Chapter 12 Forces and Motion

Chapter 12 Forces and Motion Objectives: • Demonstrate that any object does not accelerate unless an unbalanced force acts on it. • Explain the change of motion of an object. Demonstrate that the acceleration is proportional to the net force acting on the object and inversely proportional to the mass of the object. • Demonstrate that whenever one objects exerts a force on another, an equal amount of force is exerted back on the first object. • Demonstrate the ways in which frictional forces constrain the motion of objects. the motion of objects. 12.1 FORCES • What is a force? 1. A force causes a resting object to move, and it can accelerate a moving object by changing the object’s speed or direction. • Measuring a Force 1. Ex. A spring scale at the grocery store. • Unit of Force 1. Newtons = the force that is needed to move 1 Kg at a rate of 1 meter per second. 2. Named after Sir Isaac Newton. 12.1 Forces • Representing Force 1. Use arrows: a. Direction tells you direction of force. b. Length tells you how much force. 12.1 Forces E. Combining Forces 1. Net Force = overall force after all forces are combined. F. Balanced Forces = When forces are balanced, net force = 0 and there is no change in motion. Ex. Stalemate in Tug of War.

• Sliding Friction 1. Opposes the direction of motion after an object starts moving. 2. Since Sliding friction is less than static friction = less force is needed to keep the object moving

• Rolling friction – opposite force that acts on a rolling object. 1. 100 to 1000 times less than static or sliding friction. • Fluid friction – friction that acts on an object in water or air. L. Gravity – acts downward to the center of the earth. 1. Gravity causes objects to accelerate downward.

M. Projectile Motion 1. The combination of initial forward velocity + gravity causes an object to follow a curved path. 12.2 Newton’s First and Second Laws of Motion • 1st Law of Motion – The motion of an object does not change as long as the net force acting on the object is zero. 1. Inertia – an objects tends to resist change. 2. A moving objects tends to stay moving. An object that is at rest tends to stay at rest. Ex. If you are in a car when it stops suddenly you go forward because of inertia.

• Newton’s 2nd Law of Motion 1. The Acceleration of an object is equal to the net force acting on it divided by the object’s mass. 2. Acceleration = Force / Mass 3. The acceleration of an object is always in the same direction as its mass. 4. Math practice pg. 367 Weight • Weight is the force of gravity acting on an object. formula: W = mg m= mass g= gravity (9.8 m/s2) 12.3 Newton’s 3rd Law of Motion A. Newton’s 3rd Law of Motion: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

• Momentum 1. The product of mass x velocity 2. Momentum = mass x velocity C. Law of Conservation of Momentum – in a closed system the loss of momentum of one object equals the gain of momentum in another object. Momentum is Always Conserved

Gravitational Force • Newton’s law of universal gravitation says that every object is attracted to one another. example: moon stays near the earth because of gravity