Conceptual questions Flashcards
to understand the physics principles
an elevator suspended by a cable is descending at constant velocity. How many force vectors would be shown on a free-body diagram? Name them.
2 forces
tension and weight
the net force will be 0 on a free body diagram
since the elevator is descending with constant velocity , the forces must be balanced.
equal magnitude.
A compressed spring is pushing a block across a rough horizontal table. How many force vectors would be shown on a free-body diagram? Name them.
4 forces
normal force , weight, spring force and kinetic friction
A brick is falling from the roof of a three-story building. How many force vectors would be shown on a free-body diagram? Name them.
weight force
In Figure Q5.4, block B is falling and dragging block A across a table. How many force vectors would be shown on a free-body diagram of block A? Name them.
4 forces
weight, tension, kinetic friction and normal force
You toss a ball straight up in the air. Immediately after you let go of it, what force or forces are acting on the ball? For each force you name, (a) state whether it is a contact force or a long-range force and (b) identify the agent of the force.
gravitational force , its a long range force
maybe drag, its a contact force
Is the statement “An object always moves in the direction of the net force acting on it” true or false? Explain.
False.
This is because, if a object is already moving with constant velocity and the force is applied in the opposite direction. The object will first slow down, goes to 0 and then moves in the opposite direction
think of it as force = acceleration
Newton’s second law says F⃗ net=ma⃗ . So is ma⃗ a force? Explain.
no, ma is not a force.
if we go back to the definition of what is a force,
In otherwords, force is the CAUSE, and acceleration is the EFFECT. Newton’s second law says that the net force is the CAUSE of an object with mass to have an acceleration.
So mass multiplied by acceleration itself isn’t a force. It is a quantity that is caused to be that way, because of the net force acting upon the object.
Is it possible for the friction force on an object to be in the direction of motion? If so, give an example. If not, why not?
yes, it is possible.
check chegg for answers.
suppose you press your physics book against a wall hard enough to keep it from moving. Does the friction force on the book point (a) into the wall, (b) out of the wall, (c) up, (d) down, or (e) is there no friction force? Explain.
you have to draw this out.
once, you do, you realize that the answer is c
Static friction points opposite the direction in which the object would move if there were no friction. That is, it points in the direction necessary to prevent motion.
if there was no friction, it will fall down due to gravity
therefore, the arrow points up
Which, if either, of the basketballs in Figure Q5.16 are in equilibrium? Explain.
in equilibrium means that the net force is zero.
so, you will have to vector addition to check if fnet is zero or not
A car is parked on a steep hill. Identify the forces on the car.
there are three forces :
normal force, weight and static friction
A chandelier hangs from a chain in the middle of a dining room. Identify the forces on the chandelier.
2 forces :
Tension and weight
A baseball player is sliding into second base. Identify the forces on the baseball player.
3 forces :
kinetic friction, normal force and weight
A jet plane is speeding down the runway during takeoff. Air resistance is not negligible. Identify the forces on the jet
4 forces :
thrust, normal , weight and drag
An arrow has just been shot from a bow and is now traveling horizontally. Air resistance is not negligible. Identify the forces on the arrow.
2 forces :
drag and weight
there is no contact force
two rubber bands pulling on an object cause it to accelerate at 1.2 m/s2.
a) What will be the object’s acceleration if it is pulled by four rubber bands?
What will be the acceleration of two of these objects glued together if they are pulled by two rubber bands?
a)
remember that a = f
therefore, if acc is 1.2 force is also 1.2
if the force is doubled by (2 more bands) , than the acceleration is also doubled.
so, 2 x 1.2 = 2.4 m/s ^2
b) now the acceleration is 1.2 and force is also 1.2
and the mass is doubled
1.2 / 2 = 0.6 m/s ^2
Two rubber bands cause an object to accelerate with acceleration a. How many rubber bands are needed to cause an object with half the mass to accelerate three times as quickly?
remember that a = f
a1 = 1 f (a1) / m
a2 = 3F
shows acceleration-versus-force graphs for two objects pulled by rubber bands. What is the mass ratio m1/m2?
check figure
1) you need to calculate the mass of the objects
you can do this by calculating the slope of acc vs force graph
2) its asking for m1/m2
so, 5/3 divided by 4/5
5/3 * 5/4 = 25/12
need to double check this
Question 12 :
• Figure EX5.12 shows an acceleration-versus-force graph for a 200 g object. What force values go in the blanks on the horizontal scale?
answer is 1 N
and 2 N
Question 12 :
• Figure EX5.12 shows an acceleration-versus-force graph for a 200 g object. What force values go in the blanks on the horizontal scale?
answer is 1 N
and 2 N
question 13 :
shows an acceleration-versus-force graph for a 500 g object. What acceleration values go in the blanks on the vertical scale?
answer :
a) 4
b) 2
question 14 :
shows the acceleration of objects of different mass that experience the same force. What is the magnitude of the force?
the mass and the acceleration changes but the force remain the same
due to this relationship : f=ma
the magnitude at each different mass is 1.5 N
Question 15:
shows an object’s acceleration-versus-force graph. What is the object’s mass?
you can take the “mass” from the slope of the line
acc / f
= 2/50
flip this equation in order to have the following equation :
m= f/ a
= 50/2
= 25 kg
Exercises 23 through 27 describe a situation. For each, identify all forces acting on the object and draw a free-body diagram of the object.
23) • A cat is sitting on a window sill.
the forces on the free body diagram would be the normal force and the weight
both of these forces should be equal in magnitude
and thus fnet = 0
For each, identify all forces acting on the object and draw a free-body diagram of the object.
An ice hockey puck glides across frictionless ice.
there are only 2 forces : normal and weight
there is no friction due to frictionless ice
therefore, the normal and weight have both the same magnitude in opposite direction and therefore cancels each other out.
fnet = 0
normal force points upwards
gravity downward
Your physics textbook is sliding across the table.
there are 3 forces : normal, weight, and kinetic friction
the vertical component cancels out
the kinetic friction is the only force that oppose the force of motion
therefore, f net = point towards the left
A steel beam, suspended by a single cable, is being lowered by a crane at a steadily decreasing speed.
decreasing speed : meaning the velocity is changing
and has acceleration
f net = acceleration
therefore, we know that the net force should be pointing upwards , in order to decrease the speed
A jet plane is accelerating down the runway during takeoff. Friction is negligible, but air resistance is not.
airplane is accelerating therefore , the net force is not 0
there are 4 forces since friction is negligible
normal
weight
thrust
drag
vertical cancels out
thrust must be larger than drag in order to accelerate
f net = point to the right
