Chapter 3Chapter 3

Forces

ForcesForces, 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

ForcesSample 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?

ForcesSample 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?

ForcesSample 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

ForcesTypes of Friction

1.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

ForcesTypes of Friction

2.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)

ForcesTypes of Friction

3.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

GravityGravitational Acceleration

• the gravitational attraction of Earth causes falling objects to have an acceleration of 9.8 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

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

Gravityprojectile 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 (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

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

ForcesLaw 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.