CH 3—Forces

Force, Mass, and Acceleration

• For any object, the greater the force is that’s applied to it, the greater its acceleration will be.

• The acceleration of an object depends on its mass, as well as the force exerted on it.

Newton’s 2nd Law of Motion

• The net force acting on an object causes the object to accelerate in the direction of the net force.

• Force = mass x accelerationF = ma

• SI unit—kg m/s2 = Newton (N)

F

m x a

• You are pushing a friend on a sled. You push with a force of 40 N. Your friend and the sled together have a mass of 80 kg. Ignoring friction, what is the acceleration of your friend the sled?0.5 m/s2

• A tennis ball with a mass of 0.06 kg had an acceleration of 5,500 m/s2 as it raced across the court. How much force would the tennis racket have to exert to give the ball this acceleration?330 N

Friction

• friction—force that opposes motion between two surfaces that are touching each other

• The amount of friction between two surfaces depends on two factors:– kind of surfaces– force pressing the surfaces together

Air Resistance• When an object falls toward Earth, it is

pulled downward by the force of gravity. Another force, called air resistance, acts in the upward direction on all falling objects.

• The amount of air resistance depends on the speed, size, and shape of the object.

Terminal Velocity

• The force of air resistance increases until it becomes large enough to cancel the force of gravity.

• The forces on the falling objects are balanced, so the object no longer accelerates.

• It then falls at a constant speed called the terminal velocity (highest velocity a falling object will reach).

Section 2—Gravity

• According to the law of gravitation, any two masses exert an attractive force on each other.

• The attractive force depends on:– Mass of two objects– Distance between two objects

• Near Earth’s surface, the gravitational attraction of the Earth causes all falling objects to have an acceleration of 9.8 m/s2.

• acceleration due to gravity = 9.8 m/s2

• velocity of a falling object = acceleration due to gravity x time

• v = a x t

Acceleration Due to Gravity

• An acorn falls from a tree and hits the ground in 1.5 s. What is the acorn’s velocity?– v = a x t– (9.8 m/s2) (1.5 s)– 14.7 m/s

• Josh is climbing on the rafters. Brittany distracts him and he falls to the ground in 0.5 s. What is Josh’s velocity when he hits the ground?– 4.9 m/s

Weight

• Whether you are standing, jumping, or falling, Earth exerts a gravitational force on you.

• weight—gravitational force exerted on an object

• You can determine the weight of anything by using Newton’s 2nd Law:– F = m x a W = m x g

• How much does a person weigh on Earth if he has a mass of 70 kg?– (70 kg) (9.8 m/s2) = 686 N

• Gravity on the moon is 1/6 what it is on Earth, so your weight on the moon would be 1/6 what it is on Earth.– A person weighs 480 N on Earth. What does

he weigh on the moon?– w = (480 N)(1/6) = 80 N

Weight and Mass

• Weight and mass are not the same. Weight is a force. Mass is the amount of matter in an object.

• The mass of an object remains the same on any planet and with or without gravity.

Projectile Motion

• projectile—anything that is thrown or shot through the air

• Because of Earth’s gravitational pull and their own inertia, projectiles follow a curved path.

• horizontal velocity—velocity parallel to Earth’s surface

• vertical velocity—velocity perpendicular to Earth’s surface

• Figure 16 page 80

Centripetal Force

• centripetal acceleration—acceleration toward the center of a curved or circular path

• centripetal force—force that causes acceleration toward the center of the circle– In the case of a car rounding the curve, the

centripetal force is the friction between the tires and the road surface.

Section 3—Newton’s 3rd Law of Motion

• Newton’s 3rd Law of Motion—When one object exerts a force on a second object, the second object exerts a force on the first that is equal in size but opposite in direction.

• “To every action, there is an equal and opposite reaction.”

• Describe a person walking on a sidewalk using Newton’s 3rd Law of Motion.– A person exerts a force on the sidewalk and

the sidewalk pushes back on the person, moving the person forward.

• Describe a fish swimming in the ocean using Newton’s 3rd Law of Motion.– A fish pushes on the water with his fins and

the reaction force of the water on the fish propels the fish forward.

Momentum

• A moving object has a property called momentum that is related to how much force is needed to change its motion.

• momentum = mass x velocityp = m x v

• SI unit: kg m/s

Law of Conservation of Momentum

• The momentum of an object does not change unless its mass, velocity or both change. Momentum can, however, be transferred from one object to another.