AP Physics 1 : Tension

Study concepts, example questions & explanations for AP Physics 1

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Example Questions

Example Question #1 : Tension

A 10kg block is suspended by two ropes. Each rope makes an angle of 45 degrees to the horizontal.

What is the magnitude of the tension force in each rope?

\displaystyle g=10\frac{m}{s^2}

Possible Answers:

\displaystyle T_1=T_2=73.1N

\displaystyle T_1 = -70.7N,\ T_2 = 70.7N

\displaystyle T_1 = T_2 = 70.7N

\displaystyle T_1 = 52.4N,\ T_2 = 85.2N

\displaystyle T_1=T_2=0N

Correct answer:

\displaystyle T_1 = T_2 = 70.7N

Explanation:

Luckily enough, the angles of the two ropes are the same. Therefore, the tension in each will be the same. This immediately eliminates two of the five answers. Now we just need to calculate what that force is.

We know that together, the vertical components of the tension must equal the weight of the block. Therefore we can write:

\displaystyle T_1sin(45^{\circ})+T_2sin(45^{\circ}) =mg

\displaystyle T_1sin(45^{\circ})+T_2sin(45^{\circ}) = 100N

Since we know that the two tension forces are equal, we can rewrite:

\displaystyle 2Tsin(45^{\circ})=100

Rearranging for T, we get:

\displaystyle T = \frac{100}{2sin(45^{\circ})} = 70.7 N

Example Question #1 : Tension

Consider the following system:

Slope_2

If the mass is \displaystyle 5kg and \displaystyle A = 50^{\circ}, what is the tension, \displaystyle T? Assume no frictional forces.

\displaystyle g = 10\frac{m}{s^2}

Possible Answers:

\displaystyle 41.8N

\displaystyle 38.3N

\displaystyle 56.1N

\displaystyle 77.7N

\displaystyle 64.7N

Correct answer:

\displaystyle 38.3N

Explanation:

Since there is no friction between the mass and slope, there are only two relevant forces acting on the mass: gravity and tension. Furthermore, since the block is not in motion, we know that these forces are equal to each other. Therefore:

\displaystyle T = F_g

Substituting in an expression for the force of gravity, we get:

\displaystyle T = mgsin(\theta)

We know all of these values, allowing us to solve for the tension:

\displaystyle T = (5kg)(10\frac{m}{s^2})sin(50^{\circ})

\displaystyle T = 38.3N

Example Question #1 : Tension

Consider the following system:

Slope_2

If the force of tension is \displaystyle 25N, the force of static friction is \displaystyle 40N, the block has a mass of \displaystyle 10kg, and the block is motionless, what is the angle \displaystyle A?

\displaystyle g=10\frac{m}{s^2}

Possible Answers:

\displaystyle 40.5^{\circ}

\displaystyle 27.2^{\circ}

\displaystyle 39.4^{\circ}

\displaystyle 54.2^{\circ}

\displaystyle 31.4^{\circ}

Correct answer:

\displaystyle 40.5^{\circ}

Explanation:

There are three relevant forces acting on the block in this scenario: friction, tension, and gravity. We are given two of these values, so we simply need to develop an expression for the force of gravity in the direction of the slope. Since the block is motionless, we can write:

\displaystyle F_{net} = 0

\displaystyle F_g - T - F_f = 0

\displaystyle F_g = T + F_f

Substituting in an expression for the force of gravity, we get:

\displaystyle mgsin(\theta) = T+F_f

Rearrange to solve for the angle:

\displaystyle \theta = sin^{-1}\left ( \frac{T+F_f}{mg}\right )

We know all of these values, allowing us to solve:

\displaystyle \theta = sin^{-1}\left ( \frac{25N+40N}{(10kg)(10\frac{m}{s^2})}\right ) = sin^{-1}(0.65)

\displaystyle \theta = 40.5^{\circ}

Example Question #2 : Tension

Consider the following system:

Slope_2

If the coefficient of static friction is \displaystyle 0.25, the angle measures \displaystyle 30^{\circ}, the force of tension is \displaystyle 20N, and the block is motionless, what is the mass of the block?

