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AND chapter 12. FORCES. MOTION. What is a FORCE?. A FORCE is a push or pull that acts on an object. A force can cause a resting object to move OR… Accelerate a moving object by: changing the object’s speed direction. How do we MEASURE force?. Forces can be measured on a scale
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ANDchapter 12 FORCES MOTION
What is a FORCE? • A FORCE is a push or pull that acts on an object. • A force can cause a resting object to move • OR… • Accelerate a moving object by: • changing the object’s speed • direction
How do we MEASURE force? • Forces can be measured on a scale • WEIGHT is a FORCE • UNITS OF FORCE: • Newton (N) • A Newton is a force that causes a 1 kg mass to accelerate at a rate of 1 meter per second each second • Which is written as: 1 N = 1 kg m/s2
Representing Forces • Forces can be represented with an arrow • The length of the arrow shows strength or magnitude • Direction of the arrow shows the direction of the force • Figure 2 on page 357 shows an example of weight Normal Force Applied Force Friction Force
Combining Forces (Vectors) • Rules of Vector Addition • Forces in the same direction add together + = 1x 1x 2x • Forces in opposite direction subtract from one another + =1x + -2x = -1x
NET FORCE • NET FORCE is the overall force acting on an object after all the forces are combined + = 1x + 1x = 2x • The net force above is 2x • NOTE: Forces that are equal in strength and opposite in direction result in NO NET FORCE!!! • page 358 figure 4 + = ZERO
Balanced Forces • Question: what are some examples where the net force would equal zero? • HINT: When the forces on an object are balanced • Balanced Forces = the net force is zero, there is no change in the object’s motion
ANSWER • Net force would be zero when: • You play tug-of-war and neither team moves • You arm wrestle and neither person wins • Others???
Unbalanced Forces • Question: What happens to the object when the net force acting on an object is NOT ZERO, or unbalanced? (like the example below) + = Unbalanced Forces = the net force acting on an object does not equal zero = acceleration!
Answer • When an unbalanced force acts on an object, the object accelerates • 3 examples of a net force causing an object to accelerate • Pushing against the side of a book that is resting • A team winning a game of tug-of-war • A person in freefall
Try these problems • Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats in this problem to help you answer the question
2. Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? Draw arrows showing the individual and combined forces of the tugboats in this problem.
3. Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.
FRICTION • Friction: a force that opposes the motion of objects that touch as they move past each other • Without friction, it would be a very different world!!! • Food would not stay on your fork! • Cars would slide all over the road! • Walking would be almost impossible! • Friction acts at the surface where objects are “in contact”
The 4 Types of Friction • There are 4 main types of friction: • Static Friction • Sliding Friction • Rolling Friction • Fluid Friction
STATIC FRICTION • friction force that acts on objects that are not moving • Static friction always acts in the directionopposite to the applied force • Examples: • every time you take a step and push off • glass of water sitting stationary on the table
SLIDING FRICTION • a force that opposes the direction of motion of an object as it slides over a surface • Sliding friction is a weaker force than static friction • This is why less force is needed to keep an object moving than it is to start it moving
ROLLING FRICTION • The friction force that acts on rolling objects • Rolling friction is about 100-1000 times less than static or sliding friction • This is why we use wheeled dollies to move heavy objects! • In machines, ball bearings, are often used to reduce friction between two surfaces
FLUID FRICTION • Force that opposes the motion of an object through a fluid • Water and a mixture of gases such as air are known as fluids • Example: stirring cake batter • The motion of stirring is slowed by fluid friction • Fluid friction increases as the speed of the object moving through the fluid increase • So the faster you stir the greater the friction!!!
AIR RESISTANCE • Air resistance is a type of fluid friction • Remember…gases are considered “fluids” • Fluid Friction acting on an object moving through the air is called AIR RESISTANCE
GRAVITY • Gravity: a force that acts between any two masses • Gravity is an attractive force so it pulls objects together • Gravity does not require objects to be in contact for it to act on them • Gravity can act over large distances!!!
More GRAVITY! • Earth's gravity acts downward toward the center of the Earth. • There is usually an upward force that acts against gravity to balance out the forces and allow objects to remain still.
FALLING OBJECTS • QUESTION: What forces are acting on an object as it falls?
ANSWER: Only two forces acting on a falling object are gravity and air resistance
Forces Acting on Falling Objects • Gravity causes object to accelerate downward • Air resistance acts in the opposite direction of the motion • Which means it reduces acceleration
Recall…what happens to the amount of fluid friction as an object speeds up?
Falling Objects • If an object in freefall falls for long enough, the upward force of air resistance will become equal to the downward force of gravity. • At this point, the two forces are BALANCED • Acceleration is zero when forces are balanced • The object will continue to fall at a constant velocity
Terminal Velocity • Terminal Velocity: the constant velocity of a falling object when the force of air resistance equals the force of gravity
Projectile Motion • Projectile Motion: The motion of a falling object (projectile) after it is given an initial forward velocity
Projectile Motion • Question: What are the ONLY 2 FORCES that act on a projectile??? All 3 balls are experiencing projectile motion!
Projectile Motion • Answer: Air resistance and gravity!!! • Refer to figure 9 on page 362 in textbook • The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path
If I shoot a bullet horizontally and at the same time drop a bullet from the same height as the gun…which will hit the ground first?
FALLING OBJECTS • The two bullets WILL hit the ground at the same time!
The two balls fall with the same acceleration and strike the ground at the same time!!! Remember the “Investigating Freefall Lab” with the marbles? How did that lab compare with the previous statement???
It’s not ALL about Newton… • Aristotle incorrectly proposed that force is required to keep an object moving at a constant speed • Galileo studied how gravity produces constant acceleration. He concluded that objects not subjected friction or any other force would continue to move indefinitely • Newton built off the work of Galileo and later published his work in a book entitled Principia • Newton summarized his study of force and motion in several laws of motion
1st Law of Motion – Law of Inertia • 1st Law: The state of motion of an object does not change as long as the net force acting on the object is zero • In other words: Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction
1st Law – Law of Inertia • Example: A soccer ball resting on the grass remains motionless until a force is applied to it in the form of a kick. • The kicked ball begins rolling…and continues to roll… • Will the ball ever stop? If so, why?
Will the ball stop rolling? • YES…BECAUSE • Friction between the ball and grass acts on the ball as it rolls, the ball slows • The force of friction slows the ball and eventually brings it to a stop
Why is it called the “Law of Inertia” • INERTIA: the tendency of an object to resist a change in its motion (an object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed) • Remember…soccer ball sat motionless (forces were balanced) until an unbalanced force acted on it • The ball has inertia • Everything with mass has inertia • The more mass, the more inertia