Mechanical Comprehension on SIFT Exam

What are the types of lever?

1. First-class... MA is dependent on EFFORT and LOAD 2. Second-class... MA > 1 (E moves farther than L; multiplies force) 3. Third-class... MA < 1 (multiplies distance and speed of effort)

What are the four fundamental forces that form the basis for all other forces?

1. Gravitational 2. Weak 3. Electromagnetic 4. Nuclear

What are the types of friction?

1. Static 2. Kinetic

Convert 1 kg to N

9.81

A simple machine always has A. <100% efficiency B. 100% efficiency C. 200% efficiency D. 50% efficiency

A

A simple machine has a mechanical advantage of 6. This means that A. Output force is 600% of input B. Output is unknown C. Effort is quadruple input D. Effort is six times greater than output

A

Because a crowbar has a fulcrum in the middle of the effort and the resistance, it is an example of a A. first-class lever. B. second-class lever. C. third-class lever. D. first- and second-class lever. E. second- and third-class lever.

A

If Gear A (larger; 30) makes 15 revolutions, Gear B will make A. 30 revolutions. B. 25 revolutions. C. 22 revolutions. D. 18 revolutions. E. 15 revolutions.

A

Scalars have... A. Only magnitude B. Only direction C. Both magnitude and direction D. Either magnitude or direction

A A scalar or scalar quantity in physics is a physical quantity that only has magnitude.

An 80-pound object is placed on a scale inside an elevator that begins to travel upward. The scale will read that A. the weight of the object is still 80 pounds. B. the weight of the object is greater than 80 pounds. C. the weight of the object is less than 80 pounds. D. the mass of the object is greater than 80 pounds. E. the mass of the object is less than 80 pounds.

A A scale gives the weight of an object, not its mass. The scale reading is the same as the normal force that the object experiences. The object is moving upward, so the normal force has to be greater than the downward force of the weight. Thus, the scale will read a weight that is greater than 80 pounds.

A ball attached to a string is being twirled in a circle. If the string is cut at the top, in what direction will the ball move? A. The ball will move left. B. The ball will move right. C. The ball will move up. D. The ball will move down. E. The ball will continue moving in a circle.

A The tension in the string keeps the ball rotating in a circle. When this force is removed, the ball will continue in the direction it was moving at that moment. The ball will move to the left.

System

A collection of particles or objects that is isolated from its surroundings. All forces within a system are called internal forces, and forces outside the system are called external forces.

Free-body diagram

A diagram showing all the forces acting on an object Because all forces are vectors, it is important to consider the direction of the force when drawing a diagram

Inclined plane

A flat surface set at an angle or an incline with no moving parts that is able to lift objects by pushing or pulling the load. Reduces the amount of force needed to raise a mass to a certain height MA = length/height

Applied force

A force which is applied to an object by a person or another object (Fa)

Mass

A measure of how much matter is in an object A measure of inertia of an object... An object with a larger mass will resist a change more.

Force

A push or a pull

Wheel and axle

A simple machine consisting of two circular objects of different sizes; the wheel is the larger of the two circular objects MA = (r input) / (r output) r is the radius

Lever

A simple machine that consists of a bar that pivots at a fixed point called a fulcrum; based on the concept of torque MA = (r input) / (r output).... load/effort... effort arm/load arm

Pulley

A simple machine that consists of a rope that fits into a groove in a wheel MA of one pulley is 1.... MA = number of pulleys

Screw

A simple machine that is an inclined plane wrapped around a central cylinder to form a spiral. MA = [2 PIE r] / h

How does the directional movement of gears look like?

Adjacent gear move in opposite directions

Inertia

An object's tendency to resist a change in motion

Blocks A, B, and C are identical, but A and B are on the ramp. If the same force is applied to Blocks A, B, and C, which of the following will be true? A. The acceleration of A is greater than B, which is greater than C. B. The acceleration of B is greater than A but less than C. C. The acceleration of B is the same as C and less than A. D. The acceleration of A is the same as B, which is greater than C. E. All the blocks will experience the same acceleration.

D Blocks A and B will experience the same acceleration while they are on the incline. Both blocks will have a greater acceleration than C because gravity is increasing the net force moving the blocks down the incline.

Water flows out of a water tower at a rate of 3 gallons per minute and flows in at a rate of 140 gallons per hour. After one hour, the volume of water in the tank will be A. the same. B. 20 gallons more. C. 20 gallons less. D. 40 gallons less. E. 40 gallons more.

D Water flows into the tower at a rate of 140 gallons per hour and out at a rate of 3 gallons per minute, or 180 gallons per hour. The tower is losing 180 - 140 = 40 gallons of water per hour.

A ramp is an example of: A. a lever. B. a pulley. C. a wheel and axle. D. a screw. E. an inclined plane.

E

If a shaft is spun in the opposite direction but maintains the same speed, balls A and B will A. move down. B. move outward and downward. C. move up. D. move inward and upward. E. stay at the same level.

E The height of the balls will change when the speed of the shaft changes (a higher speed will lift the balls upward). Since the speed has not changed, the balls will stay at the same level.

Newton's second law

F=ma Force equals mass times acceleration

Newton's third law

For every action there is an equal and opposite reaction

Work Done = ?

Force * Displacement

Non-fundamental forces

Forces that can be derived from the four fundamental forces. Include weight, tension, friction, the normal force, and the buoyant force.

