# Chapter 3 Forces.

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Chapter 3 Forces

Forces Forces, Mass, and Acceleration
the greater the force applied on an object, the greater the acceleration the greater the mass of an object, the less acceleration

Forces Newton’s Second Law of Motion – the net force acting on an object causes the object to accelerate in the direction of the net force Formula for force: F = force m = mass a = acceleration   F = ma

Forces Newton’s Second Law of Motion – the net force acting on an object causes the object to accelerate in the direction of the net force units for force: Newtons F = ma

Forces Sample Problems
example: A person that weighs 70 kg jumps out of a plane at 9.8 m/s2. What is the force that gravity is exerting on the person?

Forces Sample Problems
If gravity is exerting a force of 98 Newtons on an object in air, and the acceleration due to gravity is 9.8 m/s2, what is the object’s mass?

Forces Sample Problems
If an object weighs 10 kg and has a force of 200 Newtons acting on it, what is the acceleration?

Forces friction – the force that opposes motion between two surfaces that are touching each other Newton’s first law would say that if you push a skateboard, it would continue to move in a straight line at constant speed. We know however, that the skateboard slows down due to friction

Forces Types of Friction
static friction – the friction between two surfaces that are not moving past each other example: when a box is so heavy you try and push it but it doesn’t move NOT Moving

Forces Types of Friction
sliding friction – the force that opposes the motion of two surfaces sliding past each other example: when you push a box it slides across the floor but the sliding friction makes it hard to push (but it DOES move)

Forces Types of Friction
rolling friction – the friction between a rolling object (ex. tire) and the surface that it rolls on example: a car’s tire rolling over the pavement or a train rolling on the rails usually much less friction than static or sliding friction

Forces air resistance - a force that acts opposite to the motion of an object example: running against the wind or a parachute falling to the ground air resistance depends on the speed, size, and shape of the object if no air resistance exists, two objects will fall at the same rate (even a feather and apple)

Forces

Gravity law of gravitation – states that any two masses exert an attractive force on each other the force of gravity increases when the mass of either object increases the force of gravity increases when the two objects move closer together

Gravity

F = m x 9.8 m/s2 Gravity Gravitational Acceleration
the gravitational attraction of Earth causes falling objects to have an acceleration of m/s2 remember that F = ma so the force of gravity on an object near Earth’s surface is: F = m x 9.8 m/s2

Weight = mass x 9.8 m/s2 Gravity
weight (W) – the gravitational force exerted on an object measured in newtons (N) weight and mass are NOT the same because mass does not change based on location but your weight does Weight = mass x 9.8 m/s2

Gravity example: How much does a person with a mass of 70 kg weigh on Earth? on the moon, the acceleration due to gravity is only 1.6 m/s2 so you will weigh less on the moon

Gravity projectile motion
the motion of anything tossed, thrown, shot, etc. will travel in a curved path the object (projectile) will follow a curved path because of Earth’s gravitational pull

Forces centripetal force – an unbalanced force that acts in the direction toward the center of motion centripetal acceleration then, is the acceleration of an object toward the center of a curved or circular path

Forces centripetal force – an unbalanced force that acts in the direction toward the center of motion centripetal acceleration then, is the acceleration of an object toward the center of a curved or circular path acceleration occurs during a curve because the direction is changing therefore making velocity change example: centripetal force (friction between the tires and road surface) causes a car on a curve to stay inward while the car’s inertia forces it outward

Motion Newton’s third law of motion – describes action-reaction pairs by stating that every action force has an equal and opposite reaction force. “for every action, there is an equal and opposite reaction” example: if an object hits water, water splashes back up

Motion A rocket works by igniting fuel which causes the gases to exert a downward force, resulting in air being pushed downward exerting a force upward, pushing the rocket up

Motion momentum = mass x velocity p = m x v
momentum (p) - property that a moving object has that is related to how much force is needed to change its motion the unit for momentum is kilogram meters per second momentum = mass x velocity p = m x v

momentum = mass x velocity
Motion momentum (p) - property that a moving object has that is related to how much force is needed to change its motion two trucks might have the same velocity but the bigger truck has more momentum example: Compare the momentum of a 50-kg dolphin swimming at 10.4 m/s and a 6,300-kg elephant walking 0.11 m/s. momentum = mass x velocity

Forces Law of Conservation of Momentum If no other force acts on bodies in motion, the momentum before collision is equal to momentum after collision momentum is not lost or created – it is conserved total momentum is zero.