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**What do you know about forces?**

In your own words, define the term “force”. Give two examples of forces. Do you think a force affects the motion of an object? If so, how? Describe a “real-life” situation in which you encounter a force.

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Forces

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What is a force? A force is a push or pull exerted on an object.

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**The object on which the force is exerted is called the system.**

The world around the object that exerts forces on it is called the environment. environment system environment environment

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Types of Forces Contact Force-a force that acts on an object only by touching it. Long Range Force- a force that acts on an object without contact. The force of gravity is an example of a long range force.

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**Each force has a specific, identifiable, immediate cause called the agent.**

The agent can be animate, such as a person, or inanimate, such as a desk, or a magnet. The agent for the force of gravity is the earth’s mass.

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**Force is a vector quantity.**

A force has both magnitude and direction. A force is represented with an arrow that points in the correct direction. The length of the arrow is proportional to the size of the force. The tail of the force vector is always on the object, even when the force is a push. The symbol F with a subscript label is used to identify the agent.

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**Common Types of Forces Force Definition Direction Friction Ff Normal**

Symbol Definition Direction Friction Ff The contact force that acts to oppose motion between surfaces Parallel to the surface and opposite the direction of the motion Normal FN The contact force exerted by a surface on an object. Perpendicular to and away from the surface Spring Fsp A restoring force, that is, the push of pull a spring exerts on an object. Opposite the displacement of the object at the end of a spring Tension FT The pull exerted by a string or rope when attached to an object and pulled taut. Away from the object and parallel to the string or rope at the point of contact. Thrust Fthrust Forces that move objects such as rockets, planes, cars, etc. In the same direction as the acceleration. Weight Fg Long range force due to gravitational attraction between the earth and an object. Straight down toward the center of the earth.

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Net Force Because forces are vectors, the total force on an object is the vector sum, or resultant, of all forces exerted on an object. The vector sum of two or more forces on an object is called the net force. Fman ● Fwoman ● Fnet

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Equilibrium If the net force on an object is zero, the object is in equilibrium. If an object is in equilibrium, the forces are balanced. (Equal in magnitude, but opposite in direction.) An object in equilibrium will be either at rest or moving at a constant velocity.

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Drag Force When an object moves through any fluid, such as air or water, the fluid exerts a drag force on the moving object in the direction opposite to its motion. As the speed of the object increases, so does the magnitude of the drag force. The size and shape of the object also affects the drag force, as well as the temperature and viscosity of the fluid.

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Terminal Velocity gravity An object in free fall has very little velocity at the start, and therefore a small drag force. The downward force of gravity is much stronger than the upward drag force so the object accelerates downward. As the ball’s velocity increases, so does its drag force. Eventually the drag force equals the force of gravity. When this happens there is no net force and no acceleration. The constant velocity that is reached when this state of equilibrium is achieved is called the terminal velocity. Air resistance gravity Air resistance Terminal velocity

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**Newton’s First Law of Motion**

An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if the net force acting on that object is zero. Newton’s first law is often called the Law of Inertia. Inertia is the tendency of an object to resist change.

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Newton’s First Law

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**Newton’s Second Law of Motion**

If the net force on an object is not equal to zero, the motion of the object will change. A change in motion is called acceleration. Experiments show that the acceleration of an object is proportional to the net force exerted on the object and inversely proportional to the mass of the object being accelerated. This is a statement of Newton’s second law and can be written as an equation. Newton’s Second Law a = Fnet/m or F=ma

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Measuring Force One unit of force causes a 1 kg mass to accelerate at 1 m/s2. Because force is equal to mass times acceleration, F=ma, one force unit has the dimensions 1 kg●m/s2 This unit in the SI system is known as the newton, N.

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**Using Newton’s Laws Consider a ball falling in midair:**

If it is touching nothing and air resistance is neglected, the only force acting on it is Fg. The ball’s acceleration is -9.8 m/s2, or g. Newton’s 2nd law becomes F = mg. The magnitude of the object’s weight is equal to its mass times the acceleration it would have if it were falling freely.

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Mass vs. Weight Mass, the amount of matter an object contains, does not change. (Measured in kilograms) Weight is dependent upon the acceleration due to gravity and will vary from planet to planet. (Measured in Newtons)

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**What is being measured, mass or weight?**

A bathroom scale contains springs. When you step on a scale, the scale exerts an upward force on you. Because you are not accelerating, the net force is zero. Therefore, that magnitude of Fsp = Fg. A spring scale, therefore measures weight, not mass. If you were on another planet, the compression of the spring would be different, and the scale’s reading would be different.

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Practice Problems On Earth, a scale shows that you weigh 585 N. What is your mass? What would the scale read on the moon (g=1.60 m/s2)? Your new motorcycle has a mass of 250. kg. What is its weight in newtons? A 7.50 kg television is set on a spring scale. If the scale reads 78.4 N, what is the acceleration due to gravity at that location?

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Apparent Weight If a bathroom scale supports you-it provides the only upward force-then it reads your weight. How would the measurement of the scale change if you push down on the bathroom counter while standing on the scale? Push up on the bathroom counter? Stand with one foot on the scale and one foot off? In these cases, the force exerted by the scale is the apparent weight.

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Weight in an Elevator If you stand on a scale in an elevator that is in equilibrium, the scale reads your weight. If the elevator accelerates upward, then you feel heavier and the scale reads a larger force. Why? If the elevator accelerates downward, then you feel lighter and the scale reads less. Why? If the cable holding the elevator beaks, the scale with you on it accelerates with a = -g. What would the scale read?

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**Which exerts the greater force?**

You are driving down the highway and a bug splatters on your windshield. Which is greater: the force of the bug on the windshield, or the force of the windshield on the bug?

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**Newton’s Third Law of Motion**

All forces come in pairs. The two forces in a pair act on different objects and are equal in strength and opposite in direction. In other words; for every action force, there is an equal but opposite reaction force. FA on B = -F B on A

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**Which exerts the greater force?**

You are driving down the highway and a bug splatters on your windshield. Which is greater: the force of the bug on the windshield, or the force of the windshield on the bug? The forces are equal. The deceleration of the bug is greater due to its smaller mass, but the forces are equal.

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Practice Problem A 50.0 kg bucket is being lifted by a rope. The rope will not break if the tension is 525 N or less. The bucket started at rest, and after being lifted 3.0 m, it is moving at 3.0 m/s. If the acceleration is constant, is the rope in danger of breaking?

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Practice Problem #2 Five people are playing tug-of-war. Anders and Alyson pull to the right with 45 N and 35 N respectively. Calid and Marisol pull to the left with 53 N and 38 N, respectively. With what force and in what direction does Benito pull if the game is tied?

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Practice Problem #3 A 7.25 g bullet is fired from a gun. The muzzle velocity of the bullet is 223 m/s. Assume that the bullet accelerates at a constant rate along the barrel of the gun before it emerges with constant speed. The barrel of the gun is m long. What average force does the bullet exert on the gun?

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