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