Embed Size (px)
DESCRIPTION
Lecture 7: Forces & The Laws of Motion. Questions of Yesterday. A ball is thrown vertically upwards in the air by a passenger on a train moving with a constant velocity. To a stationary observer outside the train, is the velocity of the ball at the top of its trajectory a) greater than - PowerPoint PPT Presentation
Citation preview
Lecture 7: Forces &
The Laws of Motion
Questions of Yesterday1) A ball is thrown vertically upwards in the air by a
passenger on a train moving with a constant velocity. To a stationary observer outside the train, is the velocity of the ball at the top of its trajectorya) greater than
b) Less thanc) Equal tothe velocity observed by the passenger?
2) The hang-time of a basketball player who jumps a vertical distance of 2 ft is about 2/3 second. What will the hang-time be if the player reaches the same height while jumping 4 ft horizontally?a) less than 2/3 sb) greater than 2/3 sc) equal to 2/3 s
ForcesContact Forces
Force applied to an object by direct contact
A physical push or pull
Ex. Stretching a spring,Hitting a baseball
Tug-of-war
Field Forces
Force applied to an object without direct contact
Applied over a range of space
Ex. Gravitational force,Electrical force,Magnetic force
Newton’s First LawWhat happens when…
you push your physics book sitting on your desk?You roll a basketball across the floor?You hit a hockey puck in an ice rink?
Why?
An object at rest will remain at rest unless acted on by an external force.
Also, an object in motion will remain in its original state of motion unless acted on by an external force.
Newton’s First LawWhat happens when…
you push your physics book sitting on your desk?You roll a basketball across the floor?You hit a hockey puck in an ice rink?
Why?
An object moves with a velocity that is constant in magnitude and direction unless acted on
by a nonzero net force.
Net force = sum of all external forces acting on an object
Newton’s First Law
Why is Newton’s first law true?What makes an object resist changing its state of motion?
An object moves with a velocity that is constant in magnitude and direction unless acted on
by a nonzero net force.
INERTIAtendency of an object to continue
in its original state of motion
Newton’s First Law
An object moves with a velocity that is constant in magnitude and direction unless acted on
by a nonzero net force.
Which ball do you want to have a game of toss with? Why?
MASSmeasure of object’s resistance to changes
in its motion due to an external force
Newton’s Second Law
The acceleration a of an object is directly proportional to the net force acting on it and
inversely proportional to its mass
The acceleration of an object is inversely proportional to its mass
The acceleration of an object is directly proportional to the net force acting on it
Newton’s Second Law
The acceleration a of an object is directly proportional to the net force acting on it and
inversely proportional to its mass
a = F/m
F = maSI Unit of force =
Newton
1 N = 1 kg*m/s2
F and a arevectors!
Vector equation!
Fx = max Fy = may Fz = maz
Newton’s Second Law
I apply a force F1 to my physics book to push it across the desk with a velocity of 10 m/s.
If instead I want to push the book at a velocity of 20 m/s is the force I need to apply
greater than, less than, or equal to F1?
F = ma
If an object is accelerating does that mean that there has to be a net force on it?
If an object is not accelerating does that mean that no forces are acting on it?
Gravitational ForceForce of attraction between any two objects in the
Universe. Gravitational force causes….
Objects in free fall near the Earth’s surface
to accelerate towards the Earth
the moon to orbit the earth & the planets to orbit the sun
An astronaut to be able to
jump higher on the Moon
than on Earth
Gravitational ForceNewton’s Law of Gravitational Force
m1,m2= mass of objects attracting each other
r = distance between the objectsUniversal gravitational constant = G = 6.67*10-11
N*m2/kg2
Fg = Gm1m
2 r2
WeightWhat is the magnitude of the gravitational force acting on object with mass m near
Earth’s surface?
ME = 5.98*1024
RE = 6.38*106 Fg = G
MEm
RE2
Fg = m*(9.8m/s2) = mg
WeightMagnitude of gravitational force acting on an object near
Earth’s surface
w = mg
Newton’s Third Law
Forces in nature always exist in pairs!
If object 1 and object 2 interact, the force F12
exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force
F21 exerted by object 2 on object 1
For every force there is an equal and opposite reaction force
Action-reaction force pairs always act on different objects!
F12 = -F21
Newton’s Third Law
Which vehicle experiences the greater acceleration?
F12 = -F21
If a Mack Truck and Honda Civic have a head-on collision, upon which vehicle is the impact force
greater?
Newton’s Third Law
What force(s) acts on the object during free fall?
F12 = -F21
What exerts this force on the object?
Is there a nonzero net force on a freely falling object? Why?
What is the corresponding reaction force?
What does it act on?
Newton’s Third Law
Why doesn’t the Earth accelerate towards us as we accelerate towards it?
F12 = -F21 m2a2 = -m1a1
aE = -(m0a0 )/ME
m0 << ME
aE ≈ 0
m0a0 = -MEaEFE0 = -F0E
Normal ForceIs there a nonzero net force acting on the block?
What about the gravitational force?
F
g
N
Normal Force
F
g
N
N’
Normal Force force exerted by surface on object in contact with the
surface
Are N and Fg
action-reaction pairs? Why?
Is the magnitude of N greater than, less than, or equal to Fg?
Fg’
Free Body Diagrams
F
g
N
N’
Diagram of a single object with all the forces acting ON it
DO NOT INCLUDE:
-Forces that the object exerts on
other objects or the Earth
-Other objects that the object is in contact with
Fg’
Free Body DiagramsTennis ball undergoing projectile motion
F
g
Free Body DiagramsTennis ball while a tennis player is serving it.
The player swings the racket at a 30o angle with the horizontal when in contact with the ball.
F
g
Is the ball accelerating?How do you know?
In what direction is it accelerating?
FA30o
Free Body DiagramsA block sliding down a frictionless inclined plane.
F
g
In what direction?
N
Is the block accelerating?
Free Body DiagramsA block sliding down a frictionless inclined plane.
F
g
N
+ y
+ x
- x
- y
Choose coordinate system to be in direction of incline plane!
Fgy
Fgx
Fgx = Fgsin
Fgy = Fgcos
Questions of the Day1) You must apply a force F1 to begin pushing a
crate from rest across the floor, you must apply a force F2 to keep the crate moving at a constant velocity once its in motion. Which statement is true?a) F1 = F2
b) F1 > F2
c) F1 < F2
2) When do action and reaction pairs of forces not cancel one another out?a) when one of the objects is acceleratingb) when both objects are acceleratingc) never
