Acceleration and Momentum Chapter 3 April 17 General Science Chapter 3

General Science Chapter 3 Review Newton’s first law Weight of one kilogram on Earth’s surface Gravity April 17 General Science Chapter 3

General Science Chapter 3 Newton’s second law A net force acting on an object causes the object to accelerate in the direction of the force. The acceleration is affected by two things size of force mass of object April 17 General Science Chapter 3

General Science Chapter 3 Force If the force on an object increases, acceleration will increase for that object and if force decreases the acceleration will decrease. IF THE MASS REMAINS CONSTANT April 17 General Science Chapter 3

General Science Chapter 3 Mass A more massive object will take a larger force to give it the same acceleration as a smaller massed object. April 17 General Science Chapter 3

Equation for Newton’s 2nd law We can also use the “magic” triangle April 17 General Science Chapter 3

General Science Chapter 3 Units on force Last chapter we learned that force has units of Newtons. From Newton’s second law equation, April 17 General Science Chapter 3

General Science Chapter 3 Example How much force is needed to accelerate a 70-kg rider and the 200-kg motorcycle the rider is on at 4 m/s2? F = 270 kg x 4 m/s2 1080 N April 17 General Science Chapter 3

General Science Chapter 3 You try How much force is needed to accelerate a 60-kg person and the 500-kg car the person is in at 6 m/s2? F = 560 kg x 6 m/s2 3360 N April 17 General Science Chapter 3

General Science Chapter 3 You try A 63-kg skater pushes off from a wall with a force of 300 N. What is the skater’s acceleration? 300 N = 63 kg • a a = 300 N ÷ 63 kg 4.76 m/s2 April 17 General Science Chapter 3

General Science Chapter 3 Review #1 What is Newton’s 2nd Law of Motion? What equation do we use with the 2nd Law of Motion? What 2 things will affect acceleration? What are the units for Force? True or False: A more massive object will take less force to accelerate? April 17 General Science Chapter 3

Acceleration due to gravity Near Earth’s surface, gravity causes all falling objects to accelerate at a rate of 9.8 m/s2, regardless of their mass. Acceleration due to gravity is represented by the letter g. Does this number seem familiar? April 17 General Science Chapter 3

General Science Chapter 3 Weight The weight of an object is the force of gravity acting on it. April 17 General Science Chapter 3

General Science Chapter 3 Reality check Do all objects really fall with the same acceleration? April 17 General Science Chapter 3

General Science Chapter 3 Air resistance The force air exerts on a moving object. Acts in the opposite directions to which an object is moving. For falling objects, air resistance pushes up while gravity pulls down. April 17 General Science Chapter 3

General Science Chapter 3 Air resistance Depends on the speed, size, and shape of the object. The larger the object, the more air resistance affects it. The faster an object is moving, the more air resistance affects it. April 17 General Science Chapter 3

General Science Chapter 3 Terminal velocity As an object falls, air resistance gradually increases until it equals the pull of gravity. At this point, the object stops accelerating and moves with a constant velocity – called its terminal velocity. April 17 General Science Chapter 3

General Science Chapter 3 Example A ball is dropped from the roof of a 50-m tall building. It hits the ground 3.2 s later. What is its velocity right before it hits the ground? a = v ÷ t OR v = a • t v = 9.8 m/s2 x 3.2 s 31.36 m/s April 17 General Science Chapter 3

General Science Chapter 3 Review #2 Define air resistance. What is air resistance dependent upon? What is terminal velocity? What is the acceleration of gravity near the surface of the earth? How do we determine the weight of an object? April 17 General Science Chapter 3

General Science Chapter 3 Projectiles Anything that’s shot or thrown through the air. Will always follow a curved path. April 17 General Science Chapter 3

General Science Chapter 3 Projectile motion Can be split into independent vertical and horizontal motions. April 17 General Science Chapter 3

General Science Chapter 3 The horizontal part Once the object is released, there is no force acting on it horizontally. If there is no force, it must maintain a constant horizontal velocity. April 17 General Science Chapter 3

