# Motion & Forces - Chapter 3

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• 1.

### The force that attracts two masses to each other is called

• A.

Air resistance

• B.

Friction

• C.

Gravity

• D.

Pressure

C. Gravity
Explanation
Gravity is the force that attracts two masses to each other. It is a fundamental force in nature that acts between all objects with mass. Unlike air resistance, friction, and pressure, which are forces that arise from interactions with specific substances or surfaces, gravity is a universal force that affects all objects in the universe.

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• 2.

### A hollow plastic ball and a solid metal ball are dropped from the same height in a vacuum.  With no air resistance, gravity is the only force acting on the balls.  What do you predict will happen?

• A.

The balls will fall at the same rate

• B.

The metal ball will fall faster

• C.

The plastic ball will fall faster

• D.

The larger ball will fall faster

A. The balls will fall at the same rate
Explanation
In a vacuum, there is no air resistance to affect the motion of the balls. This means that the only force acting on both the hollow plastic ball and the solid metal ball is gravity. According to the law of universal gravitation, all objects experience the same acceleration due to gravity regardless of their mass or size. Therefore, both the hollow plastic ball and the solid metal ball will fall at the same rate.

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• 3.

### After a space shuttle is launched into orbit, gravity

• A.

Gives it a speed greater than 11,000 m/s

• B.

No longer affects the astronauts on board

• C.

Makes it impossible for it to stay in orbit for a long period of time

• D.

Continues to pull it toward the center of Earth

D. Continues to pull it toward the center of Earth
Explanation
After a space shuttle is launched into orbit, gravity continues to pull it toward the center of Earth. This is because gravity is a force that attracts objects with mass towards each other. Even though the shuttle is moving at a high speed in orbit, the gravitational force is still acting on it, constantly pulling it towards the center of the Earth. This force is what keeps the shuttle in its orbit and prevents it from flying off into space.

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• 4.

### Michelle is sliding a box across the floor.  She feels the box resisting her push.  The resistance is caused by

• A.

Pressure

• B.

Friction

• C.

Gravity

• D.

Motion

B. Friction
Explanation
The resistance that Michelle feels while sliding the box across the floor is caused by friction. Friction is the force that opposes the motion of two surfaces in contact with each other. In this case, the box and the floor are in contact and the friction between them resists the motion of the box, making it harder for Michelle to push it.

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• 5.

### Mark wants to slide a chair across a carpeted floor.  He could reduce the effect of friction by

• A.

Placing the chair so that a smaller area is in contact with the floor

• B.

Putting a plastic sheet on the floor before he slides the chair

• C.

Asking a friend to sit in the chair while he pushes it

• D.

Increasing the force with which he pushes the chair

B. Putting a plastic sheet on the floor before he slides the chair
Explanation
By putting a plastic sheet on the floor before sliding the chair, Mark can reduce the effect of friction. The plastic sheet creates a smooth surface, which reduces the friction between the chair and the carpet. This allows the chair to slide more easily across the floor.

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• 6.

### Roger is designing a new model rocket.  He wants it to have as little air resistance as possible.  What should he do?

• A.

Decrease its surface area

• B.

Lower its temperature

• C.

Increase its speed

• D.

Make its surface rougher

A. Decrease its surface area
Explanation
To minimize air resistance, Roger should decrease the surface area of the model rocket. This is because air resistance is caused by the friction between the rocket and the air molecules it encounters. By reducing the surface area, there will be less contact between the rocket and the air, resulting in less friction and thus less air resistance.

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• 7.

### To find pressure, you

• A.

Multiply the mass by the gravitational constant g

• B.

Divide the force by the area over which it is exerted

• C.

Divide the total mass by the volume

• D.

Multiply the mass by the acceleration

B. Divide the force by the area over which it is exerted
Explanation
The correct answer is to divide the force by the area over which it is exerted. This is because pressure is defined as the force applied per unit area. By dividing the force by the area, we can determine the amount of force exerted on each unit of area, which gives us the pressure.

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• 8.

### As Lisa climbs a mountain, the air pressure relative to sea level

• A.

Increases; there is less air below her than above her

• B.

Stays the same no matter how high she climbs

• C.

Decreases; there is more air below her than above her

• D.

Changes depending on how fast Lisa climbs

C. Decreases; there is more air below her than above her
Explanation
As Lisa climbs a mountain, the air pressure relative to sea level decreases. This is because as she gains altitude, the weight of the air above her decreases, resulting in lower air pressure. Therefore, there is more air below her than above her.

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• 9.

### The water in a fast-moving stream exerts less pressure on the bottom than the water in a slow moving stream.  This is an example of

• A.

Pascal's principle

• B.

Buoyancy

• C.

Density

• D.

