Newton’s Laws of Motion

Newton’s 1st Law

What is a force? A force is a push or pull on an object. Force is a vector (magnitude and direction). Forces cause an object to accelerate… To speed up To slow down To change direction

Newton’s First Law A body in motion stays in motion at constant velocity and a body at rest stays at rest unless acted upon by a net external force. This law is commonly referred to as the Law of Inertia.

The First Law is Counterintuitive Aristotle firmly believed this, but physics students know better!

Implications of Newton’s First Law If there is zero net force on a body, it cannot accelerate, and therefore must move at constant velocity, which means it cannot turn, it cannot speed up, it cannot slow down.

What is Zero Net Force? SF = 0 The table pushes up on the book. FT A book rests on a table. Physics Gravity pulls down on the book. FG Even though there are forces on the book, they are balanced. Therefore, there is no net force on the book. SF = 0

Diagrams Draw a force diagram and a free body diagram for a book sitting on a table. Physics N W Force Diagram N W Free Body Diagram

Newton‘s 2nd Law

Newton’s Second Law A body accelerates when acted upon by a net external force. The acceleration is proportional to the net force and is in the direction which the net force acts.

Newton’s Second Law ∑F = ma where ∑F is the net force measured in Newtons (N) m is mass (kg) a is acceleration (m/s2)

Units of Force Newton (SI system) 1 N = 1 kg m /s2 1 N is the force required to accelerate a 1 kg mass at a rate of 1 m/s2 Pound (British system) 1 lb = 1 slug ft /s2

Working out 2nd Law Problems Draw a force or free body diagram. Set up 2nd Law equations in each dimension. SFx = max and/or SFy = may Identify numerical data. x-problem and/or y-problem Substitute numbers into equations. “plug-n-chug” Solve the equations.

Sample Problem In a grocery store, you push a 14.5-kg cart with a force of 12.0 N. If the cart starts at rest, how far does it move in 3.00 seconds?