Forces The Nature of Force Friction and Gravity Newton’s First and Second Laws Newton’s Third Law Rockets and Satellites Table of Contents

Motion Book M – Ch 2 – Section 1 (Page 36-39) The Nature of Force A force is a push or a pull. Like velocity and acceleration, a force is described by its strength and by the direction in which it acts. The strength of a force is measured in the SI unit called newton (N).

Motion - The Nature of Force Book M – Ch 2 – Section 1 (Page 36-39) Combining Forces The combination of all forces acting on an object is called the net force.

Motion - The Nature of Force Book M – Ch 2 – Section 1 (Page 36-39) Unbalanced Forces vs. Balanced Forces Unbalanced forces can cause an object to start moving, stop moving, or change directions. Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion. Balanced forces are equal forces acting on one object in opposite directions. Balanced forces acting on an object do not change the object’s motion.

Motion - The Nature of Force Book M – Ch 2 – Section 1 (Page 36-39) Unbalanced Forces vs. Balanced Forces

Motion Links on Force Click the SciLinks button for links on force. - The Nature of Force

Motion Book M – Ch 2 – Section 2 (Page 42-50) Friction and Gravity The force that two surfaces exert on each other when they rub against each other is called friction. The strength of the force of friction depends on two factors: 1.How hard the surfaces push together. 2.The types of surfaces involved.

Motion Book M – Ch 2 – Section 2 (Page 42-50) Four Types of Friction 1.Static Friction – the friction that acts on objects that are not moving. 2.Sliding Friction – occurs when two solid surfaces slide over each other. 3.Rolling Friction – occurs when an object rolls across a surface. 4.Fluid Friction – occurs when a solid object moves through a fluid.

Motion Book M – Ch 2 – Section 2 (Page 42-50) Gravity Gravity is a force that pulls objects toward each other. The law of universal gravitation states that the force of gravity acts between all objects in the universe. Two factors affect the gravitational attraction between objects: 1.Mass 2.Distance

Motion - Friction and Gravity Book M – Ch 2 – Section 2 (Page 42-50) Mass vs. Weight Mass is a measure of the amount of matter in an object. The force of gravity on a person or object at the surface of a planet is known as weight.

Motion Book M – Ch 2 – Section 2 (Page 42-50) Free Fall When the only force acting on an object is gravity, the object is said to be in free fall. In free fall, the force of gravity is an unbalanced force, which causes the object to accelerate. Acceleration due to gravity is 9.8 m/s². This means for every second an object is falling, its velocity increases by 9.8 meters per second (m/s). - Friction and Gravity

Motion Free Fall Use the graph to answer the following questions. - Friction and Gravity

Motion Free Fall Time is on the horizontal axis, and speed is on the vertical axis. Interpreting Graphs: What variable is on the horizontal axis? The vertical axis? - Friction and Gravity

Motion Free Fall The slope is 9.8. The speed increases by 9.8 m/s each second. Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? - Friction and Gravity

Motion Free Fall 58.8 m/s Predicting: What will the speed of the object be at 6 seconds? - Friction and Gravity

Motion Free Fall The speed values would not change. Drawing Conclusions: Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? - Friction and Gravity

Motion - Friction and Gravity Book M – Ch 2 – Section 2 (Page 42-50) Air Resistance Falling objects with a greater surface area experience more air resistance. This is why a leaf falls more slowly than an acorn. Air resistance increases with velocity. The greatest velocity a falling object reaches is called terminal velocity.

Motion - Friction and Gravity Book M – Ch 2 – Section 2 (Page 42-50) Projectile Motion An object that is thrown is called a projectile.

Motion FrictionGravity Comparing and Contrasting As you read, compare and contrast friction and gravity by completing a table like the one below. Effect on motion Opposes motion Pulls objects toward one another Depends on Types of surfaces involved, how hard the surfaces push together Mass and distance Measured inNewtons - Friction and Gravity

Motion Links on Friction Click the SciLinks button for links on friction. - Friction and Gravity

Motion Free Fall Click the Video button to watch a movie about free fall. - Friction and Gravity

Motion

Book M – Ch 2 Section 3 (pgs ) Inertia - the tendency of an object to resist a change in motion Dependent on mass - the more mass, the more inertia (resists change more) - Newton’s First and Second Laws Newton’s First Law of Motion States that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force.

