Free-body diagrams Pg. 17 in NB

Identify normal forces. Draw a free-body diagram with forces in vertical and/or horizontal directions. Calculate the net force on an object acted on by forces in vertical and/or horizontal directions. Objectives Physics terms net force normal force free-body diagram

If you know the forces acting on an object, you can predict its motion. Free-body diagrams are a way to figure out the forces acting on an object. -Find the magnitude (value) -Find the direction (+/-) FwFw F N1 F N2 Free-body diagrams

A free-body diagram is a sketch of an object isolated from its surroundings. ONLY the forces acting ON the object are included in the diagram. Forces are drawn as arrows. -Arrows indicate the direction the force is being applied Free-body diagrams FwFw F N1 F N2

To solve force problems, you have to choose which directions will be positive and which will be negative. Force is a vector

Types of forces On a free-body diagram, include every force that acts ON the object like: Weight  Always pulls straight down (F W or mg) Normal forces  Always perpendicular to contact surface (F N ) Applied forces  Act in direction of push or pull (not from gravity or surface)

Every place where an object is in contact with a surface creates a normal force. May be vertical, horizontal, or act at an angle. Always perpendicular to the surfaces that applied them. Surfaces always push NEVER pull Normal force

Reminders: Surfaces always push, NEVER pull Perpendicular to contact surface Normal force

Applied Force Applied forces are any forces NOT resulting from contact with a surface or gravity Examples: –Stretching a spring –Pulling up on your backpack –Pushing a box –Pulling on a rope

Making free body diagrams Using the following steps create the free body diagram for the picture on the next slide. Step 1  Draw an outline of the object Step 2  Draw the forces acting ON the object, start with weight Step 3  Add applied forces

Making free-body diagrams Real Object Free Body

This is a complete free-body diagram. It contains ALL the forces that act ON the object. Every force is identified with a label and direction. It does not have too much detail—a rough sketch is all you need. F FNFN FNFN Free-body diagrams mg

The net force In most situations there are many forces acting on an object at once. Objects respond to the net force that is acting on them. In physics “net” means total, taking directions into account. F FNFN FNFN What is the net force acting on the dumbbell?

Equilibrium = EQUAL Equilibrium exists when the net force is zero. In equilibrium there is no change in motion. An object at rest stays at rest. Equilibrium F net = 0 F FNFN FNFN F net = F + 2F N - F w = 0 mg The dumbbell is at rest so the net force on it must be zero: F

The boxes shown are at rest, so F net = 0. What is F N in these examples? Pressed down with a 4 N force Pulled up with a force of 4 N. mg = 10 N F = 4 N. F = 4 N Pressed against the ceiling with a 15 N force mg = 10 N F = 15 N. Find the normal force F N = 6 N. F N = 14 N. F N = 5 N.

An object at rest is in equilibrium. Therefore the net force is zero. What is the tension in the ropes that support the gymnast? Equilibrium problems F net = 0

Equilibrium problems The gymnast is centered, so we can assume the forces from the ropes are equal. Therefore we give them the same name, F.

1.Draw the free-body diagram of the object. 2.Name all the forces. 3.Set the net force to zero, taking account of +/- directions. 4.Solve for the unknown force. Here are the EQUILIBRIUM problem-solving steps. Solving equilibrium problems

Homework # 1 Two masses are at rest, connected by a rope that passes over two frictionless pulleys. The string tension T equals m 2 g. Which of these free body diagrams best represents the forces acting on mass m 1 ?

Homework # 1 Diagram A is best. Diagram (B) omits the normal force from the floor. Diagram (C) incorrectly shows the string tension acting downward. Which of these free body diagrams best represents the forces acting on mass m 1 ?

FNFN FNFN 10 kg Homework # 2 A 10 kg dumbbell resting on a table is partly supported by a spring that pulls upward with a force of 50 N. What force does the table exert on the dumbbell to hold it up? mg F = 50 N

Homework # 3 Which of the two sketches is the correct free- body diagram of the horizontal arm holding the weight?

Homework 4. Draw a free body diagram showing the forces acting on you when you sit on the floor. 5. Draw a free body diagram showing the forces acting on a stalled car when you’re pushing it.