, if a planet's gravitypulled on it (Note: This is a really simple way of describing a big idea. You will learn all the real details - and math - when you start taking more advanced classes in physics.).

Physicists use the terminertiato describe this tendency of an object to resist a change in its motion. The Latin root for inertia is the same root for "inert," which means lacking the ability to move. So you can see how scientists came up with the word. What's more amazing is that they came up with the concept. Inertia isn't an immediately apparent physical property, such as length or volume. It is, however, related to an object's mass. To understand how, consider the sumowrestlerand the boy shown below.