FORCES AND FREE BODY DIAGRAMS

Types of Forces Weight (gravity) – always acts down Normal – Acts perpendicular to surface (must be on surface) Friction –acts parallel to surface and opposite of motion Air Resistance – Acts upward as objects fall downward

Types of Forces Tension – Acts if a string or rope is attached, acts along string Spring – acts if a spring is attached, acts along spring Applied – a force applied by a person, acts in direction person is pulling or pushing

Steps to drawing a free body diagram Pick one object to analyze Draw a box or dot to represent the object Draw an arrow to represent each force acting on the object Make sure the arrow is labeled, and shows the direction and relative size of the force

Problem 1 A book is at rest on a table top. Diagram the forces acting on the book.

Problem 1 The forces are balanced because the book is not moving

Problem 2 An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Problem 2 The forces are unbalanced, so the egg will accelerate downward.

Problem 3 A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this squirrel looks like… Forces are balanced because constant velocity

Problem 4 A rightward force is applied to a book at rest, in order to move it across a desk. Construct a free-body diagram for the book. Right and left forces are unbalanced Up and down forces are balanced (no up and down movement)

Problem 5 A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram of the forces acting on the sled.

Problem 6 A football is moving upwards toward its peak after having been booted by the punter. Neglect air resistance. Draw a free-body diagram of the football in mid-air. Remember force does not determine movement, it determines acceleration!!

Problem 7 A car runs out of gas and coasts to a stop on flat ground. Draw a free body diagram of the forces acting on the car.

Even though the car is coasting, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces. That is all the car is in contact with so we are done!

What about the force of the car’s motion??? There is no such thing as force of motion. All forces must be exerted by an identifiable object (gravity is exerted by the earth) Only focus on what is happening NOW, not what happened earlier to get the car moving. If the motor is running, you will have a force to the right.

Problem 8 A car parks on an incline. Draw a free body diagram of the forces acting on the car.