AP Physics B : Forces
Study concepts, example questions & explanations for AP Physics B
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Example Questions
Example Question #1 : Calculating Weight
A 
This question essentially asks how much the astronaut will weigh during acceleration. The two forces acting on the astronaut are the downward gravitational force and the upward normal force. The net force acting on this astronaut is 
Notice that the net force is set to equal to 
Isolate the normal force.
Use the given values to solve.
Example Question #11 : Ap Physics 1
A potted plant is hanging from a rope attached to a hook. The plant exerts 25N of force. Assume the rope is weightless.
If the packaging says the rope is able to withstand 800N of force, what is the maximum amount of mass that can be hanging from the rope?
If the rope can only withstand 800N of force, we can solve for the mass of the object which would result in this maximum force when hanging from the rope.
Example Question #11 : Ap Physics 1
A skydiver with a mass of 100kg jumps out of an airplane and reaches a terminal velocity of 
Which of the following factors will not decrease the total force on the man as he jumps from the plane?
Increasing the man's mass
The man immediately opens his parachute
The man jumps as the plane is descending sharply
The man orients his stomach toward the ground, rather than his feet
Increasing the man's mass
The equation for force is 
All other options will result in a decreased acceleration when falling. Falling with his stomach downward increases the surface area of the diver, which increases his air resistance. A parachute will also increase the air resistance and surface area. If the plane is descending when he jumps, his time to reach terminal velocity will be reduced, as he will already have an initial velocity in the downward direction.
Example Question #1 : Force Equilibrium
A skydiver with a mass of 100kg jumps out of an airplane and reaches a terminal velocity of 
What is the total force acting on the skydiver at this point in his jump?
The man has reached a terminal velocity, which means that he has an acceleration of 
Example Question #11 : Forces
A man pushes a crate across a frictionless floor at a constant velocity. Which of the following statements is true?
The force the man exerts on the crate is equal and opposite to the force the crate exerts on the man
The force the man exerts on the crate is equal and in the same direction as the force the crate exerts on the man
The force on the crate is less than the force on the man
The net force on the man is greater than zero
The force on the crate is greater than the force on the man
The force the man exerts on the crate is equal and opposite to the force the crate exerts on the man
According to Newton's third law, the force on the crate must have the same magnitude as the force on the man. The forces must also be in opposite directions. We can conclude that the net force on the man is zero, since he is moving with the crate at a constant velocity.
Example Question #1 : Newton's Second Law
A 2000kg car with a velocity of 
The car and the truck experience equal force and the car experiences greater acceleration
Both the car and the truck experience equal force and acceleration
The truck experiences the greater force and the greater acceleration
The truck experiences the greater force and the car experiences the greater acceleration
The car experiences the greater force and the greater acceleration
The car and the truck experience equal force and the car experiences greater acceleration
The car and the truck experience equal and opposite forces, but since the car has a smaller mass it will experience greater acceleration than the truck according to the equation F = ma.
A greater mass will decrease the acceleration.
Example Question #31 : Ap Physics 1
The moon's distance from the center of the Earth was decreased by a multiple of three. How would this affect the gravitational force of the Earth on the moon?
The gravitational force would not be affected, because the moon's mass is the same
It would increase by a factor of three
It would increase by a factor of nine
It would decrease by a factor of nine
It would increase by a factor of nine
The law of gravitation is written as 
Since the radius of the two masses acting on each other is squared, and is found in the denominator, a decrease in the radius by a multiple of three will cause a nine-fold increase in the gravitational force.
Example Question #2 : Newton's Second Law
What is the acceleration due to gravity on a planet on which an object with a mass of 20.0kg has a weight of 270N?
0.074m/s2
13.5m/s2
10m/s 2
27m/s2
13.5m/s2
Solve the following equation for acceleration, using the values given in the question.
Example Question #8 : Motion In One Dimension
A ball is thrown horizontally off a cliff of height of 
First we will find the time required for the ball to reach the ground. Since the ball is thrown horizontally, it has no initial vertical component. We use the following equation to solve for the total flight time:
We are given the change in height, initial velocity, and acceleration. Using these values, we can solve for the time. Note that the change in height will be negative, since the ball is traveling downward.
Finally, we use the horizontal velocity to find the distance traveled in 
Example Question #1 : Universal Gravitation
A certain planet has three times the radius of Earth and nine times the mass. How does the acceleration of gravity at the surface of this planet (ag) compare to the acceleration at the surface of Earth (g)?
The acceleration of gravity is given by the equation 
For Earth, 
For the new planet,
So, the acceleration is the same in both cases.
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