Lesson 4.4 Everyday Forces

Essential Question: What are some of the everyday forces?

• How do you know that a bowling ball weighs more than a tennis ball? Imagine holding one ball in each hand.

What is weight?

• A measure of the gravitational force exerted on an object; its value can change with the location of the object in the universe.

𝐹 𝑔=𝑚𝑎𝑔𝑎𝑔=−9.81 m / s2

• If an astronaut weighs 800 N (180 lb) on Earth, he would weigh only about 130 N (30 lb) on the moon.

• Weight depends on location because it is equal to the magnitude of the force due to gravity.

• Imagine a television set at rest on a table. We know that the gravitational force is acting on the television. Why doesn’t the television continue to fall forward toward the center of the Earth?

What is the normal force?

• A force that acts on a surface in a direction perpendicular to the surface.

𝐹 𝑛

• The normal force is always perpendicular to the contact surface but is not always opposite in direction to the force due to gravity.

• In the absence of other forces, the normal force is equal and opposite to the component of the force of gravity that is perpendicular to the contact surface.

How do you calculate the magnitude of the normal force?

• The angle is the angle between the normal force and a vertical line and is also the angle between the contact surface and a horizontal line.

𝐹 𝑛=𝑚𝑔𝑐𝑜𝑠 𝜃

What causes things to slide or to stick?

• Friction causes our applied forces to work harder!

• Friction opposes the applied force.

• There is static friction and kinetic friction.

What is static friction?

• The force that resists the initiation of sliding motion between two surfaces that are in contact and at rest

• Static friction is abbreviated with

• The force of static friction is always equal to and opposite in direction to the component of the applied force that is parallel to the surface.

• As the applied force increases, the force of static friction also increases.

• If the applied force decreases, the force of static friction also decreases.

• When the applied force is as great as it can be without causing the object to move, the force of static friction reaches its maximum value,

What is kinetic friction?

• The force that opposes the movement of two surfaces that are in contact and are sliding over each other.

• The magnitude of the net force acting on the object is equal to the difference between the applied force and the force of kinetic friction.

• Kinetic friction is always less than the maximum static friction.

• When two surfaces are stationary with respect to each other, the surfaces stick together somewhat at the contact points.

• This adhesion is caused by electrostatic forces between molecules of the two surfaces.

In free-body diagrams…

• The force of friction is always parallel to the surface of contact.

• The force of kinetic friction is always opposite the direction of the motion.

• To determine the direction of the force of static friction, use the principle of equilibrium. For an object in equilibrium, the frictional force must point in the direction that results in a net force of zero.

A push below the horizontal. A pull above the horizontal.

The force of friction is proportional to the normal force.

• It’s easier to push a chair across the floor at a constant speed than to push a heavy desk across the floor at the same speed.

• Experimental observations show that the magnitude of the force of friction is approximately proportional to the magnitude of the normal force that a surface exerts on an object.

What does friction depend on?

• Many factors including:– The relationship between

normal force and the force of friction

– The composition and qualities of the surfaces in contact.

How can we calculate friction?

• We can approximate friction.

• The relationship between normal force and the force of friction provides a good approximation for the friction between dry, flat surfaces that are at rest or sliding past one another.

What is the coefficient of friction?

• The ratio of the magnitude of the force of friction between two objects in contact to the magnitude of the normal force with which the objects press against each other

• The coefficient of friction is represented by the symbol, μ

How do you find the coefficient of kinetic friction?

• The ratio of the force of kinetic friction to the normal force

How do you find the coefficient of static friction?

• The ratio of the maximum value of the force of static friction to the normal force.

How do you calculate the force of friction?

• If the value of μ and the normal force on the object are known, then the magnitude of the force of friction can be calculated directly.

Coefficients of Friction

• The coefficient of kinetic friction is always less than or equal to the coefficient of static friction.

• Coefficient of friction ():– Determined by the nature of the two surfaces – s is for static friction.– k is for kinetic friction.– s > k

A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Once the 24 kg crate is in motion, a horizontal force of 53 N keeps the crate moving with a constant velocity. Find , the coefficient of kinetic friction, between the crate and the floor.

A 25 kg chair initially at rest on a horizontal floor requires a 165 N horizontal force to set it in motion. Once the chair is in motion, a 127 N horizontal force keeps it moving at a constant velocity.a) Find the coefficient of static friction between the

chair and the floor.b) Find the coefficient of kinetic friction between the

chair and the floor.

http://www.youtube.com/watch?v=Vpq877lvG_M

Inclines

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sinmg

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FNFf

Tips•Rotate Axis•Break weight into components•Write equations of motion or equilibrium•Solve

A 91 kg refrigerator is placed on a ramp. The refrigerator begins to slide when the ramp is raised to an angle of 34°. What is the coefficient of static friction?

A box is sitting on a ramp. The ramp makes an angle of 40° with the horizontal. The box has a mass of 20.0 kg. What is the coefficient of static friction between the box and the ramp?

A block of wood is sitting on an inclined plane. One end of the plane is raised until the wood block slides down. The wood block slides down when the plane has an angle of 30 degrees to the horizontal. What is the coefficient of friction?

A museum curator moves artifacts into place on various different display surface. Find and Moving a 145 kg aluminum sculpture across a horizontal steel platform

A museum curator moves artifacts into place on various different display surface. Find and Pulling a 15 kg steel sword across a horizontal steel shield

A museum curator moves artifacts into place on various different display surface. Find and Pushing a 250 kg wood bed on a horizontal wood floor

A museum curator moves artifacts into place on various different display surface. Find and Sliding a 0.55 kg glass amulet on a horizontal glass display case

A student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0° with the horizontal. The coefficient of kinetic friction between the box and the sidewalk is 0.500. Find the acceleration of the box.

Two students are sliding a 225 kg sofa at constant speed across a wood floor. One student pulls with a force of 225 N at an angle of 13° above the horizontal. The other student pushes with a force of 250 N at an angle of 23° below the horizontal. What is the coefficient of kinetic friction between the sofa and the floor?

A student pulls on a rope attached to a box of books and moves the box down the hall. The student pulls with a force of 185 N at an angle of 25.0° above the horizontal. The box has a mass of 35.0 kg, and between the box and the floor is 0.27. Find the acceleration of the box.

The student moves the box of books up a ramp inclined at 12° with the horizontal. If the box starts from rest at the bottom of the ramp and is pulled at an angle of 25.0° with respect to the incline and with the same 185 N force, what is the acceleration up the ramp? Assume that .

A 75 kg box slides down a 25.0° ramp with an acceleration of 3.60 .a) Find between the box and the ramp.b) What acceleration would a 175 kg box have on this

ramp?

A box of books weighing 325 N moves at a constant velocity across the floor when the box is pushed with a force of 425 N exerted downward at an angle of 35.2° below the horizontal. Find between the box and the floor.

What is air resistance?

• A form of friction.

• Whenever an object moves through a fluid medium, such as air or water, the fluid provides a resistance to the object’s motion.

• For a falling object, when the upward force of air resistance balances the downward gravitational force, the net force on the object is zero. The object continues to move downward with a constant maximum speed, called the terminal speed.

What are the Four Fundamental Forces?

• Electromagnetic– Caused by interactions

between protons and electrons

– Produces friction– Act over long range

• Gravitational– The weakest force– Act over long range

• Strong nuclear force– The strongest force– Short range

• Weak nuclear force– Short range

These are all field forces.

• What are some of the everyday forces?