## SAT Physics Subject Test

## Chapter 3 Newton”s Laws

### Chapter 3 Review Questions

See __Chapter 17__ for solutions.

__1__. A person standing on a horizontal floor feels two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces

(A) have equal magnitudes and form an action/reaction pair

(B) have equal magnitudes but do not form an action/reaction pair

(C) have unequal magnitudes and form an action/reaction pair

(D) have unequal magnitudes and do not form an action/reaction pair

(E) None of the above

__2__. A person who weighs 800 N steps onto a scale that is on the floor of an elevator car. If the elevator accelerates upward at a rate of 5 m/s^{2}, what will the scale read?

(A) 400 N

(B) 800 N

(C) 1000 N

(D) 1200 N

(E) 1600 N

__3__. A frictionless inclined plane of length 20 m has a maximum vertical height of 5 m. If an object of mass 2 kg is placed on the plane, which of the following best approximates the net force it feels?

(A) 5 N

(B) 10 N

(C) 15 N

(D) 20 N

(E) 30 N

__4__. A 20 N block is being pushed across a horizontal table by an 18 N force. If the coefficient of kinetic friction between the block and the table is 0.4, find the acceleration of the block.

(A) 0.5 m/s^{2}

(B) 1 m/s^{2}

(C) 5 m/s^{2}

(D) 7.5 m/s^{2}

(E) 9 m/s^{2}

__5__. The coefficient of static friction between a box and a ramp is 0.5. The ramp”s incline angle is 30°. If the box is placed at rest on the ramp, the box will

(A) accelerate down the ramp

(B) accelerate briefly down the ramp but then slow down and stop

(C) move with constant velocity down the ramp

(D) not move

(E) Cannot be determined from the information given

__6__.

Assuming a frictionless, massless pulley, determine the acceleration of the blocks once they are released from rest.

(A)

(B)

(C)

(D)

(E)

__7__. If all of the forces acting on an object balance so that the net force is zero, then

(A) the object must be at rest

(B) the object”s speed will decrease

(C) the object will follow a parabolic trajectory

(D) the object”s direction of motion can change, but its speed cannot

(E) None of the above

__8__. A block of mass *m* is at rest on a frictionless, horizontal table placed in a laboratory on the surface of the earth. An identical block is at rest on a frictionless, horizontal table placed on the surface of the moon. Let **F** be the net force necessary to give the earth-bound block an acceleration of **a** across the table. Given that *g*_{moon} is one sixth of *g*_{earth}, the force necessary to give the moon-bound block the same acceleration **a** across the table is

(A)

(B)

(C)

(D) **F**

(E) 6**F**

__9__. A crate of mass 100 kg is at rest on a horizontal floor. The coefficient of static friction between the crate and the floor is 0.4, and the coefficient of kinetic friction is 0.3. A force *F* of magnitude 344 N is then applied to the crate, parallel to the floor. Which of the following is true?

(A) The crate will accelerate across the floor at 0.5 m/s^{2}.

(B) The static friction force, which is the reaction force to **F** as guaranteed by Newton”s third law, will also have a magnitude of 344 N.

(C) The crate will slide across the floor at a constant speed of 0.5 m/s.

(D) The crate will not move.

(E) None of the above

__10__. Two crates are stacked on top of each other on a horizontal floor; crate #1 is on the bottom, and crate #2 is on the top. Both crates have the same mass. Compared with the strength of the force **F1** necessary to push only crate #1 at a constant speed across the floor, the strength of the force **F**_{2} necessary to push the stack at the same constant speed across the floor is greater than *F*_{1} because

(A) the force of the floor on crate #1 is greater

(B) the coefficient of kinetic friction between crate #1 and the floor is greater

(C) the force of kinetic friction, but not the normal force, on crate #1 is greater

(D) the coefficient of static friction between crate #1 and the floor is greater

(E) the weight of crate #1 is greater

__11__. If the distance between two point particles is doubled, then the gravitational force between them

(A) decreases by a factor of 4

(B) decreases by a factor of 2

(C) increases by a factor of 2

(D) increases by a factor of 4

(E) Cannot be determined without knowing the masses

__12__. At the surface of the earth, an object of mass *m* has weight *w*. If this object is transported to an altitude that”s twice the radius of the earth, then, at the new location,

(A) its mass is and its weight is

(B) its mass is *m* and its weight is

(C) its mass is and its weight is

(D) its mass is *m* and its weight is

(E) its mass is *m* and its weight is

__13__. A moon of mass *m* orbits a planet of mass 100*m*. Let the strength of the gravitational force exerted by the planet on the moon be denoted by *F*_{1}, and let the strength of the gravitational force exerted by the moon on the planet be *F*_{2}. Which of the following is true?

(A) *F*_{1} = 100*F*_{2}

(B) *F*_{1} = 10*F*_{2}

(C) *F*_{1} = *F*_{2}

(D) *F*_{2} = 10*F*_{1}

(E) *F*_{2} = 100*F*_{1}

__14__. The planet Pluto has 1/500 the mass and 1/15 the radius of Earth. What is the value of *g* on the surface of Pluto?

(A) 0.3 m/s_{2}

(B) 1.6 m/s_{2}

(C) 2.4 m/s_{2}

(D) 4.5 m/s_{2}

(E) 7.1 m/s_{2}

**Keywords**

dynamics

force

tension

gravitational force

air resistance

normal force

frictional force

electrostatic force

Newton”s laws of motion

inertia

law of inertia

kilograms (kg)

mass

newton

action/reaction pair

weight

free body (force) diagram

normal force

friction force

static friction

kinetic (sliding) friction

coefficient of friction

pulley

inclined plane

gravitational force

universal gravitational constant

**Summary**

· Newton”s first law: An object at rest stays at rest, and an object in motion stays in motion. No force means no change in velocity.

· Newton”s second law: Acceleration (**a**) is directly proportional to the net applied force (**F**_{net}) and inversely proportional to the object”s mass (*m*): **F**_{net} = *m***a**.

· Newton”s third law: For every action (or force) there is an equal and opposite reaction (force). The forces are equal in magnitude, opposite in direction, and act on different bodies.

· The weight of an object is the gravitational pull exerted on it by the planet on which the object exists. Mass, conversely, does not change with location; it is a measure of an object”s inertia.

· The normal force is the component of the contact force that”s perpendicular to the surface when an object is in contact with the surface.

· Friction is the component of the contact force that is parallel to the surface when an object is in contact with the surface.

· Kinetic (sliding) friction occurs when there is relative motion (the object is actually sliding across the floor).

· Static friction occurs when there is no relative motion (the object is still or is rolling without slipping).

· Pulleys change the direction of the tension force in the cords that slide over them.

· An inclined plane is a ramp. When an object is on the inclined plane, then the force of gravity **F**_{w} = *m***g** has two components: one parallel to the ramp (*mg* sin *θ*) and one that”s normal (perpendicular) to the ramp (*mg* cos *θ*), where*θ* is the inclined plane.

· Newton”s law of gravitation states that any two objects in the universe exert on each other a gravitational force whose strength is proportional to the product of the object”s masses and inversely proportional to the square of the distance between them.

· A uniform sphere attracts another body as if all of the sphere”s mass were concentrated at its center.