Newton's Laws of Motion Study Guide (page 2)

Updated on Sep 28, 2011

Example 1

A 1,000 kg car starts from a traffic light and moves accelerated in the NE direction with an acceleration of 0.45 m/s2. Find out the resultant force on the car.

Solution 1

in NE direction

F = m · a

1,000 kg · 0.45 m/s2 = 450 N

F = 450 N in NE direction

Example 2

A 2-D force with components Fx = 100 N and Fy = 45 N accelerates an object of mass 5 kg. If this force is the net force on the object, find the value of the acceleration and its direction relative to the x-axis.

Solution 2

According to the second law of motion, we need to divide the force in each direction to the mass of the object to find the two components of the acceleration.

Fx = 100 N

Fy = 45 N

m = 5.0 kg

Because the two components are 90° apart, we can apply Pitagora's theorem to find the acceleration:

To find the direction relative to x-axis, α, we calculate the tangent of the angle and find the angle.

α = 24°

Newton's First, Second, and Third Laws of Motion

So, acceleration can be written in two ways:

a = (20,9) m/s2 for each component, or

a = 22 m/s2 at 24° north of east

The net force described by Newton's second law or any other force is, by definition, a measure of the interaction. Whenever you interact with an object, you will feel the effect of the interaction yourself. When you kick a ball, the ball will react (due to the interaction) by shooting into the air, but your foot will "feel" the interaction also. This effect is described by the third law of motion.

In this case, one force is called action and the other one reaction. As a very important note: The two forces are acting on different objects and so, although equal and opposite, they still have an effect on the system.

Newton's Third Law of Motion

If you act with a force on an object, the object will react with a force equal and opposite on the first object.

Example 3

Consider a car moving on a road. Determine the action and reaction forces on each tire.

Solution 3

Let's look at the situation for one tire, and the same can be repeated for the remaining tires. The car's left tire acts on the road with a force that we call action, and the road reacts with an equal and opposite force on the tire called reaction. That is how the car can move.

Newton's First, Second, and Third Laws of Motion


Let's put to work the knowledge we've accumulated and define some important types of forces in classical mechanics, one of which is weight. Whenever two objects are in proximity, they interact through a force. If you consider Earth and an apple that is about to fall from a tree, the apple is attracted by Earth with the force of gravity, so it will accelerate toward the earth for as long as it falls (if we do not consider other forces present such as friction with air). Because the gravity is the only force on the apple, it will be the only component in the net force. According to the second law, this force is also proportional to the mass of the apple and the acceleration, and we call it weight. The expression of the weight of an object of mass m is:

W = m · g

The symbols in the expression are: W is weight, m is mass, and g is the acceleration. It is very important to note that this expression gives us the argument to answer the question: Are mass and weight the same quantity? The answer is No! Weight is a force, and mass is a scalar quantity that gives a measure of the inertia of the object. In the previous example, the apple falling down due to gravity is in a free fall, and the acceleration of fall, which is the same for all objects at the same place on the earth, is called acceleration due to gravity, g. Although the acceleration due to gravity varies with the mass of the planet and the distance from an object to the center of the planet, for most purposes herein, we will take the acceleration due to gravity on Earth to be g = 9.8 m/s2. Another and last very important note for this section is that weight is a vector always pointing toward the center of Earth.


Consider the apple falling freely toward Earth. Why is Earth not falling toward the apple?


The third law of motion says that every action has an equal and opposite reaction, so the apple is acted on by gravity, and hence, Earth has to be acted on by an equal and opposite force. The reason the earth does not start moving toward the apple is that the same force acting on a huge object (Earth) has a negligible effect while having a definite effect on a smaller object (the apple).

Normal Force

The fact is that not all objects fall or fly; most of them are at rest or moving in contact with different surfaces. In such a case, we have an action, weight, and a reaction from the surface toward the object, and we call that a normal force, with a symbol N. In the simplest case, an object sits on a horizontal surface and pushes down only with its own weight; the surface responds with an equal and opposite force as shown in Figure 5.4. A drawing that includes all the forces acting on an object is called a free-body diagram.

Normal Force

When the surface is no longer horizontal, part of the weight will not act on the supporting surface, so the normal force of the object is now smaller than the weight of the object as seen in Figure 5.5. In this case, the component of the weight, called Wx , is the one acting on the surface, and normally is:

Wx= – N

Normal Force

The other component of the weight is not equalized by any other force in this case, and therefore, the object will accelerate toward the bottom of the plane. In real-life applications, there will be at least one other force acting. Yes, you guessed it: friction. But we will talk about this later.

In many real-life applications, you will have to take into consideration other forces acting on the object: pushes, pulls, and so on. In these cases, the freebody diagram will be more complex, and the calculation of the normal force can result in amounts smaller or larger than the weight, as we will show in the following example.

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