forces and the laws of motionsection 1 forces forces can change motion. –start movement, stop...

Forces and the Laws of Motion Section 1

Forces

• Forces can change motion.– Start movement, stop movement, or change the direction of

movement– Cause an object in motion to speed up or slow down


Forces• Contact forces

– Pushes or pulls requiring physical contact between the objects

– Baseball and bat

• Field forces– Objects create force fields

that act on other objects.– Gravity, static electricity,

magnetism


Units of Force• The SI unit of force is the newton (N).

– Named for Sir Isaac Newton

– Defined as the force required to accelerate a 1 kg mass at a rate of 1 m/s2

– Approximately 1/4 pound

• Other units are shown below.


The Four Fundamental Forces

• Electromagnetic– Caused by interactions between protons and electrons– Produces friction

• Gravitational– The weakest force

• Strong nuclear force– The strongest force– Short range

• Weak nuclear force– Short range


The Force Diagram

• Force Diagrams-1. Represent forces using vector arrows

2. All forces are drawn as if they act on a central point of the object

3. Free body diagrams show only forces acting on one object


Force Diagrams• Forces are vectors (magnitude and direction).• Force diagram (a)

– Shows all forces acting during an interaction• On the car and on the wall

• Free-body diagram (b)– Shows only forces acting on the object of interest

• On the car


Free-Body Diagrams

• Three forces are shown on the car.– Describe each force by

explaining the source of the force and where it acts on the car.

– Is each force a contact force or a field force?


Force Diagram


Force Diagram

• Step for drawing a free body diagram

1. Identify forces acting on the object– Tow truck exerts force on the car– Road exerts force on the car– Car is acted on by gravity

2.Draw a simple diagram


Force Diagram

3. Add magnitude of forces to arrows– Indicate force with a vector arrow on the car by the

tow truck (5800 N)– Gravitational force acting on the car (14700 N)– Road exerts an upward force on the car (13,690 N)– Interaction between the road and the tires exert a

backward force of (775 N)


Force Diagram Practice

• Draw a force diagram of a crash test dummy in a car at the moment of collision

• Forces acting on the car are

1.19,600 N downward

2.17,800 N forward

3.25,000 N backward


Newton’s First Law

• Experimentation led Galileo to the idea that objects maintain their state of motion or rest.

• Newton developed the idea further, in what is now known as Newton’s first law of motion:


Draw a force diagram of a crash test dummy in a car at the moment of collision• Forces acting on the car are

1.19,600 N downward

2.17,800 N forward

3.25,00 N backward

• Forces acting on the dummy are

1.585 N downward

2.175 N backward

3.585 N upward


Newton’s First Law

• Called the law of inertia• Inertia

– Tendency of an object to remain in its state of motion– Tendency of an object not to accelerate or decelerate– Mass is a measure of inertia

• More mass produces more resistance to a change in velocity

• Which object in each pair has more inertia?– A baseball at rest or a tennis ball at rest

• Answer: the baseball

– A tennis ball moving at 125 mi/h or a baseball at rest• Answer: the baseball

Section 2Forces and the Laws of Motion

Net Force - the Sum of the Forces

• This car is moving with a constant velocity.– Fforward = road pushing the tires

– Fresistance = force caused by friction and air

– Forces are balanced

• Velocity is constant because the net force (Fnet) is zero.

• Net external force can be determined by a change in motion


Equilibrium

• The state in which the net force is zero.– All forces are balanced.– Object is at rest or travels with

constant velocity.

• In the diagram, the bob on the fishing line is in equilibrium.– The forces cancel each other.– If either force changes,

acceleration will occur.


Net External Forces

• Net external forces = vector sum of all the forces acting on the object


Practice Problems

• A man is pulling on his dog with a force of 70.0N directed at an angle of 30.0º to the horizontal.– Find the x and y component.


Practice Problems

• The man pulls a box with a force of 25.0N at angle of 18.0º to the horizontal.– Find the x and y component


Practice Problems

• A crate is pulled to the right with a force of 85N, to the left with a force of 115N, upward with a force of 565 N, and downward with a force of 236N. – Find the net external force of x– Find the net external force of y– Find the magnitude and direction of the net external

force on the crate


Classroom Practice Problem

• An agricultural student is designing a support system to keep a tree upright. Two wires have been attached to the tree and placed at right angles to each other (parallel to the ground). One wire exerts a force of 30.0 N and the other exerts a force of 40.0 N. Determine where to place a third wire and how much force it should exert so that the net force on the tree is zero.

