The student will:
- Describe what force is and different types of forces
- Understand the meaning of inertia and Newton's First Law
- force: Any effect on the motion of another object. This includes pushing and pulling, as well as resistance to being moved across or through.
- inertia: The resistance of any object to changing its state of motion, equal to its mass.
- mass: A measure of the amount of matter in an object. Weight on Earth's surface is based on mass, but an object's mass is the same wherever it is taken.
- net force: The combination of all the forces on a single object.
Introduction to Newton
Isaac Newton was a 17th century scholar, scientist, and mathematician who formalized our present understanding of force. Our everyday experience is that moving objects always tend to stop, but Newton proved that this was the result of other things getting in the way – like air resistance or friction. If you slide a book across a table, it comes to a stop. If you slide it along ice, it will go further before it stops. In the absence any opposing force, an object will slide forever. Newton's work showed that the “natural” state of moving objects in the absence of an opposing force is not rest, but continuous motion.
In everyday English, we use “force” to mean pushing or being pushy. For example, someone is called “forceful” if they insist on getting their way. In physics, pushing is a force, but force also means resisting being pushed. When you are standing, the ground is exerting force on you that keeps you from falling. The ground is not doing anything, but is still exerting a force known as a normal force.
Common types of forces include:
- The normal force is an object's resistance to things going through it. The force always points away from the flat surface – known as the normal or perpendicular to that surface.
- Friction is a force that a surface exerts when something tries to slide across it. A hockey rink has a low friction force. That means it’s easy to slide across it. A football field has a medium friction force – if you fall you may slide a little on the grass. A tennis court has a high friction force – you won’t slide at all if you fall, though you may tumble. The friction force also depends on your weight – the harder you’re pushing on the ground, the harder it is for you to slip.
- Air resistance is the force of drag that the air has on things moving through it, like the force on your hand held out the window of a moving car. The force depends on how fast you’re going. If the car is going faster, your hand is pushed back harder. It also depends on the area facing the wind. If you hold your palm sideways, the wind pushes it harder than if you turn your palm down. A football going point-first has less air resistance than one that’s sideways or tumbling.
These three together are known as contact forces. Other forces include:
- Gravity is force that acts at a distance between any two objects. The more massive the object, the greater the force of gravity it exerts. In everyday life, only Earth itself is large enough to create noticeable force, but sensitive instruments can detect the pull of gravity from a mountain or other feature.
- Other fundamental forces include electrical force, magnetic force, and nuclear forces. These will be studied in other contexts.
A net force is not a type of force, but rather the combination of all the forces on a given object.
Newton’s First Law: Inertia
Galileo formulated what we now call Newton’s First Law of Motion.
Newton’s First Law of Motion: An object remains at rest or in a state of uniform motion unless acted upon by an unbalanced force.
A very important idea is implicit in Galileo’s statement. Objects at rest and objects in uniform motion (constant velocity) are equivalent. Both states – rest and uniform motion – are arbitrary, they are, in effect, interchangeable. Any frame of reference (reference frame) which can be said to exhibit a state of “at-rest” or uniform motion, with respect to any other frame of reference is said to be an inertial frame of reference. We usually associate a coordinate system with a frame of reference. If you’re standing on a street corner and a bus passes, you see the passengers on the bus in motion and they see you in motion. If you’re seated on the bus and a passenger gets up from her seat and walks down the length of the bus, you assume you’re at rest and she’s in motion.
Typically, an at-rest reference frame is understood to mean a frame of reference attached to Earth. (In fact, the Earth is not a perfectly inertial frame of reference since it rotates, but it’s a good approximation for most uses.) Any reference frame moving with constant velocity relative to you can be used as an at-rest reference frame. If you’re in an elevator moving with constant velocity, up or down, and conduct an experiment to determine the acceleration of gravity, you’ll measure the same value of the acceleration if you conduct the experiment standing on Earth – no matter what experiment you use! All inertial frames give rise to the same laws of physics.
Check Your Understanding
1. You’re in an elevator which is moving upward with a constant velocity of 3.0 m/s. You release a ball from waist height. You then perform the same experiment when you’re standing on the ground. The time of fall to the elevator floor compared to the time of fall to the ground is:
c. The same
Answer: C. All experimental results are the same regardless of which inertial frame is used to conduct the experiment.
Inertia is one of the most baffling ideas in physics. One of the common choices in the question above is option A. Many people reason that since the elevator floor is moving upward, the ball will impact it sooner than it would the ground. Perhaps we can explain what is happening in the elevator by asking what we would see while standing on the Earth and viewing only the motion of the ball. Initially, we see the ball rising with a constant velocity of 3 m/s. At the instant the ball is dropped, we see the ball begin to slow down, as it continues to briefly ascend no differently than had the ball left own hand with a velocity of 3 m/s. The acceleration of gravity that the ball experiences is the same whether you toss the ball into the air or simply drop the ball. In the reference frame of the elevator, the ball simply appears to drop to the floor. The elevator floor is no more “rushing up” to meet the ball then the “stationary” Earth does. (We will be more precise with this statement when we discuss Newton’s Third Law.)
We associate inertia with mass: the more mass, the more inertia. The more mass an object has, the more force is required to alter its state of motion. Pulling a table cloth out from under the dishes, putting a coin on your elbow and quickly retracting your elbow and catching the coin, and removing the support under a coin with a fast horizontal motion allowing the coin to drop through an opening it was suspended above, are all examples of tricks where an object’s inertia is responsible for the perceived magic. An unfortunate example of inertia in action is so-called “whiplash injury.” Suppose a moving car rear-ends another car that is at rest. Imagine that the driver’s seat of the “front” car does not have a head rest. In that case, the car, and the driver’s body, lurch forward, but the driver’s head tends to stay in place due to its inertia, straining the neck. The resulting damage to the neck is called a whiplash injury. This bit of physics often creeps into the courtroom.
It’s not uncommon in science fiction shows to show a space ship “towing” another disabled ship. As long as the towing ship is moving with constant velocity, there is no reason to keep towing. The disabled ship’s inertia will maintain the same velocity since there is negligible friction in deep space. In reality, we can send unmanned probes into deep space with only enough fuel to break out of Earth's orbit. Once the probe is moving fast enough to escape the Earth’s gravity, its thrusters are turned off for months or years depending upon how far the journey.
The New Horizons space probe will spend most of its year journey traveling nearly 60,000 km/h without a need of fuel as a result of its inertia. It will arrive at Pluto July 14, 2015.
We use the symbol to mean force in physics and we use the Greek letter sigma, , read as “the sum of” to express the condition under which Newton’s First Law holds, that is:
; the sum of all forces acting on an object is zero (in other words, the net force, on the object is zero, ). Under this condition the object may be at rest or have a constant velocity.
Check Your Understanding
A student holds a physics text book out the window of a helicopter ascending with a speed of 10 m/s. When the helicopter reaches a height of 100 m, she releases the text. The highest position above the ground that the book achieves is:
a. 100 m
b. Greater than 100 m
Answer: The correct answer is B. Due to the book’s inertia it continues moving upward after being released. The unbalanced force of gravity slows the book down until it reaches its highest position above the ground. (The book’s highest position is approximately 105 m.)