\displaystyle g = 10\frac{m}{s^2}

Possible Answers:

\displaystyle 8.9kg

\displaystyle 3.2kg

\displaystyle 7.1kg

\displaystyle 12.4kg

\displaystyle 5.4kg

Correct answer:

\displaystyle 7.1kg

Explanation:

There are three relevant forces acting on the block in this scenario: tension, friction, and gravity. We are given tension, so we will need to develop expressions for friction and gravity. Since the block is motionless, we can say:

\displaystyle F_{net}=0

\displaystyle F_g -F_f -T= 0

Plugging in expressions for the force of gravity and friction, we get:

\displaystyle mgsin(\theta)-\mu_smgcos(\theta)-T = 0

Rearranging for the mass, we get:
\displaystyle m=\frac{T}{g(sin(\theta)-\mu_scos(\theta))}

We know all of these values, allowing us to solve:

\displaystyle m = \frac{20N}{(10\frac{m}{s^2})(sin(30^{\circ})-(0.25)cos(30^{\circ}))}

\displaystyle m = 7.1 kg

Example Question #5 : Tension

Consider the following system:


Slope_2

If the block has a mass of \displaystyle 5kg and the angle measures \displaystyle 20^{\circ}, what is the minimum value of the coefficient of static friction that will result in a tension of \displaystyle 0N?

\displaystyle g = 10\frac{m}{s^2}

Possible Answers:

\displaystyle 0.27

\displaystyle 0.11

\displaystyle 0.40

\displaystyle 0.36

\displaystyle 0.45

Correct answer:

\displaystyle 0.36

Explanation:

Since there is no tension, there are only two relevant forces acting on the block: friction and gravity. Since the block is motionless, we can also write:

\displaystyle F_{net} = 0

\displaystyle F_g - F_f = 0

\displaystyle F_g = F_f

Substitute the expressions for these two forces:

\displaystyle mgsin(\theta) = \mu_s gcos(\theta)

Canceling out mass and gravitational acceleration, and rearranging for the coefficient of static friction, we get:

\displaystyle \mu_s = \frac{sin(\theta)}{cos(\theta)}=tan(\theta)

\displaystyle \mu_s = 0.36

Example Question #2 : Tension

Consider the following system:

Slope_2

If the mass is accelerating at a rate of \displaystyle 2\frac{m}{s^2}, the angle measures \displaystyle 30^{\circ}, the mass of the block is \displaystyle 2kg, and the coefficient of kinetic friction is \displaystyle 0.55, what is the tension \displaystyle T?

\displaystyle g = 10\frac{m}{s^2}

Possible Answers:

\displaystyle 11.7N

\displaystyle 3.4N

\displaystyle 0.12N

\displaystyle 1.1N

\displaystyle 24.6N

Correct answer:

\displaystyle 0.12N

Explanation:

There are three relevant forces acting on the block in this situtation: friction, gravity, and tension. We can use Newton's second law to express the system:

\displaystyle F_{net}=ma

\displaystyle F_g -F_f - T = ma

Substituting expressions in for the forces, we get:

\displaystyle mgsin(\theta) - \mu_kmgcos(\theta)-T = ma

Canceling out mass and rearranging to solve for tension, we get:

\displaystyle T = \frac{gsin(\theta)-\mu_kgcos(\theta)}{a}

We have values for each variable, allowing us to solve:

\displaystyle T = \frac{(10\frac{m}{s^2})sin(30^{\circ})-(0.55)(10\frac{m}{s^2})cos(30^{\circ})}{2\frac{m}{s^2}}

\displaystyle T = 0.12 N

Example Question #3 : Tension

Vt physics ramp pulley problem

A 12kg block is sliding down a \displaystyle 30^{o} incline with an acceleration of \displaystyle 1.5\: \frac{m}{s^{2}} as shown in the diagram. If the coefficient of kinetic friction of block 1 on the ramp is 0.18, what is the mass of block 2?

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

Possible Answers:

\displaystyle 3.47kg

\displaystyle 8.12kg

\displaystyle 5.84kg

\displaystyle 2.28kg

Correct answer:

\displaystyle 2.28kg

Explanation:

In order to find the mass of block 2, we're going to need to calculate a few other things, such as the tension in the rope.

To begin with, we'll need to identify the various forces on our free-body diagram. To do this, we will begin with block 1 and use a rotated coordinate system to simplify things. In such a system, the x-axis will run parallel to the surface of the ramp, while the y-axis will be perpendicular to the ramp's surface, as shown below:

Vt physics ramp pulley problem rotated coordinatesVt physics ramp pulley problem free body diagram

Now we can identify the forces acting on block 1. Along the rotated y-axis, the force of gravity acting on the block is equal to \displaystyle mg\cos(\theta), and the force of the ramp on the block is just the normal force, \displaystyle N. Since block 1 is not moving in the y direction, we can set these two forces equal to each other.

\displaystyle N=m_{1}gcos(\theta )

Now, considering the forces acting along the rotated x-axis, we have a force pointing downwards equal to \displaystyle m_{1}g\sin\left ( \theta \right ). Pointing upwards, we have the tension force \displaystyle T and we also have the frictional force, \displaystyle F_{k}.

