Forces and MotionMr. Conkey Physical Science Ch.12

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

Take 30 seconds to discuss force with your table partners

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?

Combining Forces (cont.)

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

Review the friction types with you shoulder partners for 30 seconds…

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

Example of Inertia

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

The Egg Drop Experiment