# STANDARD 2a Students know a force has both direction and magnitude.

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STANDARD 2a Students know a force has both direction and magnitude

FORCES A force is a push or a pull
Force is a vector with a magnitude (strength) and a direction in which it is exerted Arrows are used to represent force SI unit for the magnitude of a force is the newton (N) A spring scale is used to measure the amount of force on an object

FORCES 104. What can be concluded about these forces?
A They are the same because they point toward the objects B They are the same because they have the same magnitude C They are different because they have different magnitudes D They are different because they have different directions ANSWER: D

STANDARD 2b Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces

FORCES The combination of all the forces acting on an object is called the net force When the combined forces act in the same direction, add all the forces to find the net force When forces act together in opposite directions, subtract the forces to find the net force

FORCES 107. Robin and Wes push on opposite sides of a wheeled chair. Robin pushes with a force of 80 N to the left. Wes pushes with a force of 100 N to the right. What is the resulting force on the chair? A 20 N to the left B 20 N to the right C 180 N to the left D 180 N to the right ANSWER: B

FORCES Robin and Wes are now pushing the wheeled chair in the same direction. Robin pushes with a force of 80 N to the left and Wes pulls with a force of 120 N to the left. What is the resulting force on the chair? A 40 N to the left B 40 N to the right C 200 N to the left D 200 N to the right ANSWER: C

STANDARD 2c Students know when the forces on an object are balanced, the motion of the object does not change

FORCES Equal forces acting on one object in opposite directions are called balanced forces For an object that is not moving, the result of balanced forces is no motion For an object that is moving, the result of balanced forces is constant velocity

FORCES and MOTION FIRST LAW OF MOTION
An object will continue in motion or remain at rest when all forces acting on it are equal in magnitude and direction. This is also called inertia, the tendency of an object to resist change in motion

FORCES 108. Two students on opposite sides of a box apply forces to the box. What must be true if the box does not accelerate? A The forces are balanced B The forces are unbalanced and in the same direction C The forces are unbalanced and opposite, with the greater force to the left D The forces are unbalanced and opposite, with the greater force to the right ANSWER: A

FORCES 110. A person falls from a plane and reaches terminal velocity. At this point the downward force of gravity on the person is equal to the upward force of air resistance on that person. As the person falls he will A slow down B speed up C fall at a constant speed D None of the above ANSWER: C

STANDARD 2d Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction

FORCES Types of Forces Gravity – an attractive force that objects exert on each other. The force depends on the masses and distances of the objects. More mass, more gravity; greater distance, less gravity

FORCES Types of Forces Elastic – the force in an object that resists tension (stretching) or compression (pressing). Examples: rubber band or a spring

FORCES Types of Forces Friction – a force that resists the motion between two surfaces in contact. Examples: static, sliding, rolling, fluid (air or water resistance)

FORCES 112. The diagram shows a book sitting on a table. The force of the table pushing up on the book is called A gravity B tension C compression D static friction ANSWER: C

FORCES 114. Suppose the wheels on the cart in the diagram are locked in place. If the force applied by the girl is balanced, so that the cart does not move, what is the cause of the balanced force? A static friction B sliding friction C rolling friction D fluid friction ANSWER: A

FORCES 115. Emma wants to determine the strength of the force of gravity between two objects. What should she measure? A the mass of each object and the distance between them B the size and weight of each object C the electrical charge of each object D the weight of each object and the distance between them ANSWER: A

FORCES 116. A yo-yo hangs from a string and does not move. What forces act on the yo-yo? A gravity only B tension only C gravity and tension D no forces act on the yo-yo ANSWER: C

STANDARD 2e Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction)

FORCES SECOND LAW OF MOTION
When the forces on an object are unbalanced (not equal), the object will change its velocity, that is, it will speed up, slow down, or change direction.

STANDARD 2f Students know the greater the mass of an object, the more force is needed to achieve the same rate of change in motion

FORCES SECOND LAW OF MOTION Force = mass x acceleration
Acceleration = Force / Mass The greater the mass of an object the more force is needed to achieve the same rate of acceleration

FORCES and MOTION THIRD LAW OF MOTION
For every action force there is an equal and opposite reaction force Identify the action/reaction pairs?

FORCES Centripetal force is any force that causes an object to move in a circular path Bernoulli’s Principle states that the faster a fluid moves, the less pressure the fluid exerts. An airplane wing is designed so that the air above the wing moves faster than the air below the wing. The air pressure below the wing becomes greater than the air pressure above the wing. This creates unbalanced forces above and below the wing and helps produce lift, and upward force

FORCES and MOTION What forces are acting on this object? gravity & air resistance

FORCES and MOTION 117. What conclusion can you draw by looking at the diagram? A Air resistance in front of the balloon pushes it backwards B Gravity forces air out of the balloon’s open end C The force of the air leaving the balloon causes it to accelerate forward D Friction causes the balloon’s acceleration to decrease ANSWER: C

FORCES 119. What force keeps the moon in orbit around the Earth?
A inertia B gravitational force C net force D centripetal force ANSWER: B

FORCES 120. If the net force on a chair is 6 N to the right, what will occur? A The chair will accelerate to the right B The chair will accelerate to the left C The chair will have no velocity D The chair will not move ANSWER: A

FORCES 122. The two skaters push against each other’s hands. Both skaters move backward. What do you know about the forces acting on the skaters? A Skater X applied greater force than Skater Y B Skater Y applied greater force than Skater X C The forces on the skaters were balanced D The forces on the skaters were unbalanced ANSWER: D

