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Transcript
Forces Chapter 10 10.1 The Nature of force Force -a push or pull -described by its strength and by the direction in which it acts -an arrow shows you direction and the arrows length represents its strength ex. 20N , 7N Net Force -combination of all forces acting on an object - it determines whether an object moves and in what direction it moves Unbalanced Forces -forces that produce a nonzero net force -changes an objects motion Balanced Forces -equal forces acting on one object in opposite directions - Net Force = 0 - The Nature of Force Balanced Forces Balanced forces acting on an object do not change the object’s motion. - The Nature of Force Unbalanced Forces Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion. - The Nature of Force Combining Forces The combination of all forces acting on an object is called the net force. 10.2 Friction & Gravity Friction -friction acts in a direction opposite to the direction of the motion -2 Factors that affect the force of friction - how hard the surfaces push together -types of surfaces involved -4 Types of Friction -Static Friction -Sliding Friction -Rolling Friction -Fluid Friction Gravity -Law of Universal Gravitation – forces of gravity acts between all objects in the universe. -Factors affecting Gravity -Mass -Distance Gravitation Gravity - Friction and Gravity Two factors affect the gravitational attraction between objects: mass and distance. - Friction and Gravity Gravity The force of gravity on a person or object at the surface of a planet is known as weight. Gravity and Motion -The force of gravity is an unbalanced force, which causes an object to accelerate -Air resistance is a fluid friction, that is an upward force on falling objects. -Air resistance increases with velocity, as a falling object speeds up the force of air resistance increases. -When velocity and the force of air resistance are equal the force is balanced. - Friction and Gravity Free Fall Interpreting Graphs: What variable is on the horizontal axis? The vertical axis? Time is on the horizontal axis, and speed is on the vertical axis. - Friction and Gravity Free Fall Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? The slope is 9.8. The speed increases by 9.8 m/s each second. - Friction and Gravity Free Fall Predicting: What will the speed of the object be at 6 seconds? 58.8 m/s - Friction and Gravity Free Fall Drawing Conclusions: Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? The speed values would not change. - Friction and Gravity Air Resistance Falling objects with a greater surface area experience more air resistance. 10.3 Newton’s 1st & 2nd Laws Newton’s 1st Law of Motion -An object will remain at rest, and an object moving at a constant velocity will continue moving at constant velocity, unless it is acted upon by an unbalanced force. - Inertia depends on mass Newton’s 2nd Law of Motion -Acceleration depends on the object’s mass and on the net force acting on the object. Acceleration = Net Force / Mass Mass x Acceleration = Net Force Newton’s 1st Law of Motion Newton’s 2nd Law of Motion - Newton’s First and Second Laws Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s2 Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (Fnet) = __ What formula contains the given quantities and the unknown quantity? a = Fnet/m or Fnet = m X a Perform the calculation. Fnet = m X a = 55 kg X 2.0 m/s2 F = 110 kg • m/s2 F = 110 N Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. 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 Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s2? 3,000 N (1,000 kg X 3 m/s2) - Newton’s First and Second Laws Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s2? 350 N (25 kg X 14 m/s2) 10.4 Newton’s 3rd Law Newton’s 3rd Law of Motion -For every action there is an opposite but equal reaction. - Action Force = Reaction Force Momentum -Quantity of motion - It is described by its direction and quantity Momentum= Mass x Velocity (kg·m/s) Conservation of Momentum - The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects. Newton’s 3rd Law of Motion - Newton’s Third Law Conservation of Momentum In the absence of friction, momentum is conserved when two train cars collide. - Newton’s Third Law 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? 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 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? What quantities are you trying to calculate? Momentum = Mass X Velocity Smaller sledgehammer = 3.0 km X 1.5 m/s = 4.5 kg•m/s Smaller sledgehammer = 4.0 km X 0.9 m/s = 3.6 kg•m/s - Newton’s Third Law 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. 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 0.045 kg and the mass of the baseball is 0.14 kg. Which has the greater momentum? Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s The baseball has greater momentum. - Newton’s Third Law Calculating Momentum Practice Problem What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? 0.27 kg•m/s (0.018 kg X 15 m/s = 0.27 kg•m/s)