• Answer: 50.0 N at 143° from the 40.0 N force


Newton’s Second Law

• Increasing the force will increase the acceleration.– Which produces a greater acceleration on a 3-kg model airplane, a force

of 5 N or a force of 7 N?• Answer: the 7 N force

• Increasing the mass will decrease the acceleration.– A force of 5 N is exerted on two model airplanes, one with a mass of 3 kg

and one with a mass of 4 kg. Which has a greater acceleration?• Answer: the 3 kg airplane


Newton’s Second Law (Equation Form)

F represents the vector sum of all forces acting on an object. F = Fnet

– Units for force: mass units (kg) acceleration units (m/s2)– The units kg•m/s2 are also called newtons (N).


Practice Problems

• The net external force on the propeller of a 0.75 kg model airplane is 17 N forward. What is the acceleration of the plane?

• The net external force on a golf cart is 390 N north, if the cart has a total mass of 270 kg, what are the magnitude and direction of its acceleration?


Practice Problems

• A car has a mass of 1500 kg. What force is required to accelerate the car at 4.5m/s2 to the east?

• A 2.0 kg mass starts from rest at the top of an inclined plane 85 cm long and slides down to the bottom is 0.50s. What net external force acts on the mass along the incline?



• Space-shuttle astronauts experience accelerations of about 35 m/s2 during takeoff. What force does a 75 kg astronaut experience during an acceleration of this magnitude?

• Answer: 2600 kg•m/s2 or 2600 N



• The muscle responsible for closing the mouth is the strongest muscle in the human body. It can exert a force greater than that exerted by a man lifting a mass of 400 kg. Richard Hoffman of Florida recorded the force of biting at 4.33 103 N. If each force has a magnitude equal to the force of Hoffman’s bite, determine the net force. One force is along the horizontal, the second force is -90º from the horizontal, and the third force is -60 º from the horizontal.

• •



• In 1994, Vladimir Kurlovich, from Belarus, set the record as the world’s strongest weightlifter. He did this by lifting and holding above his head a barbell whose mass was 253 kg. Kurlovich’s mass at the time was roughly 133 kg. Draw a free-body diagram showing the various forces in the problem. Calculate the normal force exerted on each of Kurlovich’s feet during the time he was holding the barbell.

• •



• The net force exerted by a woodpecker’s head when its beak strikes a tree can be as large as 4.90 N, assuming that the bird’s head has a mass of 50.0 g. Assume that two different muscles pull the woodpecker’s head forward and downward, exerting a net force of 4.90 N. If the forces exerted by the muscles are at right angles to each other and the muscle that pulls the woodpecker’s head downward exerts a force of 1.70 N, what is the magnitude of the force exerted by the other muscle? Draw a free-body diagram showing the forces acting on the woodpecker’s head.

•


What do you think?

• Two football players, Alex and Jason, collide head-on. They have the same mass and the same speed before the collision. How does the force on Alex compare to the force on Jason? Why do you think so?– Sketch each player as a stick figure.– Place a velocity vector above each player.– Draw the force vector on each and label it (i.e. FJA is

the force of Jason on Alex).


What do you think?

• Suppose Alex has twice the mass of Jason. How would the forces compare?– Why do you think so?– Sketch as before.

• Suppose Alex has twice the mass and Jason is at rest. How would the forces compare?– Why do you think so?– Sketch as before.


Newton’s Third Law

• When two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the force exerted on object 2 by object 1. These two forces are equal as well as opposite.



• Forces always exist in pairs.– You push down on the chair, the chair pushes

up on you– Called the action force and reaction force– Occur simultaneously so either force is the

action force



• For every action force there is an equal and opposite reaction force.

• The forces act on different objects.– Therefore, they do not balance or cancel each other.– The motion of each object depends on the net force on that object.


Hammer Striking a Nail• What are the action/reaction pairs for a

hammer striking a nail into wood? – Force of hammer on nail = force of nail on

hammer – Force of wood on nail = force of nail on

wood

• Which of the action/reaction forces above act on the nail?– Force of hammer on nail (downward)– Force of wood on nail (upward)

• Does the nail move? If so, how?– Fhammer-on-nail > Fwood-on-nail so the nail

accelerates downward


Hammer Striking a Nail

• What forces act on the hammer?– Force of nail on hammer (upward)– Force of hand on hammer (downward)

• Does the hammer move? If so, how?– Fnail-on-hammer > Fhand-on-hammer so the hammer accelerates

upward or slows down

• The hammer and nail accelerate in opposite directions.


Action-Reaction: A Book on a Desk

• Earth pulls down on the book (force of gravity).

Reaction Force

• The book pushes down on the desk.

Action Force

• The desk pushes up on the book.

• The book pulls up on Earth.