Motion Book M – Ch 2 Section 3 (pgs ) Newton’s Second Law of Motion Acceleration depends on the object’s mass and on the net force acting on the object Force – measured in Newtons (N) or (kg  m/s 2 ) Mass – measured in kilograms (kg) Acceleration – measured in (m/s 2 ) 1 newton has the force required to give a 1-kg mass an acceleration of 1 m/s 2 - Newton’s First and Second Laws

Motion Book M – Ch 2 Section 3 (pgs ) What two ways can you increase acceleration?? Increase force Decrease mass - Newton’s First and Second Laws Let’s Practice!

Motion Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s 2 - Newton’s First and Second Laws

Motion Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (F) = __ What formula contains the given quantities and the unknown quantity? a = F/m or F = m X a Perform the calculation. F = m X a = 55 kg X 2.0 m/s 2 F = 110 kg m/s 2 F = 110 N - Newton’s First and Second Laws

Motion Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required to accelerate the water- skier. This may not seem like enough force, but it does not include the force of the speedboat's pull that overcomes friction. - Newton’s First and Second Laws

Motion Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ? 3,000 N (1,000 kg X 3 m/s 2 ) - Newton’s First and Second Laws

Motion Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s 2 ? 350 N (25 kg X 14 m/s 2 ) - Newton’s First and Second Laws

Motion More on Newton’s Laws Click the PHSchool.com button for an activity about Newton’s laws. - Newton’s First and Second Laws

Motion Book M – Ch 2 Section 4 (pgs ) Newton’s Third Law of Motion States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. For every action there is an equal but opposite reaction - Newton’s Third Law

Motion Book M – Ch 2 Section 4 (pgs ) Motion can not always be observed – Ex. Earth’s gravity pulling on a pencil vs. pencil pulling on the earth. Action-Reaction forces do not cancel (like in Newtons 1 st law of motion) because they are acting on different objects - Newton’s Third Law

Motion Book M – Ch 2 Section 4 (pgs ) Momentum – a characteristic of a moving object that is related to the mass and the velocity of the object. Mass – measured in kilograms (kg) Velocity – measured in meters per second (m/s) Momentum – measured in kilogram meters per second (kg·m/s) Described by its direction as well as its quantity An objects momentum is the same direction as it’s velocity - Newton’s Third Law

Motion Book M – Ch 2 Section 4 (pgs ) The more momentum a moving object has, the harder it is to stop. Increased mass and/or increased velocity will increase momentum Which has more momentum? Baseball traveling at 20 m/s or Car traveling at 20 m/s Which has more momentum? 2 ton car traveling at 10 m/s or 2 ton car traveling at 50 m/s - Newton’s Third Law

Motion Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Read and Understand What information have you been given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s - Newton’s Third Law

Motion Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Plan and Solve What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass X Velocity Perform the calculation. Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kgm/s LARGER sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kgm/s - Newton’s Third Law

Motion Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Look Back and Check Does your answer make sense? The 3.0-kg hammer has more momentum than the 4.0-kg one. This answer makes sense because the 3.0-kg hammer is swung at a greater velocity. - Newton’s Third Law

Motion Calculating Momentum Practice Problem A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has the greater momentum? Golf ball: kg X 16 m/s = 0.72 kgm/s Baseball: 0.14 kg X 7 m/s = 0.98 kgm/s The baseball has greater momentum. - Newton’s Third Law

Motion Calculating Momentum Practice Problem What is the momentum of a bird with a mass of kg flying at 15 m/s? 0.27 kgm/s (0.018 kg X 15 m/s = 0.27 kgm/s) - Newton’s Third Law

Motion Book M – Ch 2 Section 4 (pgs ) Law of Conservation of Momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change. Example of “outside force” – Friction - Newton’s Third Law

Motion - Conservation of Momentum In the absence of friction, momentum is conserved when two train cars collide. - Newton’s Third Law

Motion Momentum Activity Click the Active Art button to open a browser window and access Active Art about momentum. - Newton’s Third Law