FORCES 123. If an equal force is applied to four different objects, which will accelerate the most? A the one with the least acceleration B the one with the least density C the one with the least mass D the one with the least velocity ANSWER: C

FORCES 124. Four objects roll across a smooth floor at the same speed. The objects have different masses and diameters. Which object will require the greatest force to increase its speed by 1 m/s? A the object with the greatest diameter B the object with the least diameter C the object with the greatest mass D the object with the least mass ANSWER: C

STANDARD 2g Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system

FORCES The force of gravity caused matter to come together to form the first stars. Gravity caused stars to gather together to form galaxies Gravity pulled clouds of gas and dust together to form the sun and the planets Gravity is the force that keeps the planets and other bodies in the solar system in orbit around the sun The sun is stable because the forces within it are balanced. Pressure caused by nuclear fusion in the sun’s center pushes the sun’s matter outward, while gravity provides an equal and opposite force pulling the sun’s matter inward

FORCES 126. The sun was formed when
A the big bang blew up the solar system B gravity pulled matter into the center of a spinning cloud of gas and dust C fluid friction caused several planets to come together D two stars collided in the center of a cloud of gas and dust ANSWER: B

STANDARD 8a Students know density is mass per unit volume

Density Density - the amount of mass in a given volume Low Density
Density Units g/cm3 for solids g/mL for liquids 1cm3 = 1mL Low Density High Density

DENSITY A is half as much B stays the same C is two times greater
127. When a sample of liquid is divided in half, its density A is half as much B stays the same C is two times greater D is four times greater ANSWER: B

DENSITY A the force of gravity B the buoyancy of an object
128. Density is used to determine all of the following except A the force of gravity B the buoyancy of an object C the purity of a substance D the identity of an unknown substance ANSWER: A

DENSITY 129. Which unit is used to express the density of a substance?
A m/kg3 B g/cm3 C mL/cm3 D kg/cm ANSWER: B

STANDARD 8b Students know how to calculate the density of substances (regular and irregular solids and liquids) from the measurements of mass and volume

DENSITY Mass of a substance is measured on a balance
Volume of a liquid is measured with a graduated cylinder Volume of a regular solid = length x width x height Volume of an irregular solid is measured using water displacement method

DENSITY 130. A scientist has a beaker with 150 mL of gasoline. The mass of the gasoline is 108 g. What is the gasoline’s density? A 0.72 g/mL B 1.1 g/mL C 1.4 g/mL D 1.5 g/mL ANSWER: A

DENSITY 131. Pure copper has a density of 8.9 g/cm3. What is the mass of a 10-cm cube of copper? A 8.9 g B 89 g C 890 g D 8,900 g ANSWER: B

DENSITY 134. The rock in the diagram has a mass of 58.8 g. What is the rock’s density? A 1.3 g/mL B 2.0 g/mL C 2.6 g/mL D 2.8 g/mL ANSWER: D

DENSITY 135. Which of the following substances is a gas?
Substance Density (g/cm3) A .15 B .04 C D 2.1 ANSWER: C

STANDARD 8c Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced

BUOYANCY Buoyancy – upward force a fluid (liquid or gas) exerts on any object in the fluid

Buoyancy The buoyant force on an object is equal to the weight of the volume of fluid displaced by the object Buoyant force exists because there is more pressure on bottom of object than on top (pressure increases as depth increases)

BUOYANCY 136. A beach ball pops out of water because the buoyant force is A greater than the ball’s weight B equal to the ball’s weight C less than the ball’s weight D greater than the ball’s buoyant force ANSWER: A

BUOYANCY 137. Why does a ship float, when a solid block of steel of equal weight sinks? A the ship has less buoyant force B the steel block has more buoyant force C the ship displaces more water D the steel block displaces more water ANSWER: C

BUOYANCY 138. An air-filled balloon falls to the ground because
A air cannot hold up objects B it weighs more than the weight of the air it displaces C the buoyant force of the balloon is greater than the buoyant force of air D the balloon has less surface area than air ANSWER: B

BUOYANCY 139. The buoyant force in container A is
A greater than the buoyant force in container B B the same as the buoyant force in container B C less than the buoyant force in container B D zero ANSWER: A

BUOYANCY 140. In each container, the buoyant force equals
A the density of the object B the force of gravity on the object C the mass of the object D the weight of the displaced fluid ANSWER: D

STANDARD 8d Students know how to predict whether an object will float or sink

BUOYANCY An object will float if its weight is less than the buoyant force acting on it An object will sink if its weight is greater than the buoyant force acting on it

BUOYANCY An object will float if its density is less than the density of the fluid An object will sink if its density is greater than the density of the fluid

BUOYANCY 141. Will this object float in water?
A No, because it is more dense than water B No, because it is less dense than water C Yes, because it is more dense than water D Yes, because it is less dense than water ANSWER: D

BUOYANCY 142. Which item(s) in the table sink in gasoline?
A maple wood B graphite and seawater C paper, graphite, and seawater D paper, graphite, seawater, and maple wood ANSWER: C

BUOYANCY 144. A canoe floats at the edge of a pond. If the volume of pond water displaced by the canoe weighs 90 N, what is the buoyant force of the water on the canoe? A 45 N B 90 N C 135 N D 180 N ANSWER: B

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