Action-Reaction: A Falling Book

Action• Earth pulls down on the

book (force of gravity).

• What is the result of the action force (if this is the only force on the book)?– Unbalanced force

produces an acceleration of -9.81 m/s2.

Reaction• The book pulls up on

Earth.

• What is the result of the reaction force?• Unbalanced force

produces a very small upward acceleration (because the mass of Earth is so large).



• Field forces also act as pairs


Practice Problems

• A 6.0 kg object undergoes an acceleration of 2.0 m/s2.– What is the magnitude of the net external force acting

on it?– If this same force is applied to a 4.0 kg object, what

acceleration is produced?


Practice Problems

• A child pulls a wagon with a horizontal force, causing it to accelerate. Newton’s third law say that the wagon exerts an equal and opposite force on the child. How can the wagon accelerate?


Practice Problems

• Identify the action-reaction pairs in the following situations:– A person takes a step– A baseball player catches a ball– A snowball hits someone in the back– A gust of wind strikes a window


Practice Problems

• The forces acting on a sailboat are 390N north and 180N east. If the magnitude (including the crew) has a mass of 270 kg, what are the magnitude and direction of its acceleration?


Practice Problems

• David Purley, a racing driver, survived deceleration from 173 km/h to 0 km/h over a distance of 0.660 m when his car crashed. Assume that Purley’s mass is 70.0 kg. What is the average force acting on him during the crash? Compare this force to Purley’s weight. (Hint: Calculate the average acceleration first.)


Practice Problems

• A giant crane in Washington, D. C. was tested by lifting a 2.232 106 kg load.

a. Find the magnitude of the force needed to lift the load with a net acceleration of 0 m/s2.

b. If the same force is applied to pull the load up a smooth slope that makes a 30.0 angle with the horizontal, what would be the acceleration?


Practice Problems

• In 1991, a lobster with a mass of 20.0 kg was caught off the coast of Nova Scotia, Canada. Imagine this lobster involved in a friendly tug of war with several smaller lobsters on a horizontal plane at the bottom of the sea. Suppose the smaller lobsters are able to drag the large lobster, so that after the large lobster has been moved 1.55 m its speed is 0.550 m/s. If the lobster is initially at rest, what is the magnitude of the net force applied to it by the smaller lobsters? Assume that friction and resistance due to moving through water are negligible.


What do you think?

• How do the quantities weight and mass differ from each other?

• Which of the following terms is most closely related to the term friction?– Heat, energy, force, velocity

• Explain the relationship.


Weight and Mass

• Mass is the amount of matter in an object.– Kilograms, slugs

• Weight is a measure of the gravitational force on an object.– Newtons, pounds– Depends on the acceleration of gravity

• Weight = mass acceleration of gravity– W = mag where ag = 9.81 m/s2 on Earth

– Depends on location• ag varies slightly with location on Earth.

• ag is different on other planets.


Normal Force

• Force on an object perpendicular to the surface (Fn)

• It may equal the weight (Fg), as it does here.

• It does not always equal the weight (Fg), as in the second example.

• Fn = mg cos


Normal Force

• An object placed on a tilted surface will often slide down the surface.

• The rate at which the object slides down the surface is dependent upon how tilted the surface is; the greater the tilt of the surface, the faster the rate at which the object will slide down it.

• In physics, a tilted surface is called an inclined plane.


Normal Force

• As shown in the diagram, there are always at least two forces acting upon any object that is positioned on an inclined plane - the force of gravity and the normal force.

• The force of gravity (also known as weight) acts in a downward direction

• The normal force acts in a direction perpendicular to the surface (in fact, normal means "perpendicular").


Normal Force• Up to this point in the course, we

have always seen normal forces acting in an upward direction, opposite the direction of the force of gravity.

• But this is only because the objects were always on horizontal surfaces and never upon inclined planes.

• The normal forces is not always upwards, but rather that it is directed perpendicular to the surface that the object is on.


Normal Force

• The force of gravity will be resolved into two components of force - one directed parallel to the inclined surface and the other directed perpendicular to the inclined surface.


Normal Force

• The perpendicular component of the force of gravity is directed opposite the normal force and as such balances the normal force


Normal Force

• The parallel component of the force of gravity is not balanced by any other force. This object will subsequently accelerate down the inclined plane due to the presence of an unbalanced force


Normal Force

• It is the parallel component of the force of gravity that causes this acceleration. The parallel component of the force of gravity is the net force.


Normal Force

• The task of determining the magnitude of the two components of the force of gravity is a mere manner of using the equations. The equations for the parallel and perpendicular components are:

• .

forces and the laws of motionsection 1 forces forces can change motion. –start movement, stop...

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