Connecting Force and Motion, and Newton’s First Law of Motion

Connecting Force and Motion, and Newton’s First Law of Motion Question: If you were aboard the Starship Enterprise and you accelerated to light speed in an instant what would happen to you? Also, what would happen to you if you stopped in an instant? A. Let’s first examine what happens when a jet or starship turns on its engine: 1. The hot air produces thrust for the jet, and fusion reaction pushes gas and radiation backward propelling the starship forward. a. What is thrust? Thrust is a force that propels/pushes an object forward - What is a force? Force is a push or pull one object exerts on another b. Why is force important? - Force is what is going to determine what an object does Example: If an object is moving does the force always change an objects velocity? - Sometimes - 2 types of forces 1. Balanced Force – Forces on an object that are equal in in size and opposite in direction Example: You are having a tug of war match and you plant yourself firmly and lean back to push against the ground causing the ground to push back on you. Your opponent does the same. If you do not move forward or backward, the force of your opponent pulling the rope and must be balancing the force the ground pushing you back. 2. Unbalanced Force – Force on an object that are not not equal in size and opposite in direction - In this case the net force always changes the velocity of an object c. How is force calculated? - It is calculated F = M x A where F = Force, M = Mass, and A = Acceleration - The triangle can be applied to this equation - From this equation you can calculate Force, Mass, and Acceleration. - You must take note that you now have 2 ways to calculate Acceleration. This is why it is so important to recognize what information has been given to you in order to choose the correct formula - Force is directly related to mass and acceleration Example: If you had to push soccer ball or car up a hill which one would you pick and why? A soccer because has less mass and you don’t have to exert as much energy B. Newton’s First Law of Motion (also known as the Law of Inertia) 1. An object at rest tends to stay at rest and an object in motion tends to stay in motion a. What does this have to do with inertia? - Inertia is the tendency of an object to resist any change in its motion b. Let’s think back to the starship questions. What made the starship go? - The thrust of the engine: it was an unbalanced force that caused the ship to change velocity c. When a jet lands what makes the jet stop? - No more thrust being produced, air resistance, and friction from the ground - The forces go from being unbalanced to balanced once the jet stops d. How is mass related to inertia? - The greater the mass of an object the greater the interia of the object e. Think about this: Ice is very slippery. If you push a hockey puck on the ice it goes a long distance. However it eventually stops. If Newton’s First Law is true why does the puck stop? - Friction: is the force that opposed motion between two surfaces that are touching each other Newton’s Laws of Motion Newton’s First Law of Motion or Law of Inertia - Objects in motion tend to stay in motion or objects at rest tend to stay at rest unless acted upon by a force Forces acting upon a moving object: 1. Gravity 2. Friction 3. Wind 4. Other objects 5. Water current Forces acting upon a resting object: 1. Pushing 2. Pulling Newton’s Second Law of Motion - A net force acting on an object causes an object to accelerate in the direction of the force - Deals directly with accelerated motion o Example: Hitting a baseball – the direction of the force of the swinging bat cause the ball to go in that direction - Acceleration (change in speed and or direction) is determined by: 1. Size of the force 2. Mass of the object -So, the larger the mass the greater the force needed to achieve the same acceleration as an object with a smaller mass - . ` Calculating Force: F = M x A Where F = Force, M = Mass, and A = Acceleration Things you need to be aware of: 1. Mass is expresses in kg 2. Acceleration is expressed in m/s2 3. Force is expressed in kg x m/s2 or Newtons (N) Example: How much force is needed to accelerate a 70 kg rider and her 200 kg motorcycle at 4 m/s2? F=MxA M = 200 kg and 70 kg F = (200 kg + 70 kg) x 4 m/s2 F = 1080 N or 1080 kg x m/s2 A = 4 m/s2 F=? Example: A 63 kg skater pushes off from a wall with a force of 300 N. What is the skaters acceleration? A = F/M A = ? F = 300 N M = 63 kg A = 300N/63 kg A = 4.76 m/s2 Lets Introduce Gravity: Answer the following question? Which object would hit the ground first: A bowling ball or a tennis ball? Answer: Neither, they would hit at the same time Why? Answer: Earth’s gravity causes all falling objects to accelerate at 9.8 m/s2 Remember: 1 kg weighs 9.8 N on earth If you take F = M x A  W = M x A Where W = weight (remember weight is a measure of the pull of gravity on an object) If acceleration due to gravity is 9.8 m/s2, then: W = M x 9.8 m/s2 Which means 1 kg = 9.8 (kg x m/s2) or 9.8 N Example: A weightlifter raises a 440 kg barbell with an acceleration of 2 m/s2. How much force does the weightlifter exert on the barbell? F = M x A F = ? M = 440 kg A = 9.8 m/s2 + 2 m/s2 F = 440 kg x (9.8 m/s2 + 2 m/s2) F = 5200 N Note: This all depends if the object is freely falling. If the object is thrown or some other force pushes or pulls the object you must take this into account when making your calculation Question: Which object would hit the ground first: A crumpled piece of paper or a non crumpled piece of paper. Answer: A crumpled piece of paper Why is that if they both have the same mass and they are freely Falling? Answer: Air resistance!!!! – Air pushing up or against the falling object will slow the acceleration Terminal Velocity – Highest velocity that will be reached by a falling object - Basically when an object falls it will eventually stop accelerating due to the balancing force between gravity and air resistance. The object will continue to fall however it will be at a constant velocity. Newton’s Third Law of Motion Question: What will happen if you try to leap from a row boat onto the dock? III. Newton’s Third Law of Motion states when one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction 1. What does that mean? For every action there is an equal but opposite reaction Example: The force you are exerting on your seat is the same force your seat is exerting on you - You are pushing the seat down and the seat is holding you up. 2. Back to the initial question: What happens? You fall in a. Why? When you leap from the boat, the boat exerts a force on your feet, moving you forward. Your feet also exert an equal and opposite force on the boat sending it backward. 3. Is this how a rocket works? Yes!!!! a. Balloon Rockets – When the air was released from the balloon the force of the escaping air went one way the balloon went the other 1. The forces were equal and opposite 2. If you did not release the air from the balloon the force of the air inside the balloon is acting on the balloon. However there is nowhere for the air to go. When you open the balloon the air pushing against the opposite end of the opening of the balloon is pushing against the balloon (action force) causing the air to escape out the bottom thrusting the balloon forward 4.What is momentum? a. Is a property of any moving object resulting from its mass and Velocity 1. To calculate momentum: p = mv p = momentum, m = mass, v = velocity - The unit for momentum (p) is kg x m/s Momentum has a direction because velocity has a direction. However if an object is not moving, it has zero momentum. Question: If a motorcycle is traveling north at 100 mph and a small plane was traveling the same direction at 100 mph do they both have the same momentum? o No, because the mass of the plan is greater than the mass of the motorcycle Example: Which object has a greater momentum: a 3000 kg elephant traveling at 3 m/s or a 100 kg bike traveling at 25 m/s? P=MxV Elephant p = 3000 kg x 3 m/s = 9000 kg x m/s Motorcycle p = 100kg x 25m/s = 2500 kg x m/s The elephant has the greater momentum b. So the momentum of an object doesn’t change unless you change the velocity or mass of the object Law of Conservation of Momentum - States the total momentum of a group of objects does not change unless outside forces act on the objects