# CH 3—Forces.

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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. Have students restate Newton’s 1st Law of Motion. Force and motion are connected. How does force cause motion to change? Suppose your car breaks down at a red light. You need to get the car to the side of the road. You get out and start pushing. The greater the force (or push) you apply, the faster it will accelerate (or move). Suddenly, a cute guy or girl pulls up to help you in your time of distress. He or she begins to help you push. The car will reach the same velocity as you pushing it alone, only a lot faster. The acceleration is in the direction of the force. In other words, if you want the car to move forward, you push forward. If you throw a softball and a baseball as hard as you can, why don’t they have the same speed? (The difference is due to their masses). The softball has a mass of about 0.2 kg and the baseball has a mass of about 0.14 kg. The softball has less velocity as it leaves your hand than the baseball does, even though you exerted the same force. Complete Quick Demo on page 69.

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 acceleration F = ma SI unit—kg m/s2 = Newton (N) F m x a Mass—kg Acceleration—m/s2

You are pushing a friend on a sled. You push with a force of 40 N
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 BONUS: What is a nearly frictionless surface? Teflon What causes friction? Figure 4 page 71 microwelds formed between two surfaces The larger the force pushing the two surfaces together is, the stronger the microwelds will be.

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. After 1st bullet: Air resistance affects anything that has mass in Earth’s atmosphere. Objects with greater surface area, will be affected more. What would happen if no air resistance was present? (Everything would fall at the same rate.) Read Visual Learning on page 73. Automobile designers try to reduce air resistance in order to make cars more efficient. Nascar builders test the cars in wind tunnels so that they can find tricks or ways to reduce air resistance. They are constantly changing the styles or parts of the car to help with this.

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). The force of air resistance increases with speed. As an object falls, it accelerates and its speed increases.

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 Read the 1st paragraph on page 75.

Acceleration Due to Gravity
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

An acorn falls from a tree and hits the ground in 1. 5 s
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 After 1st bullet: Remember gravitational attraction depends on mass. A planet or moon that has more or less mass than Earth would have more or less of a gravitational attraction on your body. Table 1 page 78

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. After 2nd bullet: Discuss what measuring device is used to measure weight and mass. Read “Weightlessness and Freefall” and “Floating in Space” on pages 78 and 79.

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. Before 1st bullet: Describe how a football must be thrown for a long pass. Demonstrate. After 2nd bullet: That’s why quarterbacks, dart players and archers aim above their targets. Demonstrate with Nerf gun.

horizontal velocity—velocity parallel to Earth’s surface
vertical velocity—velocity perpendicular to Earth’s surface Figure 16 page 80 Before 1st bullet: Projectiles follow the curved path because they have horizontal and vertical velocities. After 1st bullet: When something is thrown, the force from your hand makes the object move forward—with horizontal motion. After you let go, no other force accelerates it forward, so horizontal velocity is constant (if you ignore air resistance). After 3rd bullet: After leaving the pitcher’s hand, the ball has velocities in two directions—a constant horizontal velocity and an accelerating vertical velocity (due to gravity). The result of these two motions is that the ball appears to travel in a curve.

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. After 1st bullet: Any object will continue in a straight-line path unless a net force acts on it. So what causes a car to round a sharp curve? At end: Anything that moves in a circle is doing so because a centripetal force is accelerating toward the center. Complete Minilab page 81 Frictional force is limited so tires can slip, especially if a road is wet or if you are driving too fast. The road may not exert enough force to accelerate the car toward the center of the circle, thus causing the car to continue in its straight path running off the road. Highways and recetracks are banked on curves to increase the available force toward the center. Read “The Moon is Falling” page 82

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.” Read 1st paragraph on page 83. Read “Action and Reaction” paragraphs.

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. Read “rocket propulsion” page 84.

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 velocity p = m x v SI unit: kg m/s

See Figure 25 page 87 The two trucks may have the same velocity, but the bigger truck has more momentum b/c of its greater mass.

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. Before the game starts, all the balls are motionless. (momentum=0) When the cue ball hits the group of motionless ballsit slows down and the rest of the balls continue to move. The total momentum before and after the collision has not changedthe momentum gained by the group of balls is equal to the momentum lost by the cue ball. Read Analogy page 87