jo asked in 科學及數學其他 - 科學 · 7 years ago

# Mechanics(action and reaction)

A ship carrying a car is initially at rest on sea water.As the car starts to move,the ship is found to move automatically.

1. Explain why the ship will move

2. If the car moves to the right,what is the direction of motion of the ship.

3. Would the car and the ship move at the same speed relative to the sea ?

Update:

Please answer Q1 with more details,the action and reaction chain and 1.what force ?2.which object act the force 3.on which object.? i know it is about action and reaction,but please tell ,4.which 2 forces are pair.(what happens when the car starts to move and it how to cause the ship to move)

Update 2:

For Q1 ,i know that it is about reaction force,but what really happens when the car starts to move and how it can cause the ship to move,pls tell me with more details(state all related forces and how they change)

Rating
• 天同
Lv 7
7 years ago

When the car starts to move on the ship, it (the car) gives a force to the ship by its rotating wheels. This is the action force given by the car onto the ship.

By Newton's Third Law (Action and Reaction Law), the ship will, in return, give a reaction force that acts on the car. Hence, it is by such reaction force that makes the car to move on the ship.

Since action and reaction force must act in opposite directions, the car and ship thus move in opposite directions.

Because action and reaction forces are equal in magnitude, under the same force magnitude, the one with small mass moves faster than the one with large mass. Therefore, the car moves at a higher speed than the ship does.

Alternatively, you can also use momentum conservation to explain this problem.

Because the forces between the car and ship are internal forces, the momentum of the system, which is initially zero (i.e. at rest) must be maintained zero afterwards.

After the car starts to move to the right, it has acquird a momentum towards the right. The ship thus needs to move to the left, so as to produce an equal momentum towards the left, in oder to balance the momentum of the car to make the system momentum to maintain zero.

Because the momenta of the car and ship are equal, hence the car (which has a small mass) possesses speed higher than that of the ship (which has a large mass).

• CK
Lv 7
7 years ago

1. Explain why the ship will move

It moves due to the reaction force from the car.

2. If the car moves to the right,what is the direction of motion of the ship.

The shipp moves to the left.

3. Would the car and the ship move at the same speed relative to the sea ?

No. the ship and the car are not of the same mass. Usually, the ship is much heavier than the car and thus moves slower.