Chapter 10 Forces Chapter Preview Questions 1. How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d. It has a reference point.
Chapter 10 Forces Chapter Preview Questions 1.How do you know an object is in motion? a. It has inertia. b. It has mass. c. It is changing position. d.
Dec 17, 2015
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Chapter 10 Forces
Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.
Chapter 10 Forces
Chapter Preview Questions
1. How do you know an object is in motion?
a. It has inertia.
b. It has mass.
c. It is changing position.
d. It has a reference point.
Chapter 10 Forces
Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time
Chapter 10 Forces
Chapter Preview Questions
2. What is velocity?
a. speed
b. speed in a given direction
c. miles per hour
d. change in speed over time
Chapter 10 Forces
Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed
Chapter 10 Forces
Chapter Preview Questions
3. Which is the best definition of acceleration?
a. change in velocity
b. change in reference point
c. increase in speed
d. decrease in speed
Chapter 10 Forces
Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed?
a. -100 m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s
Chapter 10 Forces
Chapter Preview Questions
4. If an object starts out at rest and accelerates to 100 m/s, what is its initial speed?
a. -100 m/s
b. 0 m/s
c. 100 m/s
d. 32 m/s
Chapter 10 Forces
Section 1: Friction, Gravity, and Elastic Forces
Standard 8.2.a Students know a force has both direction and magnitude.
Standard 8.2.c Students know when the forces on an object are balanced, the motion of the object does not change.
Chapter 10 Forces
What is a Force?How is a force described?
Force
Newton (N)
Net force
A force is described by its magnitude and by the direction in which it acts.
A push or a pull
The SI unit for the magnitude, or strength, of a force.
The combination of all forces acting on an object.
Chapter 10 Forces
Combining Force Vectors
The strength and direction of the individual forces determine the net force.
Chapter 10 Forces
Unbalanced ForcesUnbalanced forces acting on an object result in a net force and cause a change in the object’s velocity.
Chapter 10 Forces
Balanced Forces
Balanced forces acting on an object do not change the object’s velocity.
Chapter 10 Forces
Section 1 Quick QuizWhat happens when two forces act in the same direction?
A.They add together.
B.Their sum divided by two is the total force.
C.They cancel each other out.
D.The stronger one prevails.
Answer: A – They add together.
Chapter 10 Forces
Section 1 Quick QuizBalanced forces acting on an object
A.sometimes change the object’s motion.
B.always change the object’s motion.
C.are not related to motion.
D.never change the object’s motion.
Answer: D – never change the object’s motion
Chapter 10 Forces
Section 2: Friction, Gravity, and Elastic ForcesStandard 8.2.b Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.
Standard 8.2.d Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.
Chapter 10 Forces
Friction
What factors determine the strength of the friction force between two surfaces?
friction
The strength of the force of friction depends on the types of surfaces involved and on how hard the surfaces push together.
A force that two surfaces exert on each other when they rub against each other
Chapter 10 Forces
Types of Friction
Static Friction• Occurs with objects that
are not moving.• You must use extra force
to start the motion of stationary objects.
• Once the object is moving there is no longer any static friction.
Sliding Friction•Occurs when two solid surfaces slide over each other.•Examples:
• Sticky powder on the soles of ballet slippers.
• Rubber pads on bicycle brakes.
Chapter 10 Forces
Types of Friction
Rolling Friction•Occurs when an object rolls across a surface.•Rolling friction is less than sliding friction for similar materials.•Engineers use ball bearings to reduce friction in many products.
Fluid Friction•Occurs when a solid object moves through a fluid.•Fluid friction is usually less than sliding friction. Parts of machines are bathed in oil for this reason.•Fluid friction occurs between people and air.
Chapter 10 Forces
GravityWhat factors affect the gravitational force between two objects?
gravity
Law of Universal Gravitation
mass
The force of gravity between objects increases with greater mass and decreases with greater distance.
A force that pulls objects toward each other
The force of gravity acts between all objects in the universe.
The measure of the amount of matter in an object
The SI unit of mass is the kilogram (kg).
Chapter 10 Forces
Gravity Between Objects
The force of gravity between objects increases with greater mass and decreases with greater distance.
