Unit 4: Forces
What is the gravitational force acting on a 15 kg object? HINT: (Fgrav = m * g; g = 9.8 m/s/s) assume down is +
+ 147 N
Using newton's 2nd law answer the following question: A force of -20 N acts upon a 5 kg block. Calculate the acceleration of the object.
- 4 m/s/s
Using Newton's second law, answer the following question: The mass of a large car is 2200 kg. How much force would be required to accelerate the car at a rate of -3.1 m/s2?
-6,800N
A frictional force of 15N is used in order to keep a 39 N (Fgrav or weight) object moving at a constant speed. Calculate the coefficient of friction if it equals the the frictional force/normal force.
.38
A 100N force is being applied to the right and a 40N force is being applied to the left of a box. What is the resultant net force and motion of the box?
60N to the right
The amount of force required to keep a 6 kg object moving in with a constant velocity of 3 m/s is _____ N. Also, what's the acceleration, what's the formula, and how do you solve it?
Acceleration = 0, F = m a = (6) (0) = 0
Describe the relationship between mass or net force upon the acceleration of an object: increase mass _________. Increase force _______.
Acceleration decreases, acceleration increases
What is the g in Fg(not this one) = mg(this one)?
Acceleration due to gravity, it's 9.8 m/s^2
Newton's First Law
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
When all the forces acting on a box are balanced, the net forces are ___________.
Balanced
Explain in words how this force diagram would be for this situation: A block is being pushed across the table. Consider all the forces acting on the block.
Balanced forces in the Y direction, Fn is pointing up, while Fg is pointing down. X direction is unbalanced, Ff is pointing to the left with a little arrow, while Fapp is pointing to the right with a bigger arrow.
Explain in words how this force diagram would be for this situation: A block rests on top of a table. Consider all the forces acting on the block.
Balanced forces, Fn (the table), is pointing up while Fg is pointing down.
Explain, in words, what a Free body diagram of a car that is coasting down the street is.
Coasting = no Fapp so, Ff to the left, Fn up, and Fg down (balanced).
What is the mass of an object that was accelerating at a rate of 2.6 m/s ^2 on a level road after a force of 33,800 N what is applied? Show the equation too
Equation: F = ma so, 33,800 = m x 2.6 then, 33,800/2.6 to get 13,000kg
Newton's Third Law
Every action has an equal and opposite reaction
Suppose a drag racer has a mass of 1250 kg and accelerates around the track at a constant rate of 16.5 m/s ^2. How large is the unbalanced force acting on it? Show the equation too.
F = ma so, F = 1,250 x 16.5 to get 20,625 N
True or False: Inertia is a force
FALSE - Inertia is property of matter, NOT a force.
Explain in words how this force diagram would be for this situation: A skydiver who opened his parachute, is falling with the force of gravity of 800 N and an upward force of 100 N.
Fair pointing up with a small arrow, while Fg pointing down with the bigger arrow.
True or False: Fast-moving objects have more inertia than slow-moving objects
False
True or False: The standard metric unit of mass is the pound
False
True or False: When a chemistry student places a beaker on a balance and determines it to be 84.3 grams, they have weighed the beaker
False
You plan to lace up your sneakers to play a game of basketball with some pals. In order to increase the amount of friction between your shoe and the court, it would be important to wear your sneakers with a smallest friction coefficient.
False
True or false: On Earth, there are scenarios that exist where there are no forces on an object.
False, the earth constantly moves
What force pulls to the right?
Fapp
What force pulls to the left?
Ff
Explain in words how this force diagram would be for this situation: A ball is shot into the air with a spring loaded cannon. Consider the forces acting on the ball while it is in the air.
Fg pointing down
What force pulls down?
Fgrav
Nelly, a workhorse, doesn't feel like working today. What force must you push her forward with to get her 500 kg body accelerating at 4 m/s ^2? Ignore friction. Is acceleration a = 0 or a (doesn't =) 0? If acceleration isn't equal to 0, in what direction will the horse accelerate? Are forces balanced or unbalanced? Calculate the weight of the horse. What would the Fn = ? Also, what's the Fnet?
Fnet = ma so, Fnet = 4900(500 x 9.8) x 4 then, Fnet = 19600 N a doesn't equal 0, it equals 4. The horse will accelerate to the right/forward Forces are unbalanced Fn = 500 x 9.8 = 4900 N
An object has a Fg of 12 N (down), Fn of 12 N (up), Ff of ? N (left), and Fapp of 30 N (right). Mu = .500 What's the Fnet? Whats the mass? What's the acceleration? What's the Ff?
Fnet: = +24 N Mass: Fg = mg so, 12 = m(9.8) then, 12/9.8 = 1.22kg Acceleration: A = Fnet/m so, 24/1.22 = +19.67 m/s^2 Ff: Ff = (mu) Fn so, Ff = .500 x 12 then, Ff = 6 N
An object has a Fg of 12 N (down), Fn of 12 N (up), Ff of 10 N (left), and Fapp of 10 N (right). What's the Fnet? What's the acceleration? What's mu?
Fnet: = 0 N Acceleration: A = Fnet/m so, 15/5.10 = 0 m/s^2 so forces are balanced mu: Ff = (mu) Fn so, 10 = (mu) x 12 then, (mu) = .83
What force pulls up?
Fnorm, Ft, and Fair
Newton's Second Law
Force = mass x acceleration
The force that opposes the forces of gravity (fgrav) in order to support the object is what?
