# Questions for Forces and Dynamics (JC) Set 3

A man is standing on a beam which is resting on the ground. Which of the following pair forms an action-reaction pair of forces?

1. Contact force on man due to beam and the gravitational force on beam due to Earth.
2. Weight of man and the gravitational force on Earth due to man.
3. Contact force on man due to beam and weight of man.
4. Gravitational force on man due to Earth and gravitational force on beam due to Earth.

Action reaction pair needs to fulfil the following:

• has the same magnitude,
• acts along the same line but in opposite direction,
• acts on a different object,
• of the same type.

Thus only option 2 meets the above criteria. Option 4 is not correct because they act in the same direction.

A lorry and a motorcycle with different masses are involved in a head on collision, after which they travel in the opposite direction. The speeds of the lorry and the motorcycle before the collision are 10.0 m s-1 and 13.0 m s-1 respectively, and after the collision are 5.0 m s-1 and 28.0 m s-1 respectively.

Which of the following statements about the collision is true?

1. Motorcycle experienced a larger force than lorry during the collision.
2. Magnitudes of impulses experienced by lorry and motorcycle during the collision are the same.
3. Individual momentums of lorry and motorcycle are conserved.
4. Total kinetic energy of lorry and motorcycle is conserved.

During collision, the impulse experienced by two bodies are the same according to Newton’s third law.

A mass of 0.20 kg is suspended from the roof of a lift by a light thread. The thread is cut while the lift is ascending upwards at 3.0 m s-2. What is the magnitude of the acceleration of the 0.20 kg mass at the instant the thread is cut?

1. 0 m s-2
2. 3.0 m s-2
3. 9.8 m s-2
4. 12.8 m s-2

When the string is cut, the only acceleration acting on the mass is due to gravity which is equal to 9.81 m s-2.
A large box is pushed across a horizontal ground at constant speed. If the push is removed, the box will

1. stop at once.
2. start to slow down to a stop.
3. continue to travel at constant speed.
4. continue to travel at constant speed before slowing down to a stop

Friction must be present initially to balance the pushing force, so that the box moves with constant speed. Once the pushing force is removed, friction will cause the box to slow down to a stop.

A 50cm3 block of lead and a 50cm3 block of copper are completely under water. Each is suspended by a thread so that they both are at the same height in an aquarium filled with water. The volume of the aquarium is 300 cm3.Which of the following is true?

1. The buoyant force is greater on the lead than on the copper
2. The buoyant force is greater on the copper than on the lead
3. The buoyant force is the same on both blocks 