Lesson test: Forces in action

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Forces in action

1.[2p]
A block slides down a 30° slope with $μ_{k} = 0.20$ . What is its acceleration, in m s⁻², with $g = 9.81$ m s⁻²?

CorrectNot quite: 3.21

2.[3p]
A 10 kg crate sits on a floor with $μ_{s} = 0.50$ and nobody is touching it. What is the friction force on it?

Zero, because static friction only supplies what is needed to prevent sliding

Correct

49 N, because friction is $μ_{s} N$

The answer is: Zero, because static friction only supplies what is needed to prevent sliding

98 N, equal to its weight

The answer is: Zero, because static friction only supplies what is needed to prevent sliding

5 N

The answer is: Zero, because static friction only supplies what is needed to prevent sliding

3.[2p]
A block just begins to slide when a plane is tilted to 20°. What is $μ_{s}$ ?

CorrectNot quite: 0.364

4.[2p]
An Atwood machine carries 4.0 kg and 6.0 kg over a frictionless massless pulley. What is the magnitude of the acceleration, in m s⁻²?

CorrectNot quite: 1.96

5.[2p]
For the same 4.0 kg and 6.0 kg Atwood machine, what is the tension in the string, in newtons?

CorrectNot quite: 47.1

6.[1p]
The normal force on a body is always equal to its weight.

True

The answer is: False

False

Correct

7.[3p]
Which of these are true of the Amontons and Coulomb rules for dry friction?

Select all that apply

Kinetic friction is roughly independent of the apparent contact area

Correct

Static friction is an upper limit rather than a fixed value

Correct

They are empirical fits with a limited range of validity, not laws

Correct

Friction is always proportional to the mass of the sliding body

The answer is: Kinetic friction is roughly independent of the apparent contact area, Static friction is an upper limit rather than a fixed value, They are empirical fits with a limited range of validity, not laws

8.[3p]
A skydiver of mass 80 kg has a frontal area of 1.0 m² and $C_{d} = 1.2$ , in air of density 1.2 kg m⁻³. What is the terminal speed, in metres per second?

CorrectNot quite: 33.0

9.[2p]
Put the steps of the free body method in the order they are carried out.

Draw every force on it and name the agent of each
Write one component equation per axis and solve
Choose axes, preferring one along the acceleration
Isolate one body and draw it detached from everything it touches

Show the answer

a, b, c, d

1.[2p] A block slides down a 30° slope with μk=0.20. What is its acceleration, in m s⁻², with g=9.81 m s⁻²?

2.[3p] A 10 kg crate sits on a floor with μs=0.50 and nobody is touching it. What is the friction force on it?

3.[2p] A block just begins to slide when a plane is tilted to 20°. What is μs?

4.[2p] An Atwood machine carries 4.0 kg and 6.0 kg over a frictionless massless pulley. What is the magnitude of the acceleration, in m s⁻²?

5.[2p] For the same 4.0 kg and 6.0 kg Atwood machine, what is the tension in the string, in newtons?

6.[1p] The normal force on a body is always equal to its weight.

7.[3p] Which of these are true of the Amontons and Coulomb rules for dry friction?

8.[3p] A skydiver of mass 80 kg has a frontal area of 1.0 m² and Cd=1.2, in air of density 1.2 kg m⁻³. What is the terminal speed, in metres per second?

9.[2p] Put the steps of the free body method in the order they are carried out.

1.[2p]
A block slides down a 30° slope with $μ_{k} = 0.20$ . What is its acceleration, in m s⁻², with $g = 9.81$ m s⁻²?

2.[3p]
A 10 kg crate sits on a floor with $μ_{s} = 0.50$ and nobody is touching it. What is the friction force on it?

3.[2p]
A block just begins to slide when a plane is tilted to 20°. What is $μ_{s}$ ?

4.[2p]
An Atwood machine carries 4.0 kg and 6.0 kg over a frictionless massless pulley. What is the magnitude of the acceleration, in m s⁻²?

5.[2p]
For the same 4.0 kg and 6.0 kg Atwood machine, what is the tension in the string, in newtons?

6.[1p]
The normal force on a body is always equal to its weight.

7.[3p]
Which of these are true of the Amontons and Coulomb rules for dry friction?

8.[3p]
A skydiver of mass 80 kg has a frontal area of 1.0 m² and $C_{d} = 1.2$ , in air of density 1.2 kg m⁻³. What is the terminal speed, in metres per second?

9.[2p]
Put the steps of the free body method in the order they are carried out.