Physical Science: Ch 12 Notes In general, what will cause your car to move? A. Forces Force – a push or a pull that acts on an object What can a force do? A force can cause a resting object to move, or it can accelerate a moving object (by changing its speed or direction) Forces in the SAME direction ADD together and forces in OPPOSITE directions SUBTRACT from one another. Net force – the overall force acting on an object after all the forces are combined What happens when forces are all equal and balanced? When the net force is zero there is NO CHANGE in the object’s motion (like a tug-of-war between teams of equal strength) When an UNBALANCED force acts on an object, the object accelerates! Physical Science: Ch 12 Notes How are forces drawn and measured? Arrows are used to represent the direction and strength of a force. The arrow points the same direction as the force and the relative length of the arrow represents the strength, or magnitude, of the force. Forces are measured in Newtons! One Newton = the force that causes a 1-kilogram mass to accelerate at a rate of 1 m/s/s B. Newton’s First Law 1st Law: The state of motion of an object does NOT change as long as the net force acting on the object is zero. Unless an unbalanced force acts: 1. An object at rest remains at rest 2. An object in motion remains in motion with the same speed and direction Inertia – the tendency of an object to resist a change in its motion The greater the inertia of an object, the harder it is to change its motion. So, what characteristics would give objects a lot of inertia? Physical Science: Ch 12 Notes What influences how quickly an object accelerates? C. Newton’s Second Law 2nd Law: The acceleration of an object is equal to the net force acting on it divided by the object’s mass. a = F/m F=ma m=F/a Practice Problems: How much force is needed to accelerate a 5,250 kilogram truck at a rate of 1.5 m/s/s? How much force is needed to accelerate a 60 kilogram skater 5.3 m/s/s? Julie’s mass is 60 kilograms and as she dives into a swimming pool her acceleration is 8.14 m/s/s. What is her force as she hits the water? With a force of 12.6 Newtons, Heath hit the 0.43 kilogram baseball. What was the acceleration of the baseball? Bob pushed his motorcycle with a force of 297.2 Newtons and it accelerated at a rate of 3.1 m/s/s. If the friction force was 12.2 N, what is the motorcycle’s mass? (hint: find the net force first, then use m=F/a) Physical Science: Ch 12 Notes How did your balloon-powered cars move? Why? D. Newton’s Third Law 3rd Law: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. These two forces are called Action and Reaction forces. Action and Reaction forces do NOT cancel each other because they do not act on the same object. Example: Balloon-powered car Example: Stubbing your toe Example: Jellyfish Not all Action and Reaction forces produce motion (ex: your hand pushing against a wall) Physical Science: Ch 12 Notes Why did your cars stay on the floor as they moved? E. Gravity Gravity – a force that acts between any two masses Gravity is an attractive force, that is, it pulls objects together. Every object in the universe attracts every other object! Unlike friction, gravity can act over large distances. The greater the mass of the objects, the greater the gravitational force Ex: The gravity force between you and your textbook vs. the force between the earth and you Ex: The gravity force between the sun and Earth The closer two objects are, the greater their gravitational attraction Ex: Our moon vs. ex-planet Pluto Earth’s gravity acts downward toward the center of earth. An upward force usually balances the downward force of gravity. Gravity causes objects to accelerate downward, but air resistance acts in the direction opposite to motion and reduces acceleration. Air resistance increases with velocity and surface area. Physical Science: Ch 12 Notes Why is outer space called “weightless” but never “massless”??? F. Weight and Mass Weight – the force of gravity acting on an object An object’s weight is the product of the object’s mass (in kilograms) and acceleration due to gravity acting on it … because…WEIGHT IS A FORCE! Weight = mass x acceleration due to gravity W = mg On earth: g = 9.8 m/s/s So, weight is measured in NEWTONS and is different from mass! Practice Problems: What is the weight of a 8 kilogram bowling ball? What is the weight of a 8 kilogram bowling ball on earth? What is the weight of a 8 kilogram bowling ball on the moon? Physical Science: Ch 12 Notes Why did some cars continue to travel even after their balloon deflated? G. Momentum Momentum – the product of an object’s mass and its velocity Momentum = mass x velocity units: kg x m/s An object with large momentum is hard to stop! The momentum for any object at rest is ZERO Practice Problems: 1. Which has more momentum: a 0.032 kg golf ball traveling 45 m/s or a 5.5 kg bowling ball traveling 4.5 m/s? 2. Which has more momentum: a 0.012 kg bullet traveling 30 m/s or a 0.321 kg baseball rolling 3 m/s? Law of Conservation of Momentum – if no outside force acts on a system then the total momentum of the system does not change Momentum can be transferred between objects but is not destroyed. Physical Science: Ch 12 Notes Why did Miss Hinkhouse have wheels as a required part of the balloonpowered cars? H. Friction Friction – a force that opposes the motion of objects that touch as they move past each other Friction acts at the surface where objects are in contact Note that “in contact” includes solid objects that are directly touching one another as well as objects moving through a liquid or a gas Sliding friction – a force that opposes the direction of motion of an object as it slides over a surface Rolling friction – the friction force that acts on rolling objects Fluid friction – opposes the motion of an object through a fluid (liquid or gas) How do the effects of sliding, rolling, and fluid friction compare?