Chapter 10 Forces
Mass and Weight
The gravitational force exerted on a person or object at the surface of a planet is known as weight.
Weight = Mass X Acceleration due to Gravity
Chapter 10 Forces
Mass and WeightThe weight of a person on the Moon is one-sixth of their weight on Earth. The mass of an object is the same on the Moon as it is on Earth.
Example:
Weight on Earth 1617 N Mass on Earth 165 kg
Weight on Moon 270 N Mass on Moon 165 kg
Chapter 10 Forces
Gravity and Motion
Why do objects accelerate during free fall?
In free fall, the force of gravity alone causes an object to accelerate in the downward direction.•All objects in free fall accelerate at the same rate regardless of their masses.•The acceleration due to gravity is 9.8 meters per second for each second an object is falling. •An object’s velocity is 9.8 m/sec after 1 second•Its velocity is 19.6 m/sec after 2 seconds, etc.
Chapter 10 Forces
Free Fall
The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions.
Chapter 10 Forces
Free Fall
9.8 m/s; 29.4 m/s.
Interpreting Graphs:
What is the speed of the object at 1 second? At 3 seconds?
Chapter 10 Forces
Free Fall
The slope is 9.8. The speed increases by 9.8 m/s each second.
Calculating:
Calculate the slope of the graph. What does this number represent?
Chapter 10 Forces
Free Fall
58.8 m/s
Predicting:
Use the slope that you calculated in Step 2 to predict the object’s speed at 6 seconds.
Chapter 10 Forces
Free Fall
The object’s speed increases at a constant rate.
Drawing Conclusions:
The graph has a constant slope. What does the slope tell you about the object’s motion?
Chapter 10 Forces
Air Resistance
Falling objects with a greater surface area experience more air resistance. Air resistance is an upward force exerted on falling objects.
Chapter 10 Forces
Projectile Motion
• An object that is thrown is called a projectile. • The path of a projectile has horizontal motion due to
a lateral force and vertical motion due to gravitational force.
Chapter 10 Forces
Elastic Forces
When is matter considered to be elastic?
Two Types of Elastic Forces
Matter is considered elastic if it returns to its original shape after it is squeezed or stretched.
Compression•An elastic force that squeezes or pushes matter together.•Example: A cushion on a couch compresses due to a person’s weight until it comes close enough together to exert an equal compression force in the other direction.
Chapter 10 Forces
Elastic Forces
Two Types of Elastic Forces
Tension•An elastic force that stretches or pulls matter.•Example: The strings on a guitar.
Chapter 10 Forces
Section 2 Quick QuizWhich of the following is an example of rolling friction?
A.bike tires on the road as you ride
B.a boat on the water as it sails
C.two hands rubbing together
D.your shoes on a sidewalk as you walk
Answer: A – bike tires on the road as you ride
Chapter 10 Forces
Section 2 Quick QuizWhich of the following is an example of increasing friction intentionally?
A.waxing skis
B.throwing sand on an icy driveway
C.oiling a squeaky door
D.adding grease to gears on a bike
Answer: B - throwing sand on an icy driveway
Chapter 10 Forces
Section 2 Quick QuizThe force of gravity on a person or object on the surface of a planet is called
A.air resistance.
B.mass.
C.free fall.
D.weight.
Answer: D - weight
Chapter 10 Forces
Section 2 Quick QuizThe law of universal gravitation states that any two objects in the universe, without exception,
A.repel each other.
B.combine to provide a balanced force.
C.attract each other
D.create friction.
Answer: C – attract each other
Chapter 10 Forces
Section 2 Quick QuizWhen the only force acting on a falling object is gravity, the object is said to be
A.stationary.
B.decelerating.
C.in free fall.
D.a projectile.
Answer: C – in free fall
Chapter 10 Forces
Section 2 Quick QuizObjects falling through air experience a type of friction called
A.rolling friction.
B.terminal velocity.
C.inertia.
D.air resistance.
Answer: D – air resistance
Chapter 10 Forces
Section 2 Quick QuizWhich type of elastic force is present in the strings on a guitar?
