Chapter 4: Forces in One Dimension

Launch Lab on pg. 87 Which string broke in step 2? Which string broke in step 3?

Force and Motion Force: a push or a pull on an object Forces can cause an object to: Speed up, slow down, change directions Direction in which the force is exerted also plays a role

Force and motion F is a symbol that for a vector and represents the size and direction of a force F just represents magnitude

Considering how force affects motion Consider the object “called system” and everything around the object that exerts forces on it is called the external world

Contact force vs. field force Contact force examples: touch something to apply a force Field force examples: gravitational force from the earth acts on objects without touching them, also magnets There needs to be an agent causing a force on a system (example: your hand “agent” causes a force on the book “system”

Free-Body Diagrams The object is represented with a “dot” Represent each force with an arrow that points in the direction that the force is applied The length of the arrow should proportional to the size of the force Always draw the force arrows pointing away from the particle even when the force is a push Label each force Choose a direction that represent (+)  this is usually the direction with the greatest force because it makes the problem easier to solve and reduces negative values

Practice problems

Force and Acceleration Would an object continue to accelerate when there was no longer a force applied? Think about a spaceship in outer space…does it have velocity? Is there still a force acting on it? Could it accelerate?

Force and Acceleration The relationship between force and acceleration is a linear relationship where the greater the force is, the greater the resulting acceleration. Talk about the cart pulled by a stretched rubber band example, if you pull more carts with the same rubber band…acceleration decreases therefore… the slope in an acceleration vs. force graph is dependant on mass

F = ma a = F/m … tells you that a force applied to an object causes that object to experience a change in motion, the force causes the object to accelerate. It also tells you that for the same object if you double the force, the acceleration doubles

What is the unit for force? If F=ma (Newton’s 2nd law)… then one unit could be: kg·m/s2 1 kg·m/s2 = 1 Newton (N)

Think about different objects falling Acceleration as we learned is the same due to gravity Therefore different objects have different masses, and therefore require different amounts of forces.

Combining forces Net force: the vector sum of all the forces on an object

Equal and opposite direction forces Net force = __________N Draw a picture below

Equal or unequal forces same direction Net force = Draw a picture below

Unequal forces opposite directions Net force = Draw a picture below

Equal or Unequal forces perpendicular Net force = Draw a picture below

Newton’s First Law “an object that is at rest will remain at rest, and an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on the object is zero”

Newton’s First Law Inertia- the tendency of an object to resist change Equilibrium- if the net force on an object is zero, then the object is in equilibrium (if it is at rest or if it is moving at a constant velocity) As long as the object is not experiencing a change in speed or direction is at equilibrium

Table 4-2 Force Symbol Definition Direction

Table 4-2 Force Symbol Definition Direction

Section Review pg. 95 9-12, 14.

Section Review pg. 95 9-12, 14.