FORCES AND FREE BODY DIAGRAMS

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What is a Force? A force is a push or a pull. All forces have two properties: –Direction and Size (magnitude) A newton (N) is the unit that describes the size of a force. It is equal to a A force acts ON one object FROM another object (an object cannot exert a force on itself!!)

Types of Forces Weight (gravity) – always acts down (non-contact) Normal – a force that acts perpendicular to a surface that an object is in contact with – this usually acts up but not always (more on this in a minute) Tension – a force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends – acts away from object

Types of Forces Applied - a force that is applied to an object by a person or another object Friction – the force exerted by a surface as an object moves across it or makes an effort to move across it – acts parallel to surface and AGAINST motion Air Resistance – a special type of friction that acts upon objects as they travel through the air – acts up and is not constant, changes with speed

Types of Forces Spring - the force exerted by a compressed or stretched spring upon any object that is attached to it

Types of non-contact forces Gravitational - a force that attracts two objects to each other (we’ll cover this later) Electrical – the force that attracts or repels two charged objects (this is Physics 2) Magnetic – attraction or repulsion that arises between electrically charged particles because of their motion (this is Physics 2)

Normal force directions Up –You’re standing on level ground. Sideways –A ladder leans up against a wall. –You’re against the wall on the “Round Up” ride when the floor drops out. At an angle –A race car takes a turn on a banked track –An object on an incline Down –You’re in a roller coaster at the top of a loop.

The meaning of each of these forces will have to be thoroughly understood to be successful during this unit. Ultimately, you must be able to read a verbal description of a physical situation and know enough about these forces to recognize their presence (or absence) and to construct a free-body diagram that illustrates their relative magnitude and direction.

Net Force When more than one force acts on a body, the net force (resultant force) is the vector combination of all the forces, i.e., the “net effect.” F1F1 F2F2 F3F3 F net

Balanced vs. Unbalanced Forces Balanced forces cancel each other out causing an object to either not move or will move at a constant velocity (Net force = 0) Unbalanced forces will cause an object to accelerate (net force ≠ 0)

Important Fact!! Net force causes acceleration and will tell you what direction acceleration is in, but not necessarily what direction motion is in. If an object is speeding up, net force will be in the direction of motion, but if it is slowing down, net force will be opposite the direction of motion

Free Body Diagrams A force diagram, which is also known as a free body diagram, is a sketch in which all the force vectors acting on an object are drawn with their initial points at the location of the object.

When to use a free-body diagram Always - every problem that has to do with forces!

Steps to drawing a free body diagram 1.Pick one object to analyze 2.Draw a box or dot to represent the object 3.Draw an arrow to represent each force acting on the object 4.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 8 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.

Atwood Machine m 1 m 2 m1gm1g T T m2gm2g If the rope & pulley have negligible mass, and if the pulley is frictionless, then T is the same throughout the rope. If the rope doesn’t stretch, a is the same for both masses.

Mary pulls on a lawnmower parallel to the ground. The mower moves at a constant speed up the hill. x y x y x y Which free body diagram best represents the forces on the mower? x y 2

Stacey pulls a mower at 40-degrees relative to the ground x 40 0 y y x x y y x Which free body diagram best represents the forces on the mower?

Which diagram best represents the forces exerted on the car by the Earth and the Earth’s surface? The motion of a car is represented with the motion diagram below. x x y 1 x y 2 x y 3 x y 4 x y 5 o

1 Which FBD below best represents the forces exerted on the ball as it moves UP (ignore air resistance)? 2 34

1 Which FBD below best represents the forces exerted on the ball as it moves DOWN (ignore air resistance)? 2 34

1 Which FBD below best represents the forces exerted on the ball at the TOP of its path? No forces are exerted on the ball