Bernoulli's principle

D. Bernoulli's principle
Explanation
Bernoulli's principle states that as the speed of a fluid increases, its pressure decreases. In the case of a fast-moving stream, the water flows quickly, resulting in a decrease in pressure on the bottom. This is because the fast-moving water has a lower pressure compared to the slower-moving water. Therefore, the correct answer is Bernoulli's principle.

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• 10.

### Naomi squeezes the wide end of a tube of toothpaste.  The pressure transmitted to the opposite end of the tube is

• A.

Equal to the pressure Naomi exerts

• B.

Less to the pressure Naomi exerts

• C.

Greater to the pressure Naomi exerts

• D.

Dependent on the area of the opposite end of the tube

A. Equal to the pressure Naomi exerts
Explanation
When Naomi squeezes the wide end of the toothpaste tube, the pressure she exerts is transmitted equally to the opposite end of the tube. This is because the toothpaste inside the tube is incompressible, meaning that any pressure applied to one end of the tube will be transmitted throughout the entire volume of toothpaste. Therefore, the pressure transmitted to the opposite end of the tube is equal to the pressure Naomi exerts.

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• 11.

### What unit is used to measure pressure?

• A.

N (Newton)

• B.

Pa (Pascal)

• C.

Kg (kilogram)

• D.

M (meter)

B. Pa (Pascal)
Explanation
The unit used to measure pressure is the Pascal (Pa). Pressure is defined as the force applied per unit area, and the Pascal is the SI unit for measuring this. It is named after the French mathematician and physicist Blaise Pascal. The Pascal is equivalent to one Newton per square meter (N/m^2), which represents the amount of force exerted on a surface area of one square meter.

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• 12.

### What pressure is exerted when a force of 50N is applied to a box with and area of 4 meters squared?  The formula for pressure is P=F/A

• A.

0 Pa

• B.

2 Pa

• C.

25 Pa

• D.

12.5 Pa

D. 12.5 Pa
Explanation
The pressure exerted when a force of 50N is applied to a box with an area of 4 meters squared is 12.5 Pa. This is calculated using the formula for pressure, which is P = F/A. In this case, the force is 50N and the area is 4 meters squared. Therefore, the pressure is 50N/4m^2 = 12.5 Pa.

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• 13.

### Box A (Area = 4 square meters)Box B (Area = 0.75 square meters)Which box exerts the greatest pressure if they have the same area on the floor.

• A.

Box A

• B.

Box B

• C.

Option 3

• D.

Option 4

A. Box A
Explanation
Box A exerts the greatest pressure because pressure is defined as force divided by area. Since both boxes have the same area on the floor, the force exerted by the boxes will determine the pressure. Since Box A has a larger area, it will exert a greater force on the floor compared to Box B, resulting in a greater pressure.

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• 14.

### You are going to drop two balls from the same height.  They both weigh 50 grams.  One ball is the size of a basketball and the other ball is the size of a tennis ball.  Predict which ball will hit the ground first.

• A.

The tennis ball will hit the ground first because it has a smaller surface are so it has less air resistance

• B.

They will both go upward instead of fall to the ground

• C.

The larger ball will hit the ground first because it has more surface area and has more air resistance

• D.

Option 4

A. The tennis ball will hit the ground first because it has a smaller surface are so it has less air resistance
Explanation
The tennis ball will hit the ground first because it has a smaller surface area, which means it will experience less air resistance compared to the larger basketball. This allows the tennis ball to fall faster and reach the ground before the basketball.

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• 15.

### On land, Melanie cannot lift her little brother.  In the water, though, she can lift him easily.  What force helps her in the water?

• A.

Frictional force

• B.

Gravitational force

• C.

Buoyant force

• D.

Option 4

C. Buoyant force
Explanation
In the water, Melanie can lift her little brother easily because of the buoyant force. When an object is submerged in a fluid, it experiences an upward force called buoyant force, which is equal to the weight of the fluid displaced by the object. This buoyant force counteracts the force of gravity, making it easier for Melanie to lift her brother in water compared to on land.

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• 16.

### Robin is pushing an empty cart across the floor at a steady speed.  Is Robin fills the cart with rocks, predict how her force on the cart will compare with her force on the empty cart if she pushes with the same steady speed?

• A.

Robin's force needs to be smaller to push the more massive cart

• B.

Robin's force needs to be greater to push the more massive cart

• C.

Robin's force needs to be the same to push the more massive cart

• D.

It really doesn't matter how the mass of the cart changes

B. Robin's force needs to be greater to push the more massive cart
Explanation
When Robin fills the cart with rocks, the mass of the cart increases. According to Newton's second law of motion, the force required to move an object is directly proportional to its mass. Therefore, as the mass of the cart increases, Robin's force needs to be greater in order to push the more massive cart at the same steady speed.

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