Everyday Physics 2
Review – inertia
•Tendency of objects to resist changes in motion.•The inertia of an object is measured by its mass in kilograms (kg) – the quantity of matter in it.
•If an object is at rest it stays at rest.
•If an object is moving with constant velocity, it continues moving until something stops it.
Newton’s Second Law of Motion
•No force is required to keep an object moving with constant velocity.
•What can change the velocity of an object ?
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FORCES•acceleration is a change in velocity
•forces produce accelerations
•for example- friction or air resistance
Force is a vector quantity
•It matters not only how hard you push, but also in what direction
object
The NET Force
• What really matters is the Net Force
• The Net Force is what you get when all the forces are properly combined
• The Net Force takes into account both how strong the forces are and in what direction they act
• The Net Force determines the acceleration of the object net force
Example: Net force = 0
Net force = 0
•An object can have many forces acting on it at the same time.
•If all the forces oppose each other exactly then the net force = 0 and the object will either be at rest or move with constant velocity.
tension in string
weight of block
air resistance
Two forces act on a skydiver - gravity (his weight) and
air resistance. When they balance, he coasts down
with constant speed.
• Zero net force does not necessarily imply zero velocity
(a skydiver’s terminal speed will be greater than 100 mph)
• Zero force constant velocity, v = 0 is a special case of constant
velocity. A parachutes reduce the terminal speed to about 10 mph.
Contact and non-contact forces
•To change the velocity of an object a net force must be applied to it.
•A push
•Or a pull
Contact and non-contact forces
•Pushes, pulls, friction, and tension are contact forces- whatever exerts the force actually touches the object
•Non-contact forces:
Forces that act without contact between objects
a) electric forces
b) magnetic forces
c) gravity
•The force of gravity acting on the moon pulls it away from its otherwise straight line path.
MOON
• The moon is constantly falling toward the earth in the sense that it falls away from the straight line it would follow if the earth were not there
What does it take to get an object accelerating?
BIG FORCE
little force
Magnitude of force required to produce acceleration is
Proportion to mass such that
Which object moves faster?
FORCE
Acceleration produced by a given force (acting on an object) is inversely proportional to the mass of the object, and
What is the relationship between force and acceleration?
• Acceleration produced by a force on a given object is proportional to the force such that
• The force and acceleration have the same direction
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Newton’s 2nd LawForce = mass times acceleration
Force in
Newtons
(N)
Mass in Kilograms (kg)
Acceleration in m/s2
Newton’s 2nd Law
•In other words, if a force is applied to an object of mass then the acceleration is
•Or if a force acts on an object and the acceleration is , then the mass must be
•It is the law which explains how things move
•If a net force is applied to an object it will accelerate – change its velocity
•It includes the law of inertia if there is no force, F = 0, then the acceleration = 0 the velocity doesn’t change no net force is needed to keep an object moving with constant velocity.
Newton’s Law
Weight
•All objects near the surface of earth fall with an acceleration of about under the action of force of gravity. This acceleration due to gravity is denoted by
•The weight of an object is equal to the force of gravity acting on the object.
•Using Newton’s 2nd law of motion, the force of gravity acting on an object of mass is
So the weight of object is
Example Problem -1
•Two forces act on a 4 kg object. A 14 N force acts to the right and a 2 N force acts to the left. What is the acceleration of the object?
•Net force = 14 N x 2 N = 12 N (to the right)
So
Therefore the object accelerates to the right at in the direction of the NET force
Example Problem 2
•A 2 kg box is pushed by a 10 N force while a 2 N friction force acts on the box. What is the acceleration of the box?
•Net force = 10 N – 2 N = 8 N to the left
Acceleration = Force / mass = 8N / 2 kg = 4 m/s2
to the left.
acceleration is in the direction of the NET Force
Friction force = 2 N
Push = 10 N
2 kg
Review: Newton’s 1st & 2nd Laws
•1st law (Galileo’s principle of inertia)- no force is needed to keep an object moving with constant velocity
•2nd law (law of dynamics) – a force is needed to change the velocity (i.e., accelerate) of an object, how much:
Newton’s third law (deals with the interaction of 2 objects)
We will discuss collisions, impulse,
momentum and how airbags protect you in a crash
For every action there is an equal and opposite reaction.
Newton’s 3rd Law
•If object A exerts a force on object B, then object B exerts an equal force on object A in the opposite direction.
The bouncing ball
•Why does the ball bounce?
•It exerts a downward force on ground
•The ground exerts an upward force on it that makes it bounce
You can move the earth!
•The earth exerts a force on you
•you exert an equal force on the earth
•The resulting accelerations are not the same, because the masses are different
You have an influence on every object in the Universe!
Action/reaction forces always act on different objects
•A man tries to get his donkey to pull a cart but the donkey has the following argument:
•Why should I even try? No matter how hard I pull on the cart, the cart always pulls back with an equal force, so I can never move it.
•Explain to the donkey why his argument is wrong
Friction is essential to movement
The tires push back on the road and the road pushes the tires forward. If the road is icy, the friction force between the tires and road is reduced.
You can’t walk without friction
You push backward on the ground and the ground pushes you forward.
Impulse
•When two objects collide they exert forces on each other that last only a short time
•We call these short lasting, but usually strong forces IMPULSIVE forces.
•For example when I hit a nail with a hammer, I exert an impulsive force
What is impulse?
•If a force F acts for a time t, then the impulse is the Force time = F t
•Since force is measured in Newtons and time in seconds, impulse will be measured in Newton-seconds.
•Impulse is a vector, it is a measure of the net effect of the impulsive force
force
Momentum
•The term momentum is used quite often in everyday conversation about many things.
•Momentum is a physics term that has a very definite meaning. If an object has a mass m and moves with a velocity v, then its momentum is mass velocity = m v
•Momentum is a vector and has the same direction as the velocity
Momentum = m v
•Inertia in terms of momentum: if something has momentum, it doesn’t loose it easily and if it doesn’t have it, it doesn’t get it easily – something has to happen to an object to change its momentum
•Impulse causes the momentum of an object to change; in fact
the change in momentum = impulse
•Thus if
•If an object gets an impulse, F x t, then its momentum changes by exactly this amount
Elastic and inelastic Collisions (bouncy) (non-bouncy)
Force on The ball
Which ball experiences the largest upward force when it hits the ground?
Bouncing ball
•The force that the ball exerts on the ground is equal to and in the opposite direction as the force of the ground on the ball.
•The ball that bounces back not only must be stopped, but must also be projected back up.
•The ground exerts more force on the ball that bounces than the ball that stops.
Physics explains it!
•Beakers dropped from same height so they have the same velocity (and momentum) when they get to the bottom.
•One falls on a hard surface
•The other falls on a cushion.
•What prevents the beaker that falls on the cushion from breaking?
•First, what causes anything to break?
•If an object experiences a large enough FORCE then it might break.
•Why does the beaker that falls on the cushion experience a smaller force?
•Both beakers have the SAME change in their momentum – they both hit the bottom with the same speed and both end up with zero velocity.
•The beaker that shatters comes to rest more quickly than the one that gently slows down on the cushion this is the key point!
•According to the impulse-momentum relation: Impulse = Force x time (F x t)
= change in momentum
•F x t is the same for both. Since the one on the cushion takes longer to slow down the force on it is less, t is bigger F smaller
Air bags
•The same thing is true for airbags
•They protect you by allowing you to come to rest more slowly, than if you hit the steering wheel or the dash board.
•Since you come to rest more slowly, the force on you is less.