Physics First! Unit 1, Chapter 3

Unit One: Forces and Motion Chapter 3 Forces and Motion 3.1 Force, Mass and Acceleration 3.2 Weight, Gravity and Friction 3.3 Equilibrium, Action and Reaction

Chapter 3 Learning Goals Describe the natural motion of objects (i.e. Newton's 1st Law). Describe Mass as a measure of quantity of matter and of inertia. Describe how Friction resists the motion of objects. Explain the concept of net Force. Describe a vector quantity and the basic concepts of combining vectors. Explain Newton's 2nd Law. Contrast Mass and Weight. Compute Mass or Weight given the other. Explain gravitational force and how it affects the motion of an object. Describe the motion of an object in Free Fall, and solve simple problems. Explain that forces come in pairs when objects interact, exerting equal and opposite forces on each other.

3.1 Force, Mass and Acceleration Unless you apply force, things tend to keep on doing what they were doing in the first place. Force causes an object to accelerate, while the object’s mass resists acceleration. For every action, there is an equal and opposite reaction.

Newton’s First Law – The Law of Inertia An object at rest remains at rest, and an object in motion remains in motion in a straight line at a constant speed, unless acted upon by a net force. Straight line, constant speed…What else do we call that? Constant Velocity Inertia: The tendency of an object to resist a change in its motion.

What happens when: You’re rolling on skates or a skateboard and you hit a stone? Your feet stop, your body keeps moving, and you fall down. (Wear your helmet!) You’re riding in a car and the driver suddenly makes a sharp turn? Your body seems to move sideways.

What happens when: How does Inertia relate to these situations? In both cases, an object in motion tries to stay in motion. Describe another situation where inertia plays a part.

Mass is: A measure of the quantity of matter contained in an object. A measure of Inertia; a measure of the resistance of an object to change its motion. Which has more mass, a bowling ball or a baseball? Which has more inertia? Which is easier to get moving?

Mass is NOT: Volume: Objects of the same mass can have different volumes. A kg of marshmallows vs a kg of sand. Weight: Weight is the Force that gravity exerts.

Gravitational vs. Inertial Mass Gravitational Mass is measured using the force of gravity, as with a balance. Inertial Mass is measured using resistance to change in motion. Gravitational Mass = Inertial Mass

Concept Questions The law of inertia states that no force is required to maintain motion. Why, then, do you have to keep pedaling your bicycle to maintain motion? You must pedal to offset the friction force that resists your motion. Does a 2-kg rock have twice the mass of a 1-kg rock? Twice the inertia? Twice the weight? The 2-kg rock has twice the mass, twice the inertia, and twice the weight of the 1-kg rock.

Concept Questions Does a liter of molten lead have the same volume as a liter of soda? Does it have the same mass? The lead and soda have the same volume. The lead has much more mass. When a junked car is crushed into a compact cube, does its mass change? Its volume? Its weight? The car’s mass and weight do not change, but its volume is reduced.

Concept Questions On a jet plane flying at 500 mph, you can toss a ball straight up and it falls straight back down to your hands. Why doesn’t the ball fly backward in the plane? The ball is moving at the same speed as you and the plane. It keeps moving with you even as it goes up and back down.

Concept Questions Many automobile passengers suffer neck injuries (whiplash) when struck by cars from behind. How does Newton’s law of inertia apply here? How do headrests help to guard against this type of injury? The head has inertia and tries to keep the same motion even as the car is pushed forward. Headrests push the head forward with the rest of the car.

Concept Questions Think of and describe another situation that shows Newton’s law of inertia. My examples: Air hockey puck. Tablecloth trick. Why you should wear a seatbelt.

Units of Force The SI unit of Force is the Newton (N). 1 Newton = 1 kg.m/s2 Derived from Mass x Acceleration. The English unit of Force is the Pound (lb) 1 lb = 4.448 N

Net Force (Fnet) Net Force: The combined effect of all forces acting on an object. What forces act on: A book resting on a table? A biker riding on a road?

