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 ForceForce is defined as a push or pull acting on an object. There are several fundamental forces in the universe, 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.

Q 1 : What forces act on Carson’s scooter?

Force and MotionForces cause all motions. Every time 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 speed or direction or both. A change in the speed or direction of an object is called acceleration. Look at Carson’s brother Colton in the Figure 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 other vectors, a force can be represented with an arrow. You can see some examples in the Figure below . 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

A vector is a numerical value in a specific direction, and is used in both math and physics. The force vector describes a specific amount of force and its direction. You need both value and direction to have a vector. Both. Very important. Scientists refer to the two values as direction and magnitude(size). The alternative to a vector is a scalar. Scalars have values, but no direction is needed. Temperature, mass, and energy are 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 them end 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 the Resultant 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 no acceleration takes place. The forces are balanced. Balanced forces cause objects to keep the current type of motion or keep the same velocity. 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 these types of forces unbalanced.

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 figure below shows 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 or gravitational forces are 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 has mass exerts a gravitational pull, or force, 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 planet orbiting the 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 a swell in 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 same acceleration on 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 force accelerates objects 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 of air 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 (a vacuum), 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 up revolving around 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, but tangent to 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 a force that 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 the opposite direction 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. Cars hydroplane when they go too fast on puddles of water.

Friction and Gases

Friction only happens with solid objects, but you do get resistance to 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 as air 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 (less viscous) 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 high coefficient 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. Teflon is 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 objectsTake 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 objectsTake 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 surfaceConsider 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

A force is 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 will only accelerate if there is “net force” acting upon it. A net force is 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 5 Newtonsand drag is 3 Newtons, the net force is 2 Newtons, forward. Motion to the right is positive. Motion to the left is negative.

As we have said before, a net force is 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 15 Newtons, 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. An unbalanced 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 10 Newtons, forward. The mass will accelerate.

What is the net force if the wheels of the car apply 10 Newtons but a parachute applies 7 Newtons in the other direction?

The net force would equal 3 Newtons, forward. The mass will accelerate.

A rocket applies an additional force of 10 Newtons to the 10Newtons that are applied by the wheels. What is the net force if the parachute continues to apply 7 Newtons in the other direction?

The net force would equal 13 Newtons, 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 named Sir Isaac Newton. A little bit stuffy, bad hair, but quite an intelligent guy. He worked on developing calculus and physics at the same time. During his work, he came up with the three basic ideas that are applied to the physics of most motion (NOT modern physics). The ideas have been tested and verified so many times over the years, that scientists now call them Newton’s Three Laws of Motion.

First Law

The first law says that an object at rest tends to stay at rest, and an object in motion tends to stay in motion, with the same direction and speed. Motion (or lack of motion) cannot change without an unbalanced force acting. 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 of astronauts. 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 gravity pulled 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 term inertia to 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 sumo wrestler and the boy shown below.

Which person in this ring will be harder to move? The sumo wrestler or the little boy?

Let's say the wrestler on the left has a mass of 136 kilograms, and the boy on the right has a mass of 30 kilograms (scientists measure mass in kilograms). Remember the object of sumo wrestling is to move your opponent from his position. Which person in our example would be easier to move? Common sense tells you that the boy would be easier to move, or less resistant to inertia.

You experience inertia in a moving car all the time. In fact, seatbelts exist in cars specifically to counteract the effects of inertia. Imagine for a moment that a car at a test track is traveling at a speed of 55 mph. Now imagine that a crash test dummy is inside that car, riding in the front seat. If the car slams into a wall, the dummy flies forward into the dashboard. Why? Because, according to Newton's first law, an object in motion will remain in motion until an outside force acts on it. When the car hits the wall, the dummy keeps moving in a straight line and at a constant speed until the dashboard applies a force. Seatbelts hold dummies (and passengers) down, protecting them from their own inertia.

