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PHYSICS, M2 EQ-Bank 12

A student pushes a 5.0 kg box across a rough horizontal surface with a force of 30 N. The box moves with constant velocity.

  1. Identify the net force acting on the box. Justify your answer.   (1 mark)
  1. According to Newton’s Third Law, describe the force pair involved in this situation.   (2 marks)
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a.    The box moves at a constant velocity \(\Rightarrow\) Net force acting on the box is zero.
 

b.    The force pair is:

→ The student exerts at force of 30 N on the box.

→ The box exerts a reaction force of 30 N back on the student.

Show Worked Solution

a.    The box moves at a constant velocity \(\Rightarrow\) Net force acting on the box is zero.
 

b.    The force pair is:

  • The student exerts at force of 30 N on the box.
  • The box exerts a reaction force of 30 N back on the student.

PHYSICS, M2 EQ-Bank 6 MC

Two ice skaters, A and B, are standing still on an ice rink facing each other. Skater A pushes skater B. What happens immediately after the push?

  1. Only skater B moves because skater A applied the force
  2. Skater B moves backward, and skater A remains stationary 
  3. Both skaters move in opposite directions
  4. Skater A applies more force, so only skater B accelerates
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\(C\)

Show Worked Solution
  • When Skater A pushes Skater B (a contact force), an equal and opposite force is exerted back on Skater A by Skater B. As stated by Newton’s third law.
  • Both skaters accelerate in opposite directions due to these equal and opposite forces.

\(\Rightarrow C\)

PHYSICS, M2 2018 VCE 8

Two blocks, \(\text{A}\) of mass 4.0 kg and \(\text{B}\) of mass 1.0 kg, are being pushed to the right on a smooth, frictionless surface by a 40 N force, as shown in the diagram.
 

  1. Calculate the magnitude of the force on block \(\text{B}\) by block \(\text{A}\) (\(\left.F_{\text {on B by A}}\right)\). Show your working.   (2 marks)

  2. State the magnitude and the direction of the force on block \(\text{A}\) by block \(\text{B}\) (\(\left.F_{\text {on A by B}}\right)\).   (2 marks)

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a.    \(8\ \text{N}\)

b.    \(8\ \text{N}\) to the left.

Show Worked Solution

a.   Using \(F=ma\), calculate the acceleration of the entire system:

\(a=\dfrac{F}{m}=\dfrac{40}{5}=8\ \text{ms}^{-2}\)

\(F_{\text {on B by A }}=m \times a=1 \times 8=8\ \text{N}\)

♦♦ Mean mark (a) 36%.

 
b.   Newton’s third law of motion:

  • \(F_{\text {on A by B }}\) will be equal in magnitude and opposite in direction.
  • \(F_{\text {on A by B }}= 8\ \text{N}\) to the left.

PHYSICS, M2 2021 VCE 4

Liesel, a student of yoga, sits on the floor in the lotus pose, as shown in Figure 4. The action force, \(F_g\), on Liesel due to gravity is 500 N down.
 

Identify and explain what the reaction force is to the action force, \(F_{ g }\), shown in the diagram above.   (2 marks)

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  • As the action force, \(F_g\), is the gravitational force on Liesel due to the Earth, then the reaction force is the gravitational force on the Earth due to Liesel, by Newton’s third law of motion.
  • This would be a force of 500 N of Liesel pulling up on the Earth (the Earth being attracted\accelerated to Liesel due to her own gravitational force).
Show Worked Solution
  • As the action force, \(F_g\), is the gravitational force on Liesel due to the Earth, then the reaction force is the gravitational force on the Earth due to Liesel, by Newton’s third law of motion.
  • This would be a force of 500 N of Liesel pulling up on the Earth (the Earth being attracted\accelerated to Liesel due to her own gravitational force).
♦♦♦ Mean mark 9%.
COMMENT: Many students confused Newton’s third law with balancing forces. The 500N pushing up on Liesel is the normal force not the reaction force (which are not the same).

PHYSICS, M2 2022 VCE 6-7 MC

A railway truck \(\text{(X)}\) of mass 10 tonnes, moving at 3.0 m s\(^{-1}\), collides with a stationary railway truck \(\text{(Y)}\), as shown in the diagram below.

After the collision, they are joined together and move off at speed  \(v= 2.0\ \text{m s}^{-1}\).
 


 

Question 6

Which one of the following is closest to the mass of railway truck \(\text{Y}\)?

  1. 3 tonnes
  2. 5 tonnes
  3. 6.7 tonnes
  4. 15 tonnes

 
Question 7

Which one of the following best describes the force exerted by the railway truck \(\text{X}\) on the railway truck \(\text{Y} \left(F_{\text { X on Y}}\right)\) and the force exerted by the railway truck \(\text{Y}\) on the railway truck \(\text{X} \left(F_{\text {Y on X}}\right)\) at the instant of collision?

  1. \(F_{  \text { X on Y }  }<F_{ \text {Y on X}  }\)
  2. \(F_{ \text { X on Y }  }=F_{ \text { Y on X}  }\)
  3. \(F_{ \text { X on Y }  }=-F_{  \text { Y on X}  }\)
  4. \(F_{  \text {X on Y } }>F_{ \text {Y on X}  }\)
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\(\text{Question 6:}\ B\)

\(\text{Question 7:}\ C\)

Show Worked Solution

Question 6

  • By the law of conservation of momentum, the momentum before the collision is equal to the momentum after the collision.
\(m_Xu_X+m_Yu_Y\) \(=m_Xv_X+m_Yv_Y\)  
  \(=v(m_X + m_Y)\ \ \ (v_X=v_Y) \)  
\(10000 \times 3 +0\) \(= 2(10000 + m_Y)\)  
\(15000\) \(=10000 +m_Y\)  
\(m_Y\) \(=5000\ \text{kg}\)   
  \(=5\ \text{tonnes}\)   

\( \Rightarrow B\)

 
Question 7

  • By Newton’s 3rd law of motion, each action has an equal and opposite reaction. 
  • Hence, the force of \(F_{ \text { X on Y }  }\) is equal in magnitude to \(F_{  \text { Y on X}  }\) but opposite in direction which is indicated by the negative sign (–).

\( \Rightarrow C\)

♦ Mean mark (Q7) 47%.

PHYSICS, M2 2022 VCE 9 MC

Two students pull on opposite ends of a rope, as shown in the diagram below. Each student pulls with a force of 400 N.
 

Which one of the following is closest to the magnitude of the force of the rope on each student?

  1. \(\text{0 N}\)
  2. \(\text{400 N}\)
  3. \(\text{600 N}\)
  4. \(\text{800 N}\)
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\(B\)

Show Worked Solution
  • Newton’s 3rd Law states that every action has an equal and opposite reaction.
  • As each student exerts a force of 400 N on the rope, the rope will exert a force of 400 N on the student.

\(\Rightarrow B\)

♦ Mean mark 52%.