# Physics Exam 2

A. The net force on the object is zero
An object is moving forward with a constant velocity. Which statement about this object MUST be true?

A. The net force on the object is zero.

B. The acceleration of the object is in the forward direction.

C. The net force on the object is in the forward direction.

D. No forces are acting on the object.

B. less than the weight of the brick
A brick is resting on a rough incline. The friction force acting on the brick, along the incline, is

A. equal to the weight of the brick

B. less than the weight of the brick

C. greater than the weight of the brick

D. zero

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140 N
A 55-kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is 0.30. A horizontal 140-N force is applied to the box. What is the friction force on the box?
52 N pointing down
Two forces are acting on an object with magnitudes F1=78N and F2=26N. What third force will cause the object to be in equilibrium (acceleration equals zero)?
Ax = -3.1 m
Ay = 4.6 m
The magnitude of vector A is 5.5 m, lies in the second quadrant, and makes a 34° angle with the +y-axis. The components of vector A are closest to:
Exactly 1500 N whether or not the box moves
You swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is
9.5

Explanation:

<-6.1,-2.2>
<2.2,-6.9>
________________
<-8.3, 4.7>

sqrt(((-8.3)^2)+((4.7)^2)) = 9.5

The components of vector A are Ax = 2.2 and Ay = -6.9, and the components of vector B are given Bx = -6.1 and By = -2.2. What is the magnitude of the vector B-A?
the force of the car on the truck due to the collision is exactly equal to the force on the car.
A 20 T truck collides with a 1500-lb car. During the collision
Pcos(theta)
A push of magnitude P acts on a box of weight W. The push is directed at an angle theta below the horizontal, and the box remains at rest. The box rests on a horizontal surface that has some friction with the box. The friction force on the box is due to the floor is equal to
Less than 700 N
A person who normally weighs 700 N is riding in an elevator that is moving upward but slowing down at a steady rate. If this person is standing on a bathroom scale inside the elevator, what would the scale read?
displacement
The direction of the velocity vector is the
change of the velocity vector
The direction of the acceleration vector is the
Yes, if one is zero and one is non-zero
Can a vector have a component equal to zero and still have a nonzero magnitude? Explain.
No, to have zero magnitude, all components must be zero as well.
Can a vector have zero magnitude if one of its components is nonzero? Explain.
Projectile motion
Under the influence of gravity alone
An object @ rest, stays at rest unless a force acts against it
Newton’s 1st Law
The acceleration as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

a=F/m

Newton’s 2nd Law
always points in the direction of motion
Spring force…
always points vertically downward
Weight…
always points in the direction of the string/rope
Tension force…
always perpendicular to the surface
Normal force…
always points opposite to the direction of motion
Kinetic frictional force…
always points in the direction necessary to prevent motion
Static frictional force…
always points opposite to the direction of motion
Drag…
always points in the direction of motion
Thrust…
Every force occurs as one member of an action/reaction pair of forces, that point in opposite directions of each other.
Newton’s 3rd Law
An object moving in a straight line @ a constant speed (a=0)
Dynamic Equilibrium
an object is @ rest or moving @ a constant velocity.
a=0 when..
magnitude and direction
Two vectors are equal if they have the same…
Yf = Yi + (Vy)i(delta T)-1/2g(delta T)^2
How to determine the time interval?
a = v^2/r
How to find centripetal acceleration?
d. Gravity, a normal force, and kinetic friction
A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she’s jumped in?

a. Gravity and kinetic friction
b. Gravity and a normal force
c. Gravity and the force of the push
d. Gravity, a normal force, and kinetic friction
e. Gravity, a normal force, kinetic friction, and the force of the push

Tension and weight
A bungee jumper has leapt off a bridge and is nearing the bottom of her fall. What forces are being exerted on the bungee jumper?
Norma, tension, kinetic friction, and weight
A skier is being towed up a snow-covered hill by a tow rope. What forces are being exerted on the skier?
acceleration
An object pulled with a constant force moves with a constant
force
Acceleration is directly proportional to
mass
Acceleration is inversely proportional to an object’s
2 m/s2
A constant force causes an object to accelerate at 4 m/s2. What is the acceleration of an object with twice the mass that experiences the same force?
1 m/s2
An object, when pushed with a net force F, has an acceleration of 2 m/s2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be
The skier
A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater?
B. The rope tension equals the object’s weight.
An object on a rope is lowered at constant speed. Which is true?

