Prentice Hall Physical Science
Chapter 12 Forces and Motion
12.1 Forces
A. What is a Force?
- a force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction
- force is easy to measure – a spring scale can be used
- force is measured in NEWTONS (N). A newton is the force that causes a 1-kg mass to accelerate at a rate of 1 m/s2 (1 N = 1 kg*m/s2)
- an arrow in the direction of the force is used to represent a force
B. Combining Forces
- Forces can be combined: forces in the same direction can be added and forces in opposite directions combine by subtraction
- the result of combining forces is a net force
- when the forces on an object are balanced, the net force is zero and no change in motion will occur
- when the forces on the object are unbalanced, the net force is greater than zero and the object will accelerate in the direction of the larger force
C. Friction
- friction is a force that opposes motion (it acts in a direction opposite to the motion of an object)
- static friction
- the friction force that acts on objects that are not moving
- it acts opposite to the applied force
- must be overcome to make an object move
- it is the strongest friction force
- sliding friction
- the force that opposes the motion of an object as it slides over a surface
- it is less than static friction
- rolling friction
- the frictional force that acts on objects that are rolling
- 100 to 1000 times less force than sliding or static friction
- ball bearings are often used to turn sliding friction into rolling friction
- fluid friction
- the frictional force that acts on objects that are moving through a fluid (a fluid is any liquid or gas)
- it has the least amount of force
- fluid friction increases with increased speed through the fluid
- fluid friction acting on an object moving through the air is known as air resistance
D. Gravity
- gravity is a force that acts between any two objects with mass
- it does not require objects to be in contact for them to feel the force
- Earth’s gravity acts downward toward the center of the Earth
- Falling Objects
- gravity causes objects to accelerate downward and air resistance acts in the opposite direction and reduces acceleration
- if an object falls for a long time, the force of gravity pulling down and the force of air resistance pushing up become equal and the object can no longer accelerate causing it to continue to fall at a constant speed.
- this constant speed is called terminal velocity
E. Projectile Motion
- when you throw a ball forward, it follows a curved path
- this curved path is called projectile motion
- it is caused by the initial forward velocity combined with the downward vertical force of gravity
- Figure 9, p. 362
12.2 Newton’s First and Second Law of Motion
A. Newton’s First Law of Motion
- an object at rest will stay at rest and an object in motion will stay in motion unless an outside, unbalanced force acts on it.
- it is also called the law of inertia
- inertia is the tendency of an object to resist a change in motion
- the more mass an object has, the more inertia it has
- seatbelts are because of inertia
B. Newton’s Second Law of Motion
- an unbalanced force causes an object’s velocity to change (it causes it to accelerate)
- how much it accelerates depends on the size of the force and the amount of mass it contains (how much inertia it has)
- Acceleration = net force/mass (a = F/m; F = m*a; m = F/a)
(remember the unit for force is the N, the unit for acceleration is m/s2 and the unit for mass is kg)
EX 1: A 20 N net force acts on an object with a mass of 2.0 kg. What is the object’s acceleration?
K: f = 20 N
M = 2.0 kg
Unk: a = ?
a = F/m
a = 20 N/2.0 kg
a = 10 m/s2
EX 2: A box has a mass of 150 kg. What force would be required to accelerate the box at 20 m/s2?
K: m = 150 kg
a = 20 m/s2
unk: F = ?
F = m*a
F = 150 kg * 20 m/s2
F = 3000 N
C. Weight and Mass
- mass is how much matter is contained in something and its inertia and it is measured in kilograms
- weight is the force gravity is putting on that mass
- weight = mass * acceleration due to gravity (w = mg)
EX The mass of an average person is 75 kg. What is the weight of the average person?
K: m = 75 kg
g = 9.8 m/s2
unk: w =
w = mg
w = 75 kg * 9.8 m/s2
w = 735 N
Prentice Hall Physical Science
Chapter 12 Forces and Motion
12.3 Newton’s Third Law of Motion and Momentum
A. Newton’s Third Law
***Newton’s third law says for every action there is an equal but opposite reaction
- Forces come in pairs called action-reaction forces
- When you push on your desk, your desk pushes back
- Action and reaction forces do not act on the same object
EX: when you are swimming, you put a force on the water and the water puts a force on you pushing you forward
EX: rockets go up because they are pushing hot gases down
B. Momentum
- momentum is the product of an object’s mass and its velocity (p = m*v)
- the larger the mass or velocity, the larger the momentum
EX: What is the momentum of an object with a mass of 15 kg and a velocity of 6 m/s2?
K: m = 15 kg
V = 6 m/s2
Unk: p = ?
p = mv
p = 15 kg * 6 m/s2
p = 90 kg-m/s2
o Momentum in a closed system, with no outside forces acting on it, is always conserved. When one object loses momentum, another object gains momentum
12.4 Universal Forces
- Electromagnetic Forces
- this is the force associated with charged particles
- electric forces and magnetic forces are the only forces that can attract and repel
- in electric forces, opposite forces attract and like forces repel
- magnetic forces act on certain metals, on the poles of magnetic, and on moving charges
- like pole repel and opposite poles attract
- Nuclear forces
- these forces occur within the nucleus of an atom to hole it together
- the strong nuclear force is a powerful force that acts ONLY on the neutrons and protons in the nucleus
- it acts over a very short distance but it is 100 times stronger than the electric force at this distance
- weak nuclear force is an attractive force that acts over a very short range (less than the strong force)
- Gravitational Force
- this is the weakest universal force
- it is the force that acts between any two objects with mass
- every object in the universe attracts every other object in the universe
- it acts over very long distances, but it decreases as the distance between two objects increases
- it is proportional to the masses of the objects
- it keeps the moon in orbit causing tides
- it keeps satellites in orbit