Module 3 Lesson 4

Module 3—Effects of Force on Velocity

Lesson 4—Free-Body Diagrams and Net Force

Get Focused

A photo shows an ambulance, with emergency lights turned on, driving fast on a road.

Can you identify all the forces acting on this ambulance? Recall the three accelerators on a vehicle—engine, brakes, and steering. Each of these vehicle components can cause acceleration by generating a force that contributes to the overall net force and observed acceleration of the ambulance.

In this photo the ambulance is observed to be accelerating forward, which, according to Newton’s second law, means that the applied force of the engine is greater than the force of friction opposing the motion. Notice also that the ambulance is not sinking into the ground or taking off like an airplane. Does that mean that there are no vertical forces acting on the ambulance? Not necessarily. The ambulance has a significant weight that must be supported by the ground. So, there are vertical forces, but they are balanced.

Upon quick inspection, there is an applied force of the engine, the force of friction opposing the forward motion, the ambulance weight, and the supporting normal force—all acting at the same time.

Is there a quick and easy way to visualize all the forces at once and generate an equation that describes the net force and observed acceleration of a vehicle, such as an ambulance?

In the previous lesson you worked with free-body diagrams in limited situations. In this lesson you will construct labelled free-body diagrams for objects in a variety of physical situations and use the diagrams to derive net-force equations that describe the observed acceleration of the object.

Module 3: Lesson 4 Assignments

Your teacher-marked Module 3: Lesson 4 Assignment requires you to submit a response to the following:

Try This—TR 1, TR 2, and TR 3

You must decide what to do with the questions that are not marked by the teacher.

Remember that these questions provide you with the practice and feedback that you need to successfully complete this course. You should respond to all the questions and place those answers in your course folder.

As you work through this lesson, keep the following questions in mind:

How is the motion of an object affected by the forces applied to it?
How do unbalanced forces affect the motion of an object?
How do free-body diagrams, vector analyses, and Newton’s second law help solve moving-object problems?