Ch. 4 Forces

Describe an object’s acceleration in terms of its mass and the net force acting on it. Predict the direction and magnitude of the acceleration caused by a known net force. 3. Identify action-reaction pairs.

Newton’s 2nd Law Force is proportional to mass and acceleration. The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object’s mass. The direction of the acceleration is in the same direction as the applied force

Newton’s 2nd Law of Motion (N2), the Law of Acceleration The acceleration of an object is directly proportional to the net force acting on the object: a = ΣF m

. Comment: The essence of Newton’s 2nd Law is that an object WILL ACCELERATE if and only if the EXTERNAL FORCES acting on it are NOT BALANCED. A Free Body Diagram is used to depict the external forces and Newton’s 2nd Law ties the dynamics to the kinematics. Each external force includes a type, a direction, and a source.

F = ma Newton’s 2nd Law Force (F) has units of Newtons (N) Mass (m) has units of kilograms (kg) Acceleration (a) has units of meters per second squared (m/s2)

In other words…..using the same amount of force…. Small acceleration a Large Mass Large acceleration F a Small Mass

Roberto and Laura are studying across from each other at a wide table Roberto and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book towards Roberto. If the net external force acting on the book is 2.6 N to the right, what is the book’s acceleration?

Newton’s 3rd Law For every action there is an equal and opposite reaction Force always exist in pairs, even field forces Pairs are known as action-reaction pairs

Newton’s 3rd Law of Motion (N3), the Law of Interaction When one object exerts a force on a second object the second object exerts the same magnitude force in the opposite direction on the first object SIMULTANEOUSLY. Comment: The essence of Newton’s 3rd Law is that all forces are actually pairs in which the objects force each other simultaneously and oppositely REGARDLESS of the MOTION of EITHER. However, since we NEVER SEE FORCES we cannot see this relationship.

Action and Reaction pairs Action-reaction pairs do not imply that the net force on either object is zero. The action-reaction forces are equal and opposite but either object may still have a net force on it.

a = m Earth / Apple (cont.) a m Apple’s Earth’s little mass big mass The products are the same, since the forces are the same. a = m m a Apple’s little mass Earth’s big mass Earth’s little acceleration Apple’s big acceleration

Identify the action-reaction pair in the following situations. A person takes a step A snowball hits someone in the back

Identify the action-reaction pair in the following situations. A baseball player catches a ball A gust of wind strikes a window

The forces acting on a sailboat are 390 N north and 180 N east The forces acting on a sailboat are 390 N north and 180 N east. If the boat has a mass of 270 kg, what are the magnitude and direction of its acceleration?

If a locomotive can exert a constant pull of 7 If a locomotive can exert a constant pull of 7.5x105 N, how long would it take to increase the speed of the train from rest to 85 km/h? (disregard friction)