STATION 8: NEWTON’S SECOND LAW OF MOTION (F = M x A)

Isaac Newton’s First Law of Motion states, “A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.” What, then, happens to a body when an external force is applied to it? That situation is described by Newton’s Second Law of Motion. It states, “The force acting on an object is equal to the mass of that object times its acceleration.” This is written in mathematical form as:

F = ma

F is force, m is mass and a is acceleration. The math behind this is quite simple. If you double the force, you double the acceleration, but if you double the mass, you cut the acceleration in half.

When a constant force acts on a massive body, it causes it to accelerate, i.e., to change its velocity, at a constant rate. In the simplest case, a force applied to an object at rest causes it to accelerate in the direction of the force. However, if the object is already in motion, or if this situation is viewed from a moving inertial reference frame, that body might appear to speed up, slow down, or change direction depending on the direction of the force and the directions that the object and reference frame are moving relative to each other.

The bold letters F and a in the equation indicate that force and acceleration are vector quantities, which means they have both magnitude and direction. The force can be a single force or it can be the combination of more than one force. In this case, we would write the equation as:

∑F = ma

The large Σ represents the vector sum of all the forces, or the net force, acting on a body.

It is rather difficult to imagine applying a constant force to a body for an indefinite length of time. In most cases, forces can only be applied for a limited time, producing what is called impulse. For a massive body moving in an inertial reference frame without any other forces such as friction acting on it, a certain impulse will cause a certain change in its velocity. The body might speed up, slow down or change direction, after which, the body will continue moving at a new constant velocity (unless, of course, the impulse causes the body to stop).

There is one situation, however, in which we do encounter a constant force — the force due to gravitational acceleration, which causes massive bodies to exert a downward force on the Earth. In this case, the constant acceleration due to gravity is written as g, and Newton’s Second Law becomes F = mg. Notice that in this case, F and g are not conventionally written as vectors, because they are always pointing in the same direction, down.

The product of mass times gravitational acceleration, mg, is known as weight, which is just another kind of force. Without gravity, a massive body has no weight, and without a massive body, gravity cannot produce a force. In order to overcome gravity and lift a massive body, you must produce an upward force ma that is greater than the downward gravitational force mg.

Now that we know how a massive body in an inertial reference frame behaves when it subjected to an outside force, what happens to the body that is exerting that force? That situation is described by Newton’s Third Law of Motion.

STATION 9: NEWTON’S THIRD LAW OF MOTION

Newton’s Third Law of Motion states, “For every action, there is an equal and opposite reaction.”

Forces always occur in pairs; when one body pushes against another, the second body pushes back just as hard. For example, when you push a cart, the cart pushes back against you; when you pull on a rope, the rope pulls back against you; and when gravity pulls you down against the ground, the ground pushes up against your feet.

If one object is much, much more massive than the other, particularly in the case of the first object being anchored to the Earth, virtually all of the acceleration is imparted to the second object, and the acceleration of the first object can be safely ignored. For instance, if you were to plant your feet and throw a baseball to the west, you would not have to consider that you actually caused the rotation of the Earth to speed up slightly while the ball was in the air. However, if you were standing on roller skates, and you threw a bowling ball forward, you would start moving backward at a noticeable speed.

One might ask, “If the two forces are equal and opposite, why don’t they cancel each other out?” Actually, in some cases they do. Consider a book resting on a table. The weight of the book pushes down on the table with a force mg, while the table pushes up on the book with an equal and opposite force. In this case, the forces cancel each other because the book does not accelerate. The reason for this is that both forces are acting on the same body, while Newton’s Third Law describes two different bodies acting on each other.

Consider a horse and a cart. The horse pulls on the cart, and the cart pulls back on the horse. The two forces are equal and opposite, so why does the cart move at all? The reason is that the horse is also exerting a force on the ground, which is external to the horse–cart system, and the ground exerts a force back on the horse–cart system causing it to accelerate.

How, then, can a rocket move through space if there is nothing for it to push against? When the fuel is ignited in the rocket nozzle, the gas expands rapidly in all directions. Some of it goes backwards and has no effect on the rocket; however, some if it goes forward and crashes into the back of the rocket exerting a force that causes the rocket to accelerate in the forward direction. This is why Newton’s Third Law is considered to be the fundamental principle of rocket science.