Forces A force is a push or pull

Forces A force is a push or pull. Measured in newtons (N)

Forces can change an objects. . . 1. speed

2. direction

3. shape

Newton’s 1 st Law: ØAn object at rest remains at rest if no force acts on it. ØAn object in motion remains in straight-line motion if no force acts on it. No force no acceleration constant velocity Sometimes called the “Law of Inertia” ΣF=0 Resistance to changes in motion Web Links: Truck and ladder, Car and person

Newton’s 2 nd Law: the sum of forces on an object the = object’s mass the object’s x acceleration F = ma Notes Ø 1 N = 1 (kg)(m/s 2) Ø Remember: F=ma involves a specific object F = the sum of forces on that object m = the mass of that object a = the acceleration of that object Ø mass is a measure of inertia Ø another form of this equation: a = ( F)/m

Since force is a vector F has components : Fx = the sum of the x-direction forces Fy = the sum of the y-direction forces And since motion in the x and y directions is independent: Fx = max Fy = may These two equations are 1 Dimensional equations (easy )

5 N 5 N Fx = 0 Free Body Diagram – A picture with vectors showing all forces being exerted on an object. Ex: 3 N 5 N + - If the mass of the object is 0. 5 kg, find its acceleration. Web Link: Skydiver

Resultant Force 10 N 17 N ΣF = sum of all forces acting on an object Mass = 2 kg 2 N 25 N 12 N 23 N What is the resultant force or ΣF acting on the object? What is the objects acceleration?

If a body is in equilibrium, the resultant force acting on it must be zero, ie ΣF = 0

What happens to each car if a) It was originally stationary b) It was originally moving 10 N 9 N 4 N

Ex: 120 N If you push your car of mass 1500 kg over a distance of 250 m, what will be the final speed of the car? Hint: 2 nd law & kinematics

Newton’s rd 3 Law What would happen if the girl pushed the boy? The boy would move The girl would move For every action there is an equal and opposite reaction.

Newton’s 3 rd Law (Action-Reaction Law): If object A exerts a force on object B, then object B exerts an equal and opposite force on object A. Web Link: Action-Reaction Ex: An ice skater pushes on a wall v. Why don’t F and –F cancel? v. Why doesn’t the wall move? Web Link: Force Tutorial F -F

Review Question: In 1998, NASA’s 153 million dollar probe “Deep Space 1” was launched. On board was an experimental “ion-engine” that put out a thrust merely equal to the weight of a piece of paper. The entire spacecraft reportedly weighed 1000 pounds. Starting from rest, in space, powered only by the ion-engine, how many miles per hour would the spacecraft be going after 300 days? Web Link: Deep Space 1