Physics Exam 2

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?

Tension and weight

Newton's 2nd 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

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?

The ball

One Newton is the force that causes...

1 kg mass to accelerate at 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

1 m/s2

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?

140 N

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?

2 m/s2

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)?

52 N pointing down

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?

9.5 Explanation: <-6.1,-2.2> <2.2,-6.9> ________________ <-8.3, 4.7> sqrt(((-8.3)^2)+((4.7)^2)) = 9.5

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.

A. Greater than your true weight.

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.

A. The drag force increases.

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.

A. The net force on the object is zero

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.

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

Newton's 1st Law

An object @ rest, stays at rest unless a force acts against it

Dynamic Equilibrium

An object moving in a straight line @ a constant speed (a=0)

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:

Ax = -3.1 m Ay = 4.6 m

An elevator, lifted by a cable, is moving upward and slowing. Which is the correct free-body diagram?

Weight more than tension

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

B. 50N

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.

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

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.

B. equal to the force of Sarah on Jack.

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

B. less than the weight of the brick

Can a vector have a component equal to zero and still have a nonzero magnitude? Explain.

Yes, if one is zero and one is non-zero

How to determine the time interval?

Yf = Yi + (Vy)i(delta T)-1/2g(delta T)^2

How to find centripetal acceleration?

a = v^2/r

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.

C. Greater than the coefficient of kinetic friction.

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.

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

Change in T =

Change in V/ Acceleration

ax = _____/_____

Change in velocity over change in time

A car is parked on a negative slope. Which is the correct free-body diagram?

Equal normal force and weight Small static friction force in the -x direction.

A car is towed to the right at constant speed. Which is the correct free-body diagram?

Equal normal force, weight Equal Tension and Kinetic friction

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 tension and weight

Newton's 3rd Law

Every force occurs as one member of an action/reaction pair of forces, that point in opposite directions of each other.

You swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is

Exactly 1500 N whether or not the box moves

To find stopping time...

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

A box on a rough surface is pulled by a horizontal rope with tension T. The box is not moving. In this situation:

Fs = T

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?

Less than 700 N

Can a vector have zero magnitude if one of its components is nonzero? Explain.

No, to have zero magnitude, all components must be zero as well.

A skier is being towed up a snow-covered hill by a tow rope. What forces are being exerted on the skier?

Norma, tension, kinetic friction, and 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.

Only weight

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

Pcos(theta)

Under the influence of gravity alone

Projectile motion

mg + ma =

T

Fnet =

T - mg

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.

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.

An orangutan weighing 500 N hangs from a vertical rope. What is the tension in the rope?

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.

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?

The skier

An object pulled with a constant force moves with a constant

acceleration

Normal force...

always perpendicular to the surface

Static frictional force...

always points in the direction necessary to prevent motion

Spring force...

always points in the direction of motion

Thrust...

always points in the direction of motion

Tension force...

always points in the direction of the string/rope

Drag...

always points opposite to the direction of motion

Kinetic frictional force...

always points opposite to the direction of motion

Weight...

always points vertically downward

a=0 when..

an object is @ rest or moving @ a constant velocity.

The direction of the acceleration vector is the

change of the velocity vector

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

d. Gravity, a normal force, and kinetic friction

The direction of the velocity vector is the

displacement

An object moving in a straight line at a constant speed (a=0) is in

dynamic equilibrium

Acceleration is directly proportional to

force

apparent weight =

m(g+a)

T - mg =

ma

Two vectors are equal if they have the same...

magnitude and direction

Apparent weight

magnitude of supporting contact forces

Acceleration is inversely proportional to an object's

mass

w =

mg

Apparent weight equals your true weight w=mg...

only when the vertical acceleration is zero

Tension = __________ when net force = 0

weight

An object @ rest is in

static equilibrium

A 20 T truck collides with a 1500-lb car. During the collision

the force of the car on the truck due to the collision is exactly equal to the force on the car.

How to determine how much force a person is exerting on an object?

ukmg = F

n =

w + ma