General Science Chapter 3 The vertical part There is a force acting – gravity The object is pulled downward with a constant acceleration of 9.8 m/s2. April 17 General Science Chapter 3

General Science Chapter 3 Circular motion Objects moving in circles are constantly accelerating. This acceleration is towards the center of the circle or curve. It is called centripetal acceleration. Towards the center of a circle or a curved path. April 17 General Science Chapter 3

General Science Chapter 3 Centripetal force The force that causes an object to move in a curved or circular path. April 17 General Science Chapter 3

General Science Chapter 3 Weightlessness To be truly weightless, an object would have to be free from gravity. To feel weightless, something has to be in free fall along with its surroundings. April 17 General Science Chapter 3

General Science Chapter 3 Weightlessness How does a scale measure weight? If it can’t push back, it would read zero. You would feel weightless. April 17 General Science Chapter 3

General Science Chapter 3 Review #3 What is a projectile? What are the 2 components of a projectile? What does centripetal mean? Why are objects weightless during free fall? April 17 General Science Chapter 3

General Science Chapter 3 Newton’s 3rd law To every action, there is an equal and opposite reaction. When one object exerts a force on a second object, the second object exerts a force on the first object that is equal in size and opposite in direction. April 17 General Science Chapter 3

General Science Chapter 3 Examples Released balloon Firing a gun Jumping out of a boat rockets April 17 General Science Chapter 3

General Science Chapter 3 Balanced forces? Action-reaction pairs of forces are not balanced forces. The forces act on different objects. To be balanced forces, the equal and opposite forces must act on the same object April 17 General Science Chapter 3

General Science Chapter 3 Momentum A property a moving object has because of its mass and velocity April 17 General Science Chapter 3

General Science Chapter 3 Momentum Has units of kg∙m/s Has direction, because velocity has direction April 17 General Science Chapter 3

General Science Chapter 3 Momentum Which has a higher momentum, a 50-kg dolphin swimming at 16.4 m/s or a 6300-kg elephant walking at 0.11 m/s? Dolphin – (50 kg)(16.4 m/s) = 820 kg∙m/s Elephant – (6300 kg)(0.11 m/s) = 693 kg∙m/s The dolphin has higher momentum. April 17 General Science Chapter 3

General Science Chapter 3 Example Which has higher momentum, a 65-kg person running at 9 m/s or a 60-kg person running at 11 m/s? 1st person – (65 kg)(9 m/s) = 585 kg∙m/s 2nd person – (60 kg)(11 m/s) = 660 kg∙m/s The 60-kg person has higher momentum. April 17 General Science Chapter 3

Law of conservation of momentum The total amount of momentum of a group of objects does not change unless outside forces act on the objects. April 17 General Science Chapter 3

General Science Chapter 3 Momentum transfer Momentum can be transferred from one object to another. Think about playing pool The cue ball transfers its momentum to the ball it hits. The momentum lost by the cue ball is gained by the other ball. April 17 General Science Chapter 3

General Science Chapter 3 Example A 5-kg object with a velocity of 6 m/s strikes a motionless 10-kg ball. The 5 kg object stops moving. What is the velocity of the 10 kg ball after the collision? Momentum of object before collision: (5 kg)(6 m/s) = 30 kg∙m/s April 17 General Science Chapter 3

General Science Chapter 3 Example continued When the object stops, it loses all its momentum. The momentum it lost is transferred to the ball. Momentum of the ball after collision: (10 kg)(velocity) = 30 kg∙m/s Velocity = 3 m/s April 17 General Science Chapter 3

General Science Chapter 3 Example continued Notice that the velocity was not conserved, but momentum was. April 17 General Science Chapter 3

General Science Chapter 3 Review #4 What is Newton’s 3rd Law of Motion? Define momentum. What units does momentum have? What is the law of conservation of momentum? April 17 General Science Chapter 3

General Science Chapter 3 Chapter 3 Review State all 3 Laws of Motion What 2 things affect acceleration on an object What is air resistance dependent upon? How are the Law of Inertia and the Law of conservation of momentum related? April 17 General Science Chapter 3