- Slides: 17
Forces in One Dimension Chapter 4
Forces and Motion
Newton’s First Law of Motion
What is Mass? Mass is a measure of inertia—the greater the mass of an object, the harder it is to get it to move. Mass is not volume. For example, your weight on the Moon is different from your weight on Earth, but your mass is the same in both places, Mass is not weight. Mass is a fundamental property, weight is not. Mass is the amount of material in an object. The SI unit of mass is the kilogram (km).
Net Force and Equilibrium We may have more than one force acting on an object. The net force is the sum of all the forces acting on an object But remember that forces are vectors, so we have to add up the forces vectorially. When all the forces acting on an object add to zero (i. e. , the net force is zero) then the object is said to be in equilibrium. Forces and Motion Basics Adding Forces Force Tutorial Free Body Diagrams
Common Types of Forces Force Definition Direction Friction A contact force that acts to oppose sliding motion between surfaces Opposite to the direction of sliding Normal A contact force exerted by a surface on an object Perpendicular to and away from the surface Spring A restoring force; the push or pull a spring exerts on an object Opposite to the displacement of the object attached to the spring Tension The pull exerted by a string or rope when Away from the object and parallel attached to a body and pulled taut to the string or rope at the point of attachment Weight A field force due to the gravitational Straight down towards the center attraction between two objects, generally of the Earth and the object Electrical A field force due to the electrical charges Attractive (unlike charges) or on two or more objects repulsive (like charges) along the line between the objects
Newton’s Second Law
Mass and Acceleration
Free Fall and Air Resistance We’ve seen that objects in free fall with an acceleration equal to g. But that’s true only if we can ignore air resistance. When the air resistance on a falling object equals its weight, the net force is zero and no further acceleration occurs. At that point the object has reached its terminal velocity. Felix Baumgartner skydive Altitude 39 km (24 mi) Reached an estimated terminal velocity of 1, 357. 64 km/h (843. 6 mph), breaking the sound barrier
Free-Body Diagrams A free-body diagram is a diagram showing all the forces acting on an object. Free-Body Diagram Interactive
Force Exercises Force Interactive
Example Problem 1 Unfortunately for Vanessa, the wheels on her suitcase are not working. She pulls on the strap in an effort to budge it from rest and drag it to the curbside check-in desk. The free body diagram at the right depicts the forces acting upon the suitcase. Use force values to determine the net force, the mass and the acceleration of the suitcase. The values of the individual forces are: Fgrav = Fnorm = 207 N Ftens = 182 N Ffrict = 166 N
Example Problem 2 It's Friday night and Skyler has been assigned the noble task of baby-sitting Casey, his 2 -year old brother. He puts a crash helmet on Casey, places him in the red wagon and takes him on a stroll through the neighborhood. As Skyler starts across the street, he exerts a 52 N forward force on the wagon. There is a 24 N resistance force and the wagon and Casey have a combined weight of 304 N. Construct a free body diagram depicting the types of forces acting upon the wagon. Then determine the net force, mass and acceleration of the wagon.
Example Problem 3 Moments after making the dreaded decision to jump out the door of the airplane, Darin's 82. 5 -kg body experiences 118 N of air resistance. Determine Darin's acceleration at this instant in time.
Newton’s Third Law Action and Reaction
Forces and Interactions
Newton’s Third Law in Action EVA video Third Law Video