The formula for calculating the force due to kinetic friction is:

\displaystyle F_{k}=\mu _{k}N

Since we have already determined what the normal force is, we can substitute that expression into the above equation to obtain:

\displaystyle F_{k}=\mu _{k}m_{1}gcos(\theta )

Now, we can write an expression for the net force acting upon block 1 in the x direction:

\displaystyle F_{1(net)}=m_{1}a=m_{1}gsin\left ( \theta \right )-T-\mu _{k}m_{1}gcos(\theta )

Rearrange the above expression to solve for tension.

\displaystyle T=m_{1}gsin(\theta )-\mu _{k}m_{1}gcos(\theta )-m_{1}a

So far, we have only been looking at block 1. Now let's turn our attention to block 2 and see what forces are acting on it. In the downward direction we have the weight of the block due to gravity, which is equal to \displaystyle m_{2}g. In the upward direction, as we can see in the diagram, we have the tension of the rope, \displaystyle T. We need to write an expression that tells us the net force acting upon block 2.

\displaystyle F_{2(net)}=m_{2}a=T-m_{2}g

Since we calculated the expression for tension from the information regarding block 1, we can plug that expression into the above equation in order to obtain:

\displaystyle m_{2}a=[m_{1}gsin(\theta )-\mu _{k}m_{1}gcos(\theta )-m_{1}a]-m_{2}g

Now rearrange to solve for the mass of block 2.

\displaystyle m_{2}a+m_{2}g=m_{2}(a+g)=m_{1}gsin(\theta )-\mu _{k}m_{1}gcos(\theta )-m_{1}a

\displaystyle m_{2}=\frac{[m_{1}gsin(\theta )-\mu _{k}m_{1}gcos(\theta )-m_{1}a]}{(a+g)}

Then plugging in values, we can finally calculate block 2's mass:

\displaystyle m_{2}=\frac{[(12kg*9.8\frac{m}{s^{2}}* sin(30^{o}))-(0.18* 12kg* 9.8\frac{m}{s^{2}}* cos(30^{o}))-(12kg* 1.5\: \frac{m}{s^{2}})]}{(1.5 \frac{m}{s^{2}}+9.8 \frac{m}{s^{2}})}

\displaystyle m_{2}=2.28kg

Example Question #1 : Tension

What is the tension force on a wire holding a 10kg ball 20ft above the ground, if the ball is not moving at that height?

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

Possible Answers:

\displaystyle 6N

\displaystyle 49N

\displaystyle 98N

\displaystyle 0N

Correct answer:

\displaystyle 98N

Explanation:

Since the gravitational force must be cancelled by the tension force, as the ball is experiencing no acceleration, and no other forces are being applied to it:

\displaystyle F_{grav}=F_{tension}=mg

\displaystyle F_{grav}=10kg*9.8 \frac{m}{s^2}=98N= F_{tension}

Example Question #1 : Tension

A block weighing \displaystyle 10kg is hanging from a string. Bruce begins applying a \displaystyle 3N force up on the block. What is the force of tension in the string? 

\displaystyle g=10\frac{m}{s^2}

Possible Answers:

\displaystyle 30N

\displaystyle 97N

\displaystyle 13N

\displaystyle 7N

\displaystyle 103N

Correct answer:

\displaystyle 97N

Explanation:

The block has three forces on it: the force of tension, the force of gravity, and the force from Bruce. The force of gravity is:

\displaystyle F_{grav}= mg

\displaystyle F_{grav}=10kg*10\frac{m}{s^2}=100N

The force from Bruce plus the force of tension has to equal gravity (since Bruce's force and tension are up while gravity is down) so the block is in equilibrium. 

\displaystyle F_{grav}=F_{Bruce}+F_{T}

\displaystyle 100N=3N+F_{T}

\displaystyle F_{T}=97N

Example Question #3 : Tension

A helicopter is lifting a box of mass \displaystyle 1500kg with a rope. The helicopter and box are accelerating upward at \displaystyle 1\frac{m}{s^2}. Determine the tension in the rope.

Possible Answers:

\displaystyle 24.3kN

\displaystyle 16.2kN

\displaystyle 12.7kN

\displaystyle 18.8kN

\displaystyle 10.1kN

Correct answer:

\displaystyle 16.2kN

Explanation:

\displaystyle F_{total}=F_1+F_2+...

\displaystyle ma=F_{rope}+F_{gravity}

\displaystyle ma=T+mg

Plug in values:

\displaystyle 1500*1=T+1500*-9.8

Solve for \displaystyle T

\displaystyle T=16.2kN

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