A.tension
B.compression
C.friction
D.inertia
Answer: A - tension
Chapter 10 Forces
Section 2 Quick QuizWhich of the following examples demonstrates a compression force?
A.plucking a guitar string
B.pulling a wagon
C.dropping an apple
D.squeezing a sponge
Answer: D – squeezing a sponge
Chapter 10 Forces
Section 2 Quick QuizWhat holds up a large suspension bridge, such as the Golden Gate Bridge?
A.Fluid friction between the bridge’s supports and the water around them
B.A lack of balance between the weight of the bridge and the supports below it
C.An upward tension force in the cables that balances the weight of the bridge
D.The gravitational attraction between the bridge and its main supports
Answer: C - An upward tension force in the cables that balances the weight of the bridge
Chapter 10 Forces
Newton’s First and Second Laws
Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).
Standard 8.2.f Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion.
Chapter 10 Forces
The First Law of Motion
What is Newton’s First Law of Motion?
An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.•Inertia is the tendency of an object to resist a change in motion.•Another name for Newton’s First Law of Motion is the Law of Inertia.•The greater and object’s mass, the greater its inertia.
Chapter 10 Forces
The Second Law of Motion
What is Newton’s Second Law of Motion?
Acceleration depends on the net force acting on the object and on the object’s mass.
Acceleration = Net force / Mass•Acceleration is measured in meters per second per second.•Net force is measured in Newtons (N), and mass is measured in kilograms (kg).
The above formula can also be written as:
Net Force = Mass X Acceleration
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier to accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Plan and Solve
What quantity are you trying to calculate?
The net force (Fnet) = __
What formula contains the given quantities and the unknown quantity?
a = Fnet/m or Fnet = m x a
Perform the calculation.
Fnet = m x a = 55 kg x 2.0 m/s2
F = 110 kg • m/s2
F = 110 N
Chapter 10 Forces
Calculating Force
A speedboat pulls a 55-kg water-skier. The skier accelerates at 2.0 m/s2. Calculate the net force that causes this acceleration.
Look Back and Check
Does your answer make sense?
A net force of 110 N is required. This does not include the force that overcomes friction.
Chapter 10 Forces
Calculating Force
Practice Problem
What is the net force on a 1,000-kg object accelerating at 3 m/s2?
3,000 N (1,000 kg x 3 m/s2)
Chapter 10 Forces
Calculating Force
Practice Problem
What net force is needed to accelerate a 25-kg cart
at 14 m/s2?
350 N (25 kg x 14 m/s2)
Chapter 10 Forces
Section 3 Quick QuizThe greater the mass of an object,
A.the greater its inertia.
B.the more balanced it is.
C.the more space it takes up.
D.the easier the object starts moving.
Answer: A – the greater its inertia
Chapter 10 Forces
Newton’s Third Law
Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).
Chapter 10 Forces
Newton’s Third Law of Motion
What is Newton’s Third Law of Motion?
Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.
Chapter 10 Forces
Newton’s Third Law of Motion
How can you calculate the momentum of an object?
A characteristic of a moving object that depends on both the mass and the velocity of the object.
Momentum = Mass X Velocity
The unit for momentum is measured in kilogram-meters per second (kg-m/s)
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s, or a 4.0-kg sledgehammer swung at 0.9 m/s?
Read and Understand
What information are you given?
Mass of smaller sledgehammer = 3.0 kg
Velocity of smaller sledgehammer = 1.5 m/s
Mass of larger sledgehammer = 4.0 kg
Velocity of larger sledgehammer = 0.9 m/s
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Plan and Solve
What quantities are you trying to calculate?
The momentum of each sledgehammer = __
What formula contains the given quantities and the unknown quantity?
Momentum = Mass x Velocity
Perform the calculation.
Smaller sledgehammer = 3.0 km x 1.5 m/s = 4.5 kg•m/s
Larger sledgehammer = 4.0 km x 0.9 m/s = 3.6 kg•m/s
Chapter 10 Forces
Calculating Momentum
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?
Look Back and Check
Does your answer make sense?
It is possible for the 3.0-kg hammer to have more momentum than the 4.0-kg one because it has a greater velocity.
