Forces and Free-Body Diagrams

What do you think? What is a force? Are any forces acting on your book as it rests on your desk? If so, describe them. Make a sketch showing any forces on the book. What units are used to measure force? Can forces exist without contact between objects? Explain.

Forces Forces can change motion. Start movement, stop movement, or change the direction of movement Cause an object in motion to speed up or slow down

Forces Contact forces Field forces Pushes or pulls requiring physical contact between the objects Baseball and bat Field forces Objects create force fields that act on other objects. Gravity, static electricity, magnetism Pictured is a contact force, the bat and the ball, as well as a field force, the static electric field around charged balloon exerting a force on small pieces of paper. Ask students to identify other examples of contact forces.

Units of Force The SI unit of force is the newton (N). Named for Sir Isaac Newton Defined as the force required to accelerate a 1 kg mass at a rate of 1 m/s2 Approximately 1/4 pound Other units are shown below. 1 N = 0.225 pounds (roughly 1/4 pound) Have students determine their approximate weight in newtons to reinforce the size of the unit. When talking about problems, use both units to help them become more comfortable. For example, a 10 000 N car is about a 2500 lb car.

Force Diagrams (See Page 122) Forces are vectors (magnitude and direction). Force diagram (a) Shows all forces acting during an interaction On the car and on the wall Free-body diagram (b) Shows only forces acting on the object of interest On the car Students often have trouble isolating the forces acting on an object to draw a free-body diagram for the object. The free-body diagram of the car is analyzed in more detail in the next slide. (See Page 122)

Free-Body Diagrams Three forces are shown on the car. Describe each force by explaining the source of the force and where it acts on the car. Is each force a contact force or a field force? For simplicity, all forces are shown acting on the center of the object. Remind students that, when adding vectors, they can be moved parallel without changing the results. Even though the upward force acts on each of the 4 tires, the total is shown acting on the center of the car. Even though the wall strikes the front bumper, that force can be moved to the center of the car without changing the resultant. Gravity (the pull of Earth’s field) acts on every particle in the car but is shown as a single downward force at the center.

Show the object as a “point” Show each force separately Free Body Diagrams Show the object as a “point” Show each force separately W Fwc Frc Fk W T

Normal Force W N Force on an object perpendicular to the surface (Fn or N ) It may equal the weight (Fg or W ), as it does here. It does not always equal the weight, as in the second example. Fn = mg cos  W N Point out that the equation for normal force applies to the first example also. Because cos(0)=1, the equation reduces to Fn = mg when the forces are directly opposite one another.

Now what do you think? What is a force? What forces act on your book as it rests on your desk? Make a sketch showing any forces on the book. Are they contact forces or field forces? What SI unit is used to measure force? What equivalent basic SI units measure force? Answers: A force is a push or a pull between two objects. There are two forces acting on the book: the force of gravity downward (field force) and the force of the desk pushing upward (contact force). Forces are measured in newtons (N), a derived unit equivalent to kg•m/s2 .

Group Exercise A box is on a ramp that makes an angle of 30o with the horizontal. Draw: A sketch of the system A free-body diagram showing the forces acting on the box

 W N