AP Physics 1 : Forces

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #21 : Normal Force And Weight

A \displaystyle 10.0 kg box rests on a smooth table. An upward force of \displaystyle 200 N is applied to the box so the box is lifted off the table. What is the upward acceleration of the box?  

Possible Answers:

\displaystyle 20.0 \frac{m}{s^{2}}

\displaystyle 10.2\frac{m}{s^{2}}

\displaystyle 30.4 \frac{m}{s^{2}}

\displaystyle 9.8\frac{m}{s^{2}}

\displaystyle 6.77\frac{m}{s^{2}}

Correct answer:

\displaystyle 10.2\frac{m}{s^{2}}

Explanation:

The correct answer is \displaystyle 10.2\frac{m}{s^{2}}. There are 2 forces acting on the box: tension force (\displaystyle 200 N) and normal force (\displaystyle mass \cdot gravity). First we must calculate the force of the box:

\displaystyle F_{box}= (10.0kg)(9.8\frac{m}{s^{2}})= 98.0 N

\displaystyle F_{box} - mg = ma 

The equation can be manipulated to solve for acceleration:

\displaystyle a = \frac{F_{box} - mg}{m}

\displaystyle a = \frac{200 N - 98.0 N}{10.0 kg} = 10.2 \frac{m}{s^{2}}

\displaystyle a=\frac{100N-98}{10kg}=0.20\frac{m}{s^2} 

Example Question #21 : Normal Force And Weight

What is the weight of a car on Earth whose mass is \displaystyle 2700kg?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle W= 52260N

\displaystyle W= 26460N

\displaystyle W= 9800N

\displaystyle W= 2700N

Correct answer:

\displaystyle W= 26460N

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation 

\displaystyle W=mg

Where \displaystyle m is the mass of the object and \displaystyle g is the gravitational constant on the given planet.

On Earth, \displaystyle g=9.8\frac{m}{s^2}, so the weight of the car on Earth is:

\displaystyle W=mg=2700kg*9.8\frac{m}{s^2}= 26460N

Example Question #21 : Forces

What is the weight of a car on Mars whose mass is \displaystyle 2700kg? On Mars, \displaystyle g=3.75\frac{m}{s^2}.

Possible Answers:

\displaystyle W= 27000N

\displaystyle W= 26460N

\displaystyle W=10125N

\displaystyle W=3750N

Correct answer:

\displaystyle W=10125N

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation 

\displaystyle W=mg

Where \displaystyle m is the mass of the object and \displaystyle g is the gravitational constant on the given planet.

On Mars, \displaystyle g=3.75\frac{m}{s^2}, so the weight of the car on Mars is:

\displaystyle W=mg=2700kg*3.75\frac{m}{s^2}= 10125N

Example Question #21 : Normal Force And Weight

What is the normal force of a \displaystyle 5kg block resting on a table?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_{n}=10N

\displaystyle F_{n}=98N

\displaystyle F_{n}=5N

\displaystyle F_{n}=49N

Correct answer:

\displaystyle F_{n}=49N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting. 

Fbd 3

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity. To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes:

 \displaystyle \sum F=0

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces in that direction is

\displaystyle F_{n}-W=0

Substituting in the definition of weight, \displaystyle W=mg, and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_{n}=mg

Since \displaystyle m=5kg and \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_{n}=5*9.8=49N

Example Question #21 : Specific Forces

What is the normal force of a \displaystyle 15kg block resting on a table? 

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_{n}=15N

\displaystyle F_{n}=147N

\displaystyle F_{n}=98N

\displaystyle F_{n}=5N

Correct answer:

\displaystyle F_{n}=147N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting.

Fbd 3

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity.

To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes

 \displaystyle \sum F=0

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces in that direction is

\displaystyle F_{n}-W=0

Substituting in the definition of weight, \displaystyle W=mg, and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_{n}=mg

Since \displaystyle 15kg and \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_{n}=15*9.8=147N

Example Question #21 : Forces

What is the normal force of a \displaystyle 7.5kg block resting on a ramp, \displaystyle \theta=45^{\circ}?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_{n}=147N

\displaystyle F_n =75.7N

\displaystyle F_n =98N

\displaystyle F_n =51.97N

Correct answer:

\displaystyle F_n =51.97N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting. 

Inclined ramp fbd

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity.

Weight must be divided into two components, the parts acting parallel and perpendicular to the contact surface.

Inclined ramp resolved fbd  1

\displaystyle W_x is the x-component of the weight and is \displaystyle W_x=Wsin\theta =mgsin\theta

\displaystyle W_y is the y-component of the weight and is \displaystyle W_y=Wcos\theta =mgcos\theta

To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes

 \displaystyle \sum F=0

To find \displaystyle F_{n}, we only need to consider forces acting the same or opposite direction as \displaystyle F_{n}.