Chapter 10 Forces
Calculating Momentum
Practice Problem
A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum?
Golf ball: 0.045 kg x 16 m/s = 0.72 kg•m/s
Baseball: 0.14 kg x 7 m/s = 0.98 kg•m/s
The baseball has greater momentum.
Chapter 10 Forces
Calculating Momentum
Practice Problem
What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?
0.27 kg•m/s (0.018 kg x 15 m/s = 0.27 kg•m/s)
Chapter 10 Forces
Law of Conservation of Momentum
What is the Law of Conservation of Momentum?
The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects.
Chapter 10 Forces-
Conservation of Momentum
In the absence of friction, momentum is conserved when two train cars collide.
Chapter 10 Forces
Section 4 Quick QuizHow does the momentum of a 4.0-kg bowling ball rolling at 3.0 m/s compare to the momentum of a 6.0-kg bowling ball rolling at 2.0 m/s?
A.The 4.0-kg bowling ball has more momentum.
B.The 6.0-kg bowling ball has more momentum.
C.There is not enough information to compare the two bowling balls.
D.The bowling balls have equal momentum.
Answer: D - The bowling balls have equal momentum.
Chapter 10 Forces
Section 4 Quick QuizAccording to Newton’s third law of motion, when a hammer strikes and exerts force on a nail, the nail
A.disappears into the wood.
B.creates a friction with the hammer.
C.moves at a constant speed.
D.exerts an equal force back on the hammer.
Answer: D - exerts an equal force back on the hammer.
Chapter 10 Forces
Section 4 Quick QuizHow can you increase the momentum of an object?
A.by decreasing its velocity
B.by increasing its mass
C.by increasing its friction
D.by decreasing its acceleration
Answer: B – by increasing its mass
Chapter 10 Forces
Section 4 Quick QuizAccording to the law of conservation of momentum, when two objects collide in the absence of friction,
A.only the object with the larger mass continues on.
B.momentum is not lost.
C.velocity increases.
D.velocity decreases.
Answer: B – momentum is not lost
Chapter 10 Forces
Section 4 Quick QuizThe total momentum of a group of objects is conserved unless
A.outside forces act on the objects.
B.outside forces do not act on the objects.
C.there are more than two objects.
D.the objects are moving.
Answer: A – outside forces act on the objects
Chapter 10 Forces
Rockets and Satellites
Standard 8.2.e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).
Chapter 10 Forces
Rockets
How does a rocket lift off? • A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket.
• As long as the upward force, called thrust, is greater than the downward pull of gravity there is an unbalanced force, and the rocket will accelerate toward space.
Chapter 10 Forces
Satellites
What keeps a satellite in orbit?
Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it•A satellite is a falling projectile that keeps missing the ground.•A satellite continues to move around the Earth due to its inertia.•Gravity continuously changes the satellite’s direction.•The speed an object must be thrown in order to orbit Earth is about 7,900 meters per second.
Chapter 10 Forces
What Is a Satellite?
A projectile follows a curved path. The horizontal and vertical motions act independently.
Chapter 10 Forces
What Is a Satellite?The faster a projectile is thrown, the farther it travels before it hits the ground. A projectile with enough velocity moves in a circular orbit. Any force that causes an object to move in a circle is called a centripetal force.
Chapter 10 Forces
What Is a Satellite?
Depending on their uses, artificial satellites orbit at different heights.
Chapter 10 Forces
Section 5 Quick QuizThe achievement of lifting a rocket off the ground and into space can be explained by
A.the law of conservation of momentum.
B.Newton’s first law.
C.Newton’s second law.
D.Newton’s third law.
Answer: D – Newton’s third law
Chapter 10 Forces
Section 5 Quick QuizWhat is required for a rocket to lift off into space?
A.mass that is greater than Earth’s
B.thrust that is greater than Earth’s gravity
C.more velocity than friction
D.very little air resistance
Answer: B - thrust that is greater than Earth’s gravity
Chapter 10 Forces
Section 5 Quick QuizAny force that causes an object to move in a circle is called a(n)
A.centripetal force.
B.gravitational force.
C.unbalanced force.
D.balanced force.
Answer: A – centripetal force
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