Forces and Force Diagrams4.1 pp. 124 - 128

Mr. Richter

Agenda

Warm Up

Notes: That’s a Force! Units of Force Types of Forces Force Diagrams

Tests Back

Objectives: We Will Be Able To…

Explain how force affects the motion of an object.

Distinguish between contact forces and field forces.

Interpret and construct free-body diagrams (FBDs)

Warm-Up:

A car is driving down a flat roadway to the right. What forces do you think are acting on the car (there are at least 4)? What direction do these forces go?

Draw a sketch of a car, and then draw arrows to represent each force you think is present.

Force

Force

Force is a push or a pull acting on an object.

Another way of thinking is that a force changes an object’s velocity. Force can speed up an object or slow it down. Force can change an object’s direction.

Force causes acceleration (more on this when we discuss Newton’s Laws of Motion)

Units of Force

The units of force we will most often use in physics are Newtons.

A Newton is the amount of force required to accelerate a 1-kg object 1 m/s2. [1 N = 1 kg*m/s2]

In the British system, the pound is the unit of force. 1 pound = 4.448 N

Types of Force

All forces can be broken up into two categories.

Contact force is force that arises from the physical contact between two objects. catching a baseball, speeding up a car, etc.

Field force is force that exists in the absence of physical contact. gravity, magnetic force, etc.

Is air resistance a contact force or a field force?

Force Diagrams

Force Diagrams

Force is a vector quantity, meaning that it has both magnitude and direction.

In complex situations (like a car crashing into a wall), force diagrams help physicists to analyze the situation.

Free-Body Diagrams (FBDs)

To determine the effects of all of the forces acting on one object, a free-body diagram (FBD) is drawn.

Only the forces ACTING ON THE OBJECT are drawn, pointing in the directions in which they act.

FBD Example

Consider the forces acting on the car in the tow truck below.

FBD Practice

Sketch the FBD for the following two situations:

1. A book lying on a flat table.

2. A book lying on a ramp with an incline of 15°

Wrap-Up: Did we meet our objectives?

Explain how force affects the motion of an object.

Distinguish between contact forces and field forces.

Interpret and construct free-body diagrams (FBDs)

Homework

p. 128 #2, 3, 5 and 6

Science Fair Notebook and Research due Wednesday Rough draft of Procedure due Wednesday 12/5