Chapter 4 Forces in One Dimension Launch Lab

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 force? • If F=ma (Newton’s 2 nd law)… then one unit could be: kg·m/s 2 • 1 kg·m/s 2 = 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

Section Review pg. 95 • 9 -12, 14.