Force is a Vector Quantity That means that direction is important. What’s another Vector quantity that you have learned about? Velocity Combining Vector quantities is a bit different…

Diagramming Forces How could we show or sketch the forces acting on an object? Free Body Diagram – Represents all forces acting on an object. Forces are shown as labeled arrows. FL FT Fg FD

Equilibrium: When Net Force Equals Zero (Fnet = 0) What happens to an object when the net force on it equals zero? (Forces balance or cancel each other.) The object’s motion does not change. Acceleration = zero. An object at rest remains at rest. An object in motion remains in motion at a constant velocity.

Equilibrium: When Net Force Equals Zero (Fnet = 0) 12 20 Equal forces in opposite directions balance, or cancel each other.

3.1 Newton's Second Law a = F m force (newtons, N) acceleration (m/sec2) mass (kg)

3.1 Force, Mass, and Acceleration Key Question: What is the relationship between force, mass and acceleration? *Read text section 3.1 BEFORE Investigation 3.1

3.2 Gravity What is gravity? Gravity is a force. Gravity depends on mass. Gravity accelerates objects.

3.2 Gravity If you were on Mars, your force/mass balance would have to be adjusted. The planet is smaller than Earth and therefore Mars’s gravity is weaker.

3.2 Weight gravity (9.8 m/sec2) Fw = mg Weight force (N) mass (kg)

3.2 Weight and Galileo A legend has it that, around 1587, Galileo dropped two balls from the Leaning Tower of Pisa to see which would fall faster.

3.2 Gravity The attractive force from gravity between objects of ordinary mass is incredibly small.

3.2 Gravity You feel weight because the mass of Earth is large enough to create significant gravity forces.

3.2 Newton's Law of Universal Gravitation The force of attraction between two objects is directly related to the masses of the objects and inversely related to the square of the distance between them.

3.2 Newton's Law of Universal Gravitation gravity (9.8 m/sec2) mass 1 (kg) mass 2 (kg) Force (N) F = G m1m2 R2 distance (m) between m1 and m2

3.2 Weight, Gravity and Friction Key Question: How does increasing the mass of the car affect its acceleration? *Read text section 3.2 BEFORE Investigation 3.2

3.2 Friction Friction is a force that always opposes motion. What is left is often called the net force. What is the net force acting on the car?

3.3 Equilibrium, Action and Reaction Key Question: What is Newton's third law of motion? *Read text section 3.3 AFTER Investigation 3.3

Newton’s Third Law of Motion We know what happens to an object when forces act on it. But where do forces come from? Forces come in pairs when objects interact. When two objects interact, they exert equal and opposite forces on each other.

Newton’s Third Law of Motion Example: A hanging plant. What objects are interacting? Hook & Ceiling Plant & Hook Plant & Earth What are the forces? Hook pulls down on Ceiling. Ceiling pulls up on Hook. Plant pulls down on Hook. Hook pulls up on plant. Earth pulls down on Plant. Plant pulls up on earth. Label all forces.

Newton’s Third Law of Motion When you walk along a floor, what pushes you along? When you push backward on the floor, the floor pushes you forward.

Newton’s Third Law of Motion How can a rocket be propelled in the vacuum of space where there is no air to "push against"? The rocket pushes exhaust gases backward. The exhaust gases push the rocket forward.

Newton’s Third Law of Motion Newton’s Third Law: For every Action force, there is an equal and opposite Reaction force. Action and Reaction happen at exactly the same time. Note: Action and Reaction forces: Are always equal and opposite. Always occur on different objects. Cannot exist without the other.

Newton’s Third Law of Motion Your weight is the result of the gravitational force of the earth on your body. What is the corresponding reaction force? The objects interacting are your body and the earth. So as earth pulls down on your body, your body pulls up on the earth.

Newton’s Third Law of Motion Be careful not to confuse Force with Motion. Force is the push or pull. Motion may be the result of force, but is NOT a reaction. Action and Reaction are equal, even when the two objects are of different size or mass. Change in motion as a result of the force depends on mass. It will be different for objects of different mass.

Newton’s Third Law of Motion If a bicycle and a truck have a head-on collision, upon which vehicle is the impact force greater? They exert equal and opposite forces on each other. Which vehicle experiences a greater change in motion? The bicycle has a greater change in motion because it is much less massive.

3.3 Equilibrium, Action and Reaction Momentum explains why the speed and the direction of motion are related to the mass of the object

3.3 Momentum velocity (m/sec) Momentum (kg-m/sec) P = mv mass (kg)