5 Chapter 5: Introduction to Forces
Textbook Chapter 5: Newton’s Laws of Motion
Section 5.1: Newton’s First Law
Textbook Section 5.2: Newton’s First Law
Textbook Section 5.4: Mass and Weight
A force is defined as a push or a pull on an object. This might be due to a direct interaction between two objects (contact force) or due to a long-range force such as gravity. The contact forces we will consider include the normal, friction, and tension forces.
We make a distinction between the mass and weight of an object. Mass is a measure of how much stuff is there; it is measured in kg (SI unit). Weight, on the other hand, is a force, not a measure of mass. Weight is calculated as mass times gravity (). On Earth’s surface, with a constant gravitational force of 9.81 m/s, we can convert back and forth between mass and weight. However, while your mass remains constant anywhere you go in the universe, your weight is constantly changing due to local gravitational forces.
Forces are vector quantities with both magnitude and direction. Forces are added as vectors.
Newton’s First Law: the law of inertia. An object in motion (has a constant or zero velocity) will remain in motion unless acted upon by an outside force.
Newton’s first law tells us that any change in velocity (magnitude, direction, or both) must be due to a force acting on the object. What forces might act on a book pushed across a table to bring it to a stop? These forces can include friction with both the table and the air (we will address frictional forces in Chapter 6). Our own experience tends to disagree with Newton’s first law because we are always in the presence of frictional forces on Earth. But if you threw a book into the emptiness of space, it would continue to move at the velocity you threw it until it ran into something. There’s nothing to slow it down.
Newton’s first law is only valid in inertial reference frames (where the object is not accelerating – moving at constant velocity or not moving at all). We will only consider these cases.
Section 5.2: Newton’s Second Law
Textbook Section 5.3: Newton’s Second Law
Newton’s Second Law: a net force on an object causes it to accelerate in the direction of the force ().
Any change in the velocity (magnitude or direction or both) of an object is caused by a force, and the magnitude of that force is equal to the mass of the object times the acceleration of the object. Forces are measured in units of Newtons (N), which are equal to a kg m/s.
Section 5.3: Newton’s Third Law
Textbook Section 5.5: Newton’s Third Law
Newton’s Third Law: if object A exerts a force on object B, object B exerts an equal and opposite force on object A.
This law refers to the interaction between two objects. If one person pushes something, the object pushes back. You feel this every time you interact with something — heavy objects put up a lot of resistance to motion, and lighter objects put up less resistance to motion, but everything pushes back at least a little bit