SmarterEd

Aussie Maths & Science Teachers: Save your time with SmarterEd

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 Figure 10.
 

  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)

Show Answers Only

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:

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

→ \(F_{\text {on B by A }}\) \(=ma\)  
  \(=1 \times 8\)  
  \(=8\ \text{N}\)  
♦♦ 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 Figure 4.  (2 marks)

Show Answers Only

→ 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 normal force of 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  \(v= 2.0\ \text{m s}^{-1}\).
 


 

\(\text{Question 6}\)

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

 
\(\text{Question 7}\)

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. \(F_{  \text {X on Y } }>F_{ \text {Y on X}  }\)
Show Answers Only

\(\text{Question 6:}\ B\)

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

Show Worked Solution

\(\text{Question 6}\)

→ By the law of conservation of momentum, the momentum before the collision is equal to the momentum after the collision.

\(m_Xu_X+m_Yu_Y\) \(=m_Xv_X+m_Yv_Y\)  
  \(=v(m_X + m_Y)\ \ \ (v_X=v_Y) \)  
\(10000 \times 3 +0\) \(= 2(10000 + m_Y)\)  
\(15000\) \(=10000 +m_Y\)  
\(m_Y\) \(=5000\ \text{kg}\)   
  \(=5\ \text{tonnes}\)   

\( \Rightarrow B\)

 
\(\text{Question 7}\)

→ 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. \(\text{800 N}\)
Show Answers Only

\(B\)

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%.