Kinetic friction

Friction between moving surfaces increases with weight

Static friction

Friction that acts on objects that are not moving The amount of force that needs to be overcome for an object to move.

Nuclear (or strong) force

Holds together the nucleus in an atom. The nuclear force has a larger magnitude than the electromagnetic force that pushes protons (positive charges) away from each other in the nucleus.

Gear ratio

Speed ratio; the ratio of angular velocity of the input gear to the angular velocity of the output gear At the point of contact, the linear velocity must be the same [W input / W output] = [r output / r input] = [N output / N input]

What is the acceleration due to gravity?

g = 9.8 m/s^2 Always points to the center of the earth

A brake pedal with the load between the fulcrum and the effort is an example of a A. first-class lever. B. second-class lever. C. third-class lever. D. first- and second-class lever. E. first- and third-class lever.

B

Which of these statements is true about a car's transmission? A. Higher gears have a higher mechanical advantage. B. Lower gears have a higher mechanical advantage. C. All gears have the same mechanical advantage. D. The mechanical advantage depends on the weight of the car.

B

Pulley C > Pulley B > Pulley A. Pulley C and A are the same size and larger than B. Pulley A, will rotate: A. faster than both Pulley B and Pulley C. B. slower than both Pulley B and Pulley C. C. slower than Pulley B but with the same speed as Pulley C. D. faster than Pulley B but with the same speed as Pulley C. E. faster than Pulley C but with the same speed as Pulley B.

B The closer the string is to the center of the pulley, the faster the pulley rotates. Pulleys A and C are the same size, but the string is on the outer edge of Pulley A, so it will rotate slower than both C and B.

A 140-pound woman jumps off a 700-pound stationary raft that is in a river and travels a distance of 10 feet to right. If the water in the river is at rest, the raft will move A. 2 feet to the right. B. 2 feet to the left. C. 4 feet to the left. D. 4 feet to the right. E. 8 feet to the left.

B The overall momentum of the system must be conserved. The product of force and distance for the woman will be equal to the raft but in the opposite direction (left). 140(10) = 700(d) d= 1400/700 = .2 ft to the left

Increasing the slant of an inclined plane increases its A. Power B. Output C. Mechanical Advantage D. None of the Above

C

Three cubes of equal volume are put in a hot oven. The cubes are made of three materials: wood, iron, and silver. Which cube will heat the fastest? A. wooden cube B. iron cube C. silver cube D. cubes all heat up at the same rate

C

A steel ball with a temperature of 150°C is dropped into a liquid with a temperature of 120°C. Which of the following statements about the equilibrium temperature has to be true? A. The equilibrium temperature is less than 120°C. B. The equilibrium temperature is exactly 135°C. C. The equilibrium temperature is between 120°C and 150°C. D. The equilibrium temperature is greater than 150°C. E. The equilibrium temperature is exactly 270°C.

C Heat will transfer from the steel ball to the liquid. The equilibrium temperature depends on the masses of the objects and the specific heat of each object. Without knowing these values, the only guarantee is that the temperatures of the objects will be somewhere between 120°C and 150°C.

A rubber ball, a metal ball, and a wooden ball are placed in the same pot of boiling water. Which ball will have the highest temperature? A. The wooden ball will have the highest temperature. B. The rubber ball will have the highest temperature. C. The metal ball will have the highest temperature. D. The wooden and rubber balls will have the highest temperature. E. All three balls will have the same temperature.

C The material that transfers heat the best will have the highest temperature. Wood and rubber are insulators, and metal is a conductor; therefore, the metal will have the highest temperature.

An object in equilibrium

It is either at rest or moving at a constant speed. No acceleration... a=0

Newton's first law

Law of Inertia 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.

What is the unit of force?

Newton

Normal force

Occurs when an object is resting on another object (Fn or N) Always equal and opposite to the force pushing onto the surface

Wedge

Often used to separate material (e.g., knife) MA = L/W L is the length of the inclined plane on wedge W is the width (length of the back edge)

On the bicycle shown in the figure, there are 12 teeth on the rear sprocket (smaller gear) and 24 teeth on the front sprocket (larger gear). Each time the rear wheel goes around twice, the pedals go around?

Once 12(2)=24(r) r=24/24... 1

What characteristic are needed for gears to mesh well?

Opposite orientation and similar in size

Tension

Pressure; stress that occurs when forces act to stretch an object (Ft or T)

Mechanical advantage

Ratio of the output force exerted by a machine to the input force applied to the machine. MA= (F output) / (F input)

When shooting a basketball, at what point(s) will the basketball move the fastest?

Release point and entrance into hoop An object traveling in an arc has a velocity of zero at the top of its path and is moving slower the closer it is to that point. The object will also have the same speed at both points on the path that have the same height. So, the ball will be moving the fastest at points A and E, since both have the same height and are farthest from the top of the arc.

Electromagnetic force

The force between electric charges. Repulsive when the charges are the same sign and attractive when the charges are opposite. Holds the positive nuclei and negative electrons of atoms together.

Gravitational force

The force that pulls mass together

Weight

The gravitational force exerted on an object (Fw) W=mg

Friction

The resistance that one surface or object encounters when moving over another (Ff)

Buoyant force

The upward (vertical) force exerted by a fluid on a submerged object Magnitude of force is calculated by considering the volume of the object that is submerged in fluid

Gears

Wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed MA = [T output / T input] = [N output / N input] T is the torque from each gear N is the number of teeth on each gear

Equilibrium condition

When a=0; therefore, F=0