Exam 1 Flashcards
Force, Velocity (20 cards)
Suppose that a car traveling to the East (+x direction) begins to slow down as it approaches a traffic light. Make a statement concerning its acceleration
- The acceleration is zero
- The car is decelerating, and its acceleration is positive
- The car is decelerating and its acceleration is negative
- A statement cannot be made with the information given
The car is decelerating and its acceleration is negative
**If it is traveling in the positive x direction, and it slows down its acceleration will be negative because opposite the direction the car is traveling.
Can an object have increasing speed while its acceleration is decreasing?
- Yes, and an example would be an object released from rest in the presence of air friction.
- No, because if acceleration is decreasing the object will be slowing down
- No, this is impossible because of the way in which acceleration is defined
- Yes, and an example would be an object falling in the absence of air friction.
Yes, and an example would be an object released from rest in the presence of air friction.
** An object falling experiences a drag force, or air friction. It is proportional to [velocity]^2 of the object. As the object falls, it experiences a constant force due to gravity acting downward, and a velocity-dependent drag force acting upward. As the object’s speed increases, the drag force becomes larger, and so the total acceleration acting on the object decreases as its speed increases.
Objects A and B both start from rest. They both accelerate at same rate. However, object A accelerates for twice the time as object B. What is the final speed of object A compared to that of object B?
- Twice as fast
- Four times as fast
- Three times as fast
- The same speed
Twice as fast
**Using the equation v=v0+a*t, you can see that as the time period increases, the value of the velocity increases as well.
Ball A is dropped from the top of a building. One second later, ball B is dropped from the same building. As time progresses, their difference in speeds:
- Increases
- Remains constant
- Decreases
- Cannot be determined
remains constant
**Using the equation v=v0+a*t, you can see that the time will affect the velocity, and the speeds of the two objects will remain what they are from the start.
If the velocity versus time graph of an object is a straight line making an angle of 30 degrees with the time axis, the object is
- Moving with infinite speed
- Moving with constant non-zero speed
- Moving with constant non-zero acceleration
- At rest
Moving with constant non-zero acceleration
** By stating the object is making a straight line, this shows constant velocity which is equal to non-zero acceleration
An object starts from rest and undergoes uniform acceleration. During the first second it travels 5m. How far will it travel during the third second?
- 5m
- 45m
- 15m
- 25m
25m
** a = 2x/t² = 2*5/1² = 10 m/s²
D = ½at² = ½10*(3² - 2²) = 25 m
Where “during the third second” means between 2 and 3 seconds
A soccer ball is kicked with a velocity of 25m/s at an angle of 45deg above the horizontal. What is the vertical component of its acceleration as it travels along its trajectory.
- (9.8m/s^2)*sin(45deg)Upward
- (9.8m/s^2)upward
- (9.8m/s^2)*sin(45deg)Downward
- (9.8m/s^2)downward
(9.8m/s^2)downward
** Acceleration due to gravity will be pushing DOWN on the ball along its trajectory
An Olympic athlete throws a javelin at four different angles above the horizontal, each with the same speed: 30deg, 40deg, 60deg, and 80deg. Which two throws cause the javelin to land the same distance away?
- 30 and 60
- 30 and 80
- 40 and 80
- 40 and 60
30 and 60
**since sin 60º = cos 30º and vice versa, either 60º or 30º gives the same result.
A stone is thrown horizontally from the top of a tower at the same instant a ball is dropped vertically. Which object is traveling faster when its hits the level ground below?
- It is impossible to tell
- The stone
- the ball
- Neither, since both are traveling at same speed
Neither, since both are traveling at same speed
**Horizontal motion has no impact on vertical motion. if no air resistance there is equal acceleration, remember a bowling ball falls as fast as a feather.
Both accelerate at equal speeds
A pilot drops a bomb from a plane flying horizontally at a constant speed. Neglecting air resistance, when the bomb hits the ground the horizontal location of the place will
- be over the bomb
- depend on the speed of the plane when the bomb is released
- be in front of the bomb
- be behind the bomb
The plane should be directly over the package when it finally reaches the ground.
**Since the plane is flying at a constant speed, the X component of velocity is the same throughout the flight. If the package is dropped from the plane, the X component of velocity will be the same for the package as it falls (since air resistance is neglected) so the only difference is the height.