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces in that direction is

\displaystyle F_{n}-W_y=0

Substituting in \displaystyle W_y , and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_n =mgcos\theta

Since \displaystyle 7.5kg\displaystyle \theta=45^{\circ} and \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_n =7.5*9.8cos(45) \approx 51.97N

Example Question #22 : Specific Forces

What is the normal force of a \displaystyle 10kg block resting on a ramp, \displaystyle \theta=30^{\circ}?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_n =51.97N

\displaystyle F_n =112.37N

\displaystyle F_n =54.27N

\displaystyle F_n =84.87N

Correct answer:

\displaystyle F_n =84.87N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting.

Inclined ramp fbd

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity.

Weight must be divided into two components, the parts acting parallel and perpendicular to the contact surface.

Inclined ramp resolved fbd  1

\displaystyle W_x is the x-component of the weight and is \displaystyle W_x=Wsin\theta =mgsin\theta

\displaystyle W_y is the y-component of the weight and is \displaystyle W_y=Wcos\theta =mgcos\theta

To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes

 \displaystyle \sum F=0

To find \displaystyle F_{n}, we only need to consider forces acting the same or opposite direction as \displaystyle F_{n}.

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces is

\displaystyle F_{n}-W_y=0

Substituting in \displaystyle W_y , and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_n =mgcos\theta

Since \displaystyle m=10kg\displaystyle \theta=30^{\circ} and \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_n =10*9.8cos(30) \approx 84.87N

Example Question #24 : Normal Force And Weight

What is the normal force acting on a \displaystyle 10kg book with \displaystyle 10N pressing down on it?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_{n}=98N

\displaystyle F_{n}=122N

\displaystyle F_n =84N

\displaystyle F_{n}=108N

Correct answer:

\displaystyle F_{n}=108N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting.

Fbd force down

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity. \displaystyle F is the force pressing down on the book.

To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes

 \displaystyle \sum F=0

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces in that direction is

\displaystyle F_{n}-W-F=0

Substituting in the definition of weight, \displaystyle W=mg, and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_{n}=mg+F

Since \displaystyle m=10kg,  \displaystyle F=10N and \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_{n}=10*9.8+10=108N

Example Question #23 : Specific Forces

What is the normal force acting on a \displaystyle 12kg television with \displaystyle 15N pulling up on it?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle F_{n}=133.9N

\displaystyle F_{n}=102.6N

\displaystyle F_{n}=98N

\displaystyle F_{n}=87.8N

Correct answer:

\displaystyle F_{n}=102.6N

Explanation:

To begin solving our problem, we will create a free body diagram, labeling all our forces and the direction in which they are acting.

Fbd force up

The normal force, \displaystyle F_{n}, is the component of force perpendicular to the surface of contact. Weight, \displaystyle W, is a force that gravity exerts on an object, acting in the same direction as gravity.  \displaystyle F is the force pulling up on the book.

To find the normal force, we will begin with Newton's 2nd law:

\displaystyle \sum F=ma 

Where \displaystyle \sum F is the sum of all forces in the same direction, \displaystyle m is mass, and \displaystyle a is the acceleration in the direction of the forces.

Since our object is at rest, \displaystyle a=0 and Newton's 2nd law becomes

 \displaystyle \sum F=0

Assuming \displaystyle F_{n} is in the positive direction, the sum of our forces acting in that direction is

\displaystyle F_{n}+F-W=0

Substituting in the definition of weight, \displaystyle W=mg, and solving the expression for \displaystyle F_{n} gives us 

\displaystyle F_{n}=mg-F

Since \displaystyle m=12kg,  \displaystyle F=15Nand \displaystyle g=9.8\frac{m}{s^2}

\displaystyle F_{n}=12*9.8-15=102.6N

Example Question #22 : Forces

What is the mass of a \displaystyle 60000N plane?

\displaystyle g=9.8\frac{m}{s^2}

Possible Answers:

\displaystyle m=3438kg

\displaystyle m=1124kg

\displaystyle m=7264kg

\displaystyle m=6122kg

Correct answer:

\displaystyle m=6122kg

Explanation:

Weight is the force that gravity exerts on an object. Weight is determined using the equation \displaystyle W=mg where \displaystyle m is the mass of the object and \displaystyle g is the gravitational constant on the given planet.

Given the weight of an object and the gravitational constant acting on it, we find the mass using

\displaystyle W=mg\Rightarrow m=\frac{W}{g}=\frac{60000N}{9.8\frac{m}{s^2}} \approx 6122kg

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