Forces Chapter 11

11.1 Laws of Motion

Newton’s First Law An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force. It is also called the law of inertia Inertia is the tendency of an object to resist being moved or changed All objects resist changes in motion, so all objects have inertia

Inertia is related to an object’s mass Mass is the measure of inertia An object with a small mass has a smaller amount of inertia, so it is easier to change its motion because it requires a small force An object with a large mass has a larger amount of inertia, so it requires a larger force to change its motion.

Seat belts and car seats provide protection Why does a seat belt help you stop when a car stops suddenly? Why do we place babies in a special backward-facing car seat?

Newton’s Second Law of motion The unbalanced force acting on an object equals the object’s mass times its acceleration. Force = mass x acceleration F=ma

Force is measured in newtons, which is the SI base unit for Force 1 N = 1 kg x 1 m/s2

Newton Second Law Practice Problem Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is 0.657 m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and stretcher?

11.2 Gravity

Gravity – any two masses that exert an attractive force on each other. Gravity depends on mass & distance between objects Law of Universal Gravitation- All objects in the universe attract each other through gravitational force.

Law of Universal Gravitation The greater the object mass, the stronger the gravitational force on it. The greater the distance, the weaker the gravitational force.

Free fall – the motion of an object when only the force of gravity is acting on the object Free fall acceleration near Earth’s surface is constant Since we live on Earth we assume that objects accelerate toward Earth at 9.8m/s2

weight= mass x free-fall acceleration •Weight – gravitational force exerted on an object; measured in units called Newtons weight= mass x free-fall acceleration w = mg g=9.8m/s2

Weight is different from mass Mass is the amount of matter an object has and will not change Weight is the amount of gravitational force on an object and will change depending on where you are. Weight influences shape

Velocity is constant when air resistance balances weight Must have balanced forces Terminal velocity – the constant velocity of a falling object when the force of air resistance is equal in size and opposite in direction to the force of gravity Example: Skydivers

Projectile Motion Projectile is anything that is thrown or shot through the air. A projectile follows a curved path that has both horizontal and vertical motion Horizontal motion- motion parallel to Earth’s surface. Caused by a force. Vertical motion- motion perpendicular to Earth’s surface. Caused by gravity.

Horizontal and vertical velocities are independent on each other. Horizontal velocity Depends on inertia Remains constant Vertical velocity Depends on gravity Accelerate downward at 9.8 m/s2

•A moving truck launches a ball vertically (relative to the truck) •A moving truck launches a ball vertically (relative to the truck). If the truck maintains a constant horizontal velocity after the launch, where will the ball land (ignore air resistance)? A) In front of the truck B) Behind the truck C) In the truck

11.3 Newton’s Third Law

Newton’s third Law For every action force, there is an equal and opposite reaction force. Also called the Law of Action and Reaction

Forces always occur in pairs called force pairs. They will be equal in size and opposite in direction Force pairs do not act on the same object and their names will be opposites of each other Equal forces don’t always have equal effects, think of gravity

Action and reaction pairs are everywhere List 5 examples of action and reaction pairs you see in your everyday life.

Momentum Momentum is a property of moving objects resulting from its mass and velocity. Momentum describes the tendency of objects to keep going in the same direction with the same speed. Changes in momentum result from forces or create forces.

Momentum = mass x velocity p=mv Units for momentum are kg m/s

Momentum Example problem Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley toward the pins.

Momentum is conserved in collisions Total momentum before a collision is the same total momentum after the collision

When one object hits another object, some or all of the momentum of the first object is transferred to the second object and the rest of the momentum stays with the first object.

Example of momentum being transferred The cue ball is moving forward with momentum. The billiard ball’s momentum is zero. When the cue ball hits the billiard ball, the action force makes the billiard ball move forward. The reaction force stops the cue ball. Because the cue ball’s momentum was transferred to the billiard ball, the cue ball’s momentum after the collision is zero.