You are trying to cross a river that flows due south with a strong current. You start out in your motorboat on the east bank desiring to reach the west bank directly west from your starting point. You should head your motorboat
- In a southwesterly direction
- In a northwesterly direction
- Due west
- Due North
-In a northwesterly direction
**You should point your boat in the opposite direction of the flow, as well as towards the west (to where you want to end up), in order to neglect the effects of the strong flow.
A projectile is launched with an initial velocity of 60m/s at an angle of 30deg above the horizontal. What is the maximum height reached by the projectile?
- 46m
- 23m
- 92m
- 69m
46m
**find the initial vertical velocity: v0 sin(theta) = 60 sin 30 = 30m/s at max height, the vertical velocity is zero, so use vf^2=v0^2 + 2ad , which gives you vf=final vel =0 v0=initial vertical velocity = 30m/s a=g=-9.8m/s/s d= max ht 0=30^2 -2*9.8*d => d= 45.9 m ~46m
You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. What force caused you to fall forward?
- Normal force due to your contact with the floor of the bus
- Gravity
- Force due to friction between you and the floor of the bus
- There is not a force leading to your fall
There is not a force leading to your fall
**The only force on you is through your feet on the floor of the bus (and your hand if you were smart enough to hold onto a bar). Since the bus is slowing, those are forces are backwards. And, so, your acceleration is backwards. But you already had speed and momentum forwards. Your motion forwards, relative to the bus, is because you are not accelerating (backwards) quite as fast as the bus.
A 20 ton truck collides with a 1500lb car and causes a lot of damage to the car. Since a lot of damage is done on the car
- The force on the truck is greater than the force on the car
- The force on the truck is equal to the force on the car
- The truck did not slow down during the collision
- The force on the truck is smaller than the force on the car
The force on the truck is equal to the force on the car
** Due to Newtons third law, there is an equal action to every reaction force, therefore, the truck force exerted on the truck is equal to that of the force exerted on the car.
The acceleration due to gravity is lower on the Moon than on the Earth. Which of the following is true about the mass and weight of an astronaut on the Moon’s surface, compared to the Earth?
- Both mass and weight are less
- Mass is the same, weight is less
- Both mass and weight are the same
- Mass is less, weight is the same
Mass is the same, weight is less
**Weight is the mass x g, if it is lower on the moon, then the weight on the moon will be lower than the weight on Earth, however, the mass will not change
A 20N weight and a 5N weight are dropped simultaneously from the same height. Ignore air resistance. Compare their accelerations
- The 5N weight accelerates faster because it has a smaller mass
- They both accelerate at the same rate because they have the same weight to mass ratio
- The 20N weight accelerates faster because it has more inertia
- The 20N weight accelerates faster because it is heavier
They both accelerate at the same rate because they have the same weight to mass ratio
**When object is dropped at some height, the acceleration due to gravity ( g = 9.8m/s^2) are same regardless its mass.
A packing crate slides down an inclined ramp at constant velocity. Thus we can deduce that
- it may be accelerating
- it is not acted on by appreciable gravitational force
- a frictional force is acting on it
- a net downward force is acting on it
a frictional force is acting on it
**It cannot be accelerating because the velocity is constant, however, there is a frictional force that acts on the box to try and stop it from moving.
Its more difficult to start moving a heavy carton from rest than it is to keep pushing it with constant velocity, because
-uk
uk
A person of weight 480N stands on a scale in an elevator. What will the scale be reading when the elevator is accelerating downward at 4m/s^2?
- 196N
- 676N
- 284N
- 480N
676N
when an elevator is accelerating, you add its acceleration value to the gravity acceleration
(9.8+4)=13.8
Finding the mass using 480=9.8m, you get 13.8m which is 676N.
The coefficient of static and kinetic frictions between a 3kg box and a desk are 0.4 and 0.3, respectively. What is the net force on the box when a 15N horizontal force is applied to the box?
- zero
- 6.2N
- 8.8N
- 12N
8.8N
**Using Newtons Law, you find that Fk=normal force*uk, since the box will be moving
using the mass given you find the weight of the box to be 29.4, and multiplying this by .3 (uk) gives you 8.82N.
