The book is the system. The table, the hand, and Earth’s mass (through gravity) all exert forces on the book. We must consider SYSTEMS and INTERACTIONS when we look at forces.

Forces result from interactions; thus, each force has a specific and identifiable cause called the agent. You should be able to name the agent exerting each force, as well as the system upon which the force is exerted. For example, when you push your textbook, your hand (the agent) exerts a force on the textbook (the system).

To make a physical model of the forces acting on an object, apply the particle model and draw an arrow to represent each force. Label each force, including its agent.

Because the rubber band is stretched a constant amount, it applies a constant force on the cart, which is designed to be low-friction The cart’s motion can be graphed and shown to be a linear relationship

In this example, you would have to stretch the rubber band farther to get a greater amount of force. The slopes of the lines depend upon the number of carts; that is, the slope depends on the total mass of the carts. If the slope, k in this case, is defined as the reciprocal of the mass (k = 1/m), then a = F/m, or F = ma.

What are the proper units for measuring force? Because F = ma, one unit of force causes a 1-kg mass to accelerate at 1 m/s 2, so one force unit has the dimensions 1 kgm/s 2. The unit 1 kgm/s 2 is called the Newton, represented by N. One Newton of force applied to a 1-kg object will cause it to have an acceleration of 1 m/s 2.

Net Forces Pushing on the table with equal force in opposite directions (a) results in no net force on the table, as shown by the vector addition in the free-body diagram (b). However, there is a net force applied in (c) and (d), as shown by the free-body diagrams.

Newton’s 2 nd Law of Motion (Accelerated Motion)