Force Normal
Fair=
Force due to air
Fapp=
Force due to applied force
Ff=
Force due to friction
What is Fg?
Force due to gravity(when you have newtons,
Ft=
Force due to tension
Explain in words how this force diagram would be for this situation: a block hangs at rest from the ceiling by a piece of rope.
Ft is pointing up with a large arrow while Fg is pointing down with a smaller arrow.
An object has a Fg of 50 N (down), Fn of 50 N (up), Ff of 15 N (left), and Fapp of 30 N (right). What's the mass? What's the Fnet? What's the acceleration?
Mass: Fg = mg so 50 = m(9.8) then, m = 5.10kg Fnet: = +15 N Acceleration: A = Fnet/m so, 15/5.10 = +2.94 m/s^2
An object has a Fg of 50 N (down), Fn of 50 N (up), Ff of 10 N (left), and Fapp of 40 N (right). What's the mass? What's the Fnet? What's the acceleration?
Mass: Fg = mg so 50 = m(9.8) then, m = 5.10kg Fnet: = +30 N Acceleration: A = Fnet/m so, 30/5.10 = +5.88 m/s^2
An object has a Fg of 50 N (down), Fn of 50 N (up), and Ff of 20 N (left). What's the mass? What's the Fnet? What's the acceleration?
Mass: Fg = mg so 50 = m(9.8) then, m = 5.10kg Fnet: = -20 N Acceleration: A = Fnet/m so, -20/5.10 = -3.92 m/s^2
An object has a Fg of 50 N (down), Fn of 50 N (up), Ff of 50 N (left), and Fapp of 50 N (right). What's the mass? What's the Fnet? What's the acceleration?
Mass: Fg = mg so 50 = m(9.8) then, m = 5.10kg Fnet: = 0 N Acceleration: A = Fnet/m so, 0/5.10 = 0 m/s^2
Explain, in words, what a Free body diagram of an elevator that is suspended motionlessly, held up by two cables.
Motionlessly = balanced so, Fg is down and balanced with Ft and Fn which is up.
Is the following action one of newtons laws? If so, which one?: A marble will continue to roll in a straight line at a constant speed unless the force of friction causes it to stop, according to
Newton's first law
Is the following action one of newtons laws? If so, which one?: A penny at rest falls into the cup when the card it rests on is removed.
Newton's first law
Is the following action one of newtons laws? If so, which one?: The ball continuing to move when the runner stops.
Newton's first law
Is the following action one of newtons laws? If so, which one?: A 5 kg object having an acceleration of 10 m/s ^2 has a force of 50 N because of _____.
Newton's second law
Is the following action one of newtons laws? If so, which one?: A ball with more mass will have a greater force.
Newton's second law
Is the following action one of newtons laws? If so, which one?: When one fall in Domino bumps into another in that one bumps into the next, the chain reaction occurs because of ______.
Newton's third law
During a class discussion, Jimmy suggest that the object is moving to the left since the horizontal force acting upon it is a leftward force. Is he right? Explain.
No, he's slowing down because the left word forces friction
True or False: A more massive object has more inertia than a less massive object
True
True or False: All objects have inertia
True
True or False: If a skateboarder is traveling at a constant velocity, the resulting net force is 0, meaning no additional force is needed to be applied to keep the skateboarder in motion.
True
True or False: If all other variables are equal, then an object with a greater mass would have a more difficult time accelerating
True
True or False: Mass depends on how much stuff is present in an object
True
True or False: People in Weight Watcher's are really concerned about their mass (they're mass watchers).
True
True or False: The mass of an object is mathematically related to the weight of the object
True
True or False: The weight of an object is dépendant upon the value of the acceleration of gravity
True
True or False: The weight of an object would be less on the Moon than on the Earth
True
True or False: Two objects of the same mass can weigh differently on two different planets
True
True or False: Weight refers to a force experienced by an object
True
True or false: In order for an object to be moving, there must be unbalanced forces.
True
True or false: in order for an object to experience a change in motion, there must be an unbalanced force.
True
True or false? As you sit in your seat in the physics classroom, the earth pulls down on your body with a gravitational force; the reaction force is the chair pushing upwards on your body with an equal magnitude. Explain.
True, you don't fall through the chair. There is an equal amount of Fn pushing up and an equal amount of Fg pushing down. These are the interacting force pairs, they are equal in magnitude and opposite in direction, which keeps you sitting in your seat and not falling through.
What happens when you have more inertia?
You resist changes more
How do you find acceleration?
a=f/m
If the net force acting upon an object is 0 N, then the object must be __________.
at rest or at a constant speed
A numerical value given to a surface to show how much friction there is between that surface and another surface is called the ____________________________.
coefficient of friction
The resistance that one surface or object encounters when moving over another is called ____________________.
friction
Fnet=
m x a (mass x acceleration)
The amount of inertia possessed by an object is dependent solely upon its ________.
mass
The weight of an object is equal to the ___________ of an object times the acceleration due to gravity.
mass
Fg=
mg, where g is 9.8 m/s^2
Fn=
normal force
The type of friction that exists between 2 objects at rest is called ___________________ friction and the type of friction that exists between to objects in motion is called _________________ friction.
static, kinetic
Inertia can best be described as __________.
the tendency of any object to resist change and keep doing whatever it's doing
The force of gravity (fgrav) of an object is equal to the ______ of an object.
weight