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Transcript
Chapter 11 Force and Newton’s Laws Section 1 – Pages 310 - 315 Newton’s 1st Law Mass and Inertia The mass of an object affects how easy it is to change its motion. motion. – Recall that mass is the amount of matter in an object. – The tendency of an object to resist a change inertia.. in motion is called inertia – The inertia of an object increases as the increases.. object’s mass increases Momentum Just as increasing the mass of an object makes it harder to stop, so does increasing the speed or velocity of an object. The momentum of an object is a measure of how hard it is to stop the object, and it depends on the object’s mass and velocity. The symbol for momentum is p. The units for momentum are kg ● m/s in the direction of the velocity. velocity. Momentum (Practice Problems) A 10,000 kg train is traveling east at 15 m/s. Calculate the momentum of the train. p=m*v p = 10,000 kg * 15 m/s East p = 150,000 kg · m/s East What is the momentum of a car with a mass of 900 kg traveling north at 27 m/s? p=m*v p = 900 kg * 27 m/s North p = 24,300 kg · m/s North Law of Conservation of Momentum In any collision, momentum is transferred from one object to another. Example – Cue ball hits other billiard balls. The Law of Conservation of Momentum states that the total momentum of a group of objects remains constant unless outside forces act on the group. FORCES The Nature of Force: – A force is a PUSH or a PULL that gives energy to an object. – EXAMPLES OF FORCES: Combining Forces The combination of all the forces acting on an object is the net force. force. When these forces are unbalanced unbalanced,, this causes an object to start moving, stop moving or change direction. direction. Unbalanced forces acting on an object will change the object’s motion. Combining Forces Unbalanced Forces (change motion) – Two forces acting in the same direction combine by adding together. EX - dogs pulling a sled team Combining Forces Unbalanced Forces (change motion) – Two forces acting in opposite direction combine by subtraction subtraction.. – If one force is greater than the other force, the overall force is in the direction of the greater force Ex. – Pushing open a door. Combining Forces Balanced Forces (no change in motion) – Equal forces acting on one object in opposite directions are called balanced forces. forces. – Balanced forces acting on an object will NOT change the objects motion. Example – Pushing on a wall. Explain the Example Unbalanced Forces in opposite direction (Push) Explain the Example Balanced Forces in opposite directions (Pull) Explain the Example Tailwind Unbalanced Forces in same direction (Push) Balanced vs. Unbalanced Forces Newton’s 1st Law of Motion Newton’s 1st Law of Motion states: An object at rest will continue to stay at rest AND an object in motion will continue to stay in motion unless acted on by an outside force. Therefore, since inertia is the tendency of an object to resist motion….. motion….. Newton’s 1st Law of Motion is also known as the Law of Inertia. Newton’s 1st Law of Motion Remember, the amount of inertia an object has depends on its mass mass.. The greater the mass of an object the greater the inertia. Which is easier to move? Which has more inertia? FRICTION The force that opposes motion is known as friction. Friction is a force that acts to oppose sliding between two surfaces that are touching. FRICTION Creates…… Rubbing your hands together creates friction TRY IT! What Happens? Friction creates heat! Apply more pressure to your hands, what does it do? Pressure increases the amount of friction! Now try rubbing only ONE FINGER along your palm? What happens to friction? A smaller surface area decreases friction! Factors that affect friction Surface Area – As surface area increases between surfaces, so does friction (more rubbing = more friction). Texture – A rougher surface = greater friction. A smoother surface = less friction. Mass/Weight – The greater the mass or weight of an object, the more friction it will create. Fluids – Fluids reduce friction by preventing surfaces from coming into contact. Galileo’s Ideas About Motion It is the force of friction that usually makes moving objects slow down and come to a stop. To keep objects moving, a force has to be applied to overcome the force of friction. If friction could be removed, an object in motion would continue to move with a constant velocity. Different types of Friction There are a few different types of Friction: – 1. Static Friction – 2. Sliding Friction – 3. Rolling Friction – 4. Fluid Friction STATIC FRICTION #1 STATIC FRICTION: – Have you ever tried to push something HEAVY? – The type of friction that prevents a stationary object from moving when a force is applied is static friction. (The object is still and you are trying to move it!) The rougher the surface, the larger the surface area, or the greater the mass = MORE STATIC friction Sliding and Rolling Friction #2 Sliding Friction – When solid surfaces slide over one another Example: Match striking a surface #3 Rolling Friction – When an object rolls over a surface (less force needed than sliding friction – WHY?) Example: Bicycle tire on sidewalk Fluid Friction #4 Fluid Friction – When an object moves through a liquid or a gas, the fluid reduces friction by preventing objects from coming into contact. Why can Friction be useful? Slows things down! Without friction you could not – Walk around Or – Move around Or – Drive a car Imagine playing basketball in your socks or walking with sneakers on ice! How can we reduce friction? THE END