Transcript Insert the title here

Forces
Newton’s Second Law
Newton’s Second Law
• States that the greater the force applied, the greater the
acceleration and the greater the mass the less the acceleration
• F=ma
Weight
• Mass and weight are not the same!
• Weight is equal to the force of gravity
• F=ma
• Acceleration due to gravity is 9.8 m/s2
• Force due to gravity (weight)=mass x 9.8 m/s2
Example of Weight
• A boy has a mass of 50 kilograms, what is his weight?
• An object exerts a force of 40 N on the ground, what is the mass?
Example of Weight
• Tina exerts 513.4 Newtons of force, what is her weight in pounds?
• 1 pound=.45 kilograms
Net Force
• When you have two forces
• Add them- when they are working together
• Subtract them- when they are opposing each other
Newton’s Second Law
• Arnold needs to lift a 35.0 kg rock. If he exerts an upward force
of 502 Newtons on the rock, what is the rock’s acceleration?
An Example Using a Constant Velocity
• A student raises their 15 kg backpack from the floor at a constant
velocity of 5.0 m/s. How much force must the student apply?
Example
• Annie is holding a stuffed dog, with a mass of 0.30 kg, when
Sarah decides she wants it and tries to pull it away. If Sarah pulls
horizontally on the dog with a force of 10.0 N and Annie pulls with
a horizontal force of 11.0 N, what is the horizontal acceleration
of the dog?
Apparent Weight
• How does a scale work?
• Apparent weight is the force exerted by the scale
• Weightlessness is when there are no contact forces pushing up
on the object
Elevator Problem
• Your mass is 75.0 kg and you are standing on a bathroom scale in
an elevator. Starting from rest the accelerates 2.0 m/s2 for 2.00
seconds and then continues at a constant speed. Is the scale
reading during acceleration greater than, equal to, or less than
the scale reading when the elevator is at rest?
Elevator Problems Tips
• Apparent weight changes when you are accelerating or
decelerating, NOT when you are at rest OR going at a constant
speed
• Accelerating upwards= Weight is greater
• Accelerating downwards= weight is less
• Decelerating upwards=weight is less
• Decelerating Downwards= Weight is greater
Drag Force and Terminal Velocity
• Drag Force- force exerted by a fluid (liquid OR a gas) on an
object moving through the fluid
• Depends on
• Motion of the object
• Properties of object and fluid
• As speed increases for the object the drag force also increases
• When drag force is equal to the force of gravity a terminal
velocity is reached
• Terminal Velocity is the highest velocity can reach when falling
An example with UAM
• A ball is kicked and slides to a stop. The ball has an initial velocity
of -4.5 m/s and the force of friction is 1.4 N. The ball has a mass of
47 grams. What is the displacement of the ball.
Newton’s
rd
3
Law
Newton’s Thrid Law Recap
• For every force there is an opposing force in the opposite
direction
• F(a on b)=-F(b on a)
Action Reaction Pairs
• A string pulls a book
• A tennis racket hits a tennis ball
• Chalk writes on a chalk board
Example
• When a softball with a mass of 0.18 kg is dropped, its
acceleration toward Earth is equal to g, the acceleration due to
gravity. What is the force on Earth due to the ball and what is
Earth’s resulting acceleration? Earth’s mass is 6.0 x 10 24 kg.
Example of
rd
3
Law and Tension
• A 50.0 kg bucket is being lifted by a rope. The rope will not break
if the tension is 525 N or less. The bucket started at rest, and after
being lifted 3.0 m, it is moving at 3.0 m/s. If the acceleration is
constant, if the rope in danger of breaking?
Friction
Static Friction Versus Kinetic Friction
• Friction- force exerted by two surfaces rubbing against each
other
• Static Friction- No motion
– Acts in response to other forces, but there is a limit to how large static
friction can be (maximum)
– Once the maximum point has been met and movement starts static
friction kinetic friction
• Kinetic Friction- Motion
– Is less than the static friction
Equations
• Kinetic Friction
– fk=μk Fn
– Kinetic Friction= kinetic coefficient of friction times the normal force
• Static friction
– Fs=μs Fn
– Static friction= static coefficient of friction time the normal force
Example
• A force of 98 N is exerted on a 25.0 kg box and the box is moving.
If the coefficient of friction (kinetic) is 0.20, what is the
acceleration?
Tips for friction problems
• Use static coefficient when not moving
• Use kinetic coefficient when moving
• The normal force is equal to the weight of the object