Ch 4 Newton’s Laws

This is a “big” chapter

Newton’s Laws

#1: Law of Inertia or Laziness Objects want to keep doing what they are already

doing. Objects at rest stay at rest or objects moving at constant speed in a straight line continue doing this unless an outside thing (force ) acts on them.

Newton’s Laws

#2: F = ma (or better yet, a = F/m) An object changes its motion (accelerates) when

a net unbalanced force acts. The amount of change (acceleration) also depends on the mass of the object

Newton’s Laws

#3: Forces exist in pairs For every force there is an equal and opposite

force acting on another object. Forces are interactions between 2 things. Golf

ball/golf club, tire/road, etc.

Mass vs. Weight

Mass (kg) is a measure of inertia. It’s a property of the material which doesn’t change whether you are in outer space or on mars.

Weight is a force which depends on the gravitational pull (or gravitational acceleration) Fg = mg

Common Forces

Fg = mg ( weight) Fnormal = contact force perpendicular to plane

of contact Ffriction acts parallel to the plane of surface

contact. Frictional force can be static or kinetic. Ffk = u Fn ( u = coefficient of friction, a property of the

materials in contact)

Free Body Diagrams

FBD’s are essential to solving problems involving multiple forces.

Isolate the object and draw arrows to represent each separate force acting.

Force Arrows typically point away from the object and should be drawn approximately to scale.

FBD’s show forces but not direction of motion.

Free Body Diagrams

Method for solving Newton’s 2nd Law problems Choose a coordinate system Draw a Force Diagram (free body diagram) Resolve forces into their x- and y-

components (if necessary) For each direction write Newton’s 2nd law a = F/m (for both x- and y- directions) Solve