A. The rope tension is greater than the object’s weight.

B. The rope tension equals the object’s weight.

C. The rope tension is less than the object’s weight.

D. The rope tension can’t be compared to the object’s weight.

A. The rope tension is greater than the object’s weight.
An object on a rope is lowered at a steadily decreasing speed. Which is true?

A. The rope tension is greater than the object’s weight.

B. The rope tension equals the object’s weight.

C. The rope tension is less than the object’s weight.

D. The rope tension can’t be compared to the object’s weight.

1 kg mass to accelerate at 1 m/s2
One Newton is the force that causes…
Weight more than tension
An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram?
Only weight
A ball has been tossed straight up. Which is the correct free-body diagram just after the ball has left the hand? Ignore air resistance.
Equal tension and weight
A ball, hanging from the ceiling by a string, is pulled back and released. Which is the correct free-body diagram just after its release?
Equal normal force and weight

Small static friction force in the -x direction.

A car is parked on a negative slope. Which is the correct free-body diagram?
Equal normal force, weight

Equal Tension and Kinetic friction

A car is towed to the right at
constant speed. Which is the
correct free-body diagram?
The block is not accelerating because the force of friction acts to oppose the pushing force, resulting in a net force of zero and no acceleration. The size of the pushing force must equal the size of the friction force if the block is moving at a steady speed.
Consider pushing a block across the table at a steady speed. Since you’re exerting a force on it, why isn’t it accelerating? Identify all the forces and draw a free-body diagram. Compare the size of the pushing force and the size of the friction force.
B. equal to the force of Sarah on Jack.
10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is

A. greater than the force of Sarah on Jack.

B. equal to the force of Sarah on Jack.

C. less than the force of Sarah on Jack.

C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito.
A mosquito runs head-on into a truck. Splat! Which is true during the collision?

A. The mosquito exerts more force on the truck than the truck exerts on the mosquito.

B. The truck exerts more force on the mosquito than the mosquito exerts on the truck.

C. The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

D. The truck exerts a force on the mosquito but the mosquito does not exert a force on the truck.

E. The mosquito exerts a force on the truck but the truck does not exert a force on the mosquito.

static equilibrium
An object @ rest is in
A. Greater than your true weight.
You are riding in an elevator that is accelerating upward. Suppose you stand on a scale. The reading on the scale is

A. Greater than your true weight.
B. Equal to your true weight.
C. Less than your true weight.

C. Greater than the coefficient of kinetic friction.
In general, the coefficient of static friction is

A. Smaller than the coefficient of kinetic friction.

B. Equal to the coefficient of kinetic friction.

C. Greater than the coefficient of kinetic friction.

A. The drag force increases.
The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force?

A. The drag force increases.

B. The drag force stays the same.

C. The drag force decreases.

B. 50N
Two boxes are suspended from a rope over a pulley. Each box has weight 50 N. What is the tension in the rope?

A. 25N
B. 50N
C. 100N
D. 200N

dynamic equilibrium
An object moving in a straight line at a constant speed (a=0) is in
The orangutan is at rest, so it is in static equilibrium. The net force on it must then be zero.

Neither force has an x-component, so we need to examine only the y-components of the forces. In this case, the y-component of Newton’s second law is

1. Sigma(Fy) = Ty + wy = may = 0

2. T – w = 0

3. T = w = 500 N

Tension in the rope equals the weight of the orangutan.

An orangutan weighing 500 N hangs from a vertical rope. What is the tension in the rope?
Change in velocity over change in time
ax = _____/_____
weight
Tension = __________ when net force = 0
T – mg
Fnet =
ma
T – mg =
T
mg + ma =
mg
w =
magnitude of supporting contact forces
Apparent weight
w + ma
n =
Find acceleration

Then use Vf = Vi + ay(Change in T)

To find stopping time…
Change in V/ Acceleration
Change in T =
m(g+a)
apparent weight =
Fs = T
A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation:
ukmg = F
How to determine how much force a person is exerting on an object?
only when the vertical acceleration is zero
Apparent weight equals your true weight w=mg…
The ball
A ball is thrown horizontally from the top of a tower at the same instant that a stone is dropped vertically. Which object is traveling faster when it hits the level ground below if neither of them experiences any air resistance?

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