Forces and Motion - Washoeschools.net
Transcript of Forces and Motion - Washoeschools.net
Forces
• Force – a push or pull that acts on an object
• A force can cause a restingobject to move, or it can accelerate a moving object by changing the objects speed or direction
Measuring Units of Force
• Force is measured in Newtons(N)
• Newton – the force that causes a 1 kg mass to accelerate at a rate of 1 m/s²
• 1 N = 1 kg • m/s²
• Named for Sir Isaac Newton
Representing Force/Combining Forces
• Force can be represented by an arrow
• The direction of the arrow represents the direction of the force
• The length of the arrow represents the strength, or magnitude of the force
• Net force – the overall force acting on an object after all forces are combined
For example 1: What do you think will
happen to the object?
For example 2: What about now?
Balanced vs Unbalanced Forces
• When forces on an object are balanced, the net force is zeroand the objects motion doesn’t change
• When an unbalanced force acts on an object, the object accelerates
What is the net force in this
example?
Friction
• Friction – a force that opposes the motion of objects that touch as they move past each other
• Four types: static friction; sliding friction; rolling friction; and fluid friction
• Static friction – the friction force that acts on objects that are notmoving
• Sliding friction – a force that opposes the direction of motion of an object sliding over a surface
Friction (cont.)
• Rolling friction – the slight bend out of shape of the floor and a round object due to the friction force acting on them
• Fluid friction – the force that opposes the motion of an object through a fluid
Gravity
• Gravity – a force that acts between any two masses
• An attractive force, pullingobjects together
• Objects do not have to be in contact for it to act on them
• Earth’s gravity acts downwardtoward the center of the Earth
• Gravity causes objects to accelerate downward; air resistance opposes this and reduces acceleration
Gravity (cont.)
• Larger surface area when falling increases air resistance, slowing the effects of gravity
• What are some examples of this?
• Terminal velocity – the constant velocity of a falling object when the forces of gravity and air resistance are equal
Projectile Motion
• Projectile motion – the motion of a falling object (projectile) after it is given an initial forward velocity
• A ball follows a curved path due to the combination of forward velocity and the downward vertical force of gravity
Newton’s Laws of Motion (background)
• Aristotle made mistakes too!• He incorrectly proposed that
force is required to keep an object moving at a constant speed; people didn’t question this for 2,000 years!
• Galileo – concluded that moving objects without the influence of friction or any other force would move indefinitely (forever); found this by rolling balls down wooden ramps
Newton’s First Law of Motion
• Newton’s First Law of Motion – the state of motion of an object does not change if the net force acting on that object is zero
• Inertia – the tendency of an object to resist a change in it’s motion
Newton’s Second Law of Motion
• Mass – a measure of the inertiaof an object; depends on the amount of matter the object has
• Newton’s 2nd Law – the acceleration of an object is equal to the net force acting on it divided by the objects mass
F = net force; m = mass; a =
acceleration
Newton’s Second Law Sample Problem
• A car with a mass of 1,000 kilograms (kg) accelerates when the stop light turns green. If the net force on the car is 4,000 newtons(N), what is the car’s acceleration? (Note: N/kg is equal to m/s²)
Mass, m = 1,000 kg
Force, F = 4,000 N (in the forward direction)
Acceleration = Net force
Mass, a =
𝐹
𝑚
a = 4,000 N
11,00 kg=
4 N
kg= 4 kg•𝑚/𝑠²
kg= 4 m/s²
Weight and Mass
• Weight is not the same as mass • Weight – the force of
gravity acting on an object
• This formula is similar to the formula for Newton’s
second law: a = F
m, but
instead a = g (gravity = 9.8 m/s²) and F = W
• Mass is a measure of the inertia of an object; weight is a measure of the force of gravity on an object
Sample Problem
• An astronaut has a mass of 112 kg, what is her weight on Earth where the acceleration due to gravity is 9.8 m/s²?
• Weight = Mass x Acceleration due to gravity
= 112 kg x 9.8 m/s²
= 1100 kg • m/s² = 1100 N
• What is her weight on the Moon? And her mass? (hint: gravity is about 1/6th that of the Earth)
Newton’s Third Law of Motion and Momentum
• Forces always exist in pairs
• Newton’s third law – whenever one object exerts a force on a second object, the second object exerts an equal and oppositeforce on the first object
• Action and reaction forces –action force = you pushing against a wall; reaction force = the wall pushing back against you
• Action and reaction forces don’tcancel – only when both forcesact on the same object do they cancel
Momentum
• Momentum – the product of an objects mass and its velocity
• An object has a large momentum if the product of its mass and velocity is large
• Momentum is measured in Kilograms • meters per second
• Sample Problem: Which has more momentum, a 0.046 kg golf ball with a speed of 60.0 m/s, or a 7.0 kg bowling ball with a speed of 6.0 m/s?
Sample Problem (cont.)
• Momentum golf ball = 0.046 kg x 60.0 m/s = 2.8 kg • m/s
• Momentum bowling ball = 7.0 kg x 6.0 m/s = 42 kg • m/s
Conservation of Momentum
• Law of conservation of momentum – if no net force acts on a system, then the total momentum of the system does not change
• In a closed system, the loss of momentum of one object equals the gain in momentumof another object; momentum is conserved
Universal Forces
• Electromagnetic forces –associated with charged particles
• Electric force and magnetic force are the only forces that can both attract and repel
• Electric forces – deal with electrons and protons; opposites attract and likes repel
• Magnetic forces – act on certain metals, poles of magnets, and moving charges
• Same principles as electric forces
Nuclear Forces
• Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus of an atom to hold it together
• Strong nuclear force – a powerful force of attractionthat acts only on the neutronsand protons in the nucleus, holding them together
• Weak nuclear force – an attractive force that acts only over a short range (less range than strong)
Gravitational Force
• Gravitational force – an attractiveforce that acts between any two masses
• Newton’s law of universal gravitation – every object in the universe attracts every other object
• Proportional to the objects’ masses; decreases rapidly as distance between then increases
• Centripetal force – a center-directed force that continuouslychanges the direction of an object causing it to move in a circle