Laws of Motion Test

Result

Laws of Motion Test - 9

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Question 1
1 / -0

Neglecting air resistance, in which of the following cases the net force acting on a stone of mass 0.1 kg is not acting downward?

A
just after it is dropped from the window of a train accelerating with 1 m s⁻²

B
just after it is dropped from the window of a stationary train

C
just after it is dropped from the window of a train running at a constant velocity of 36 km/h

D
lying on the floor of a train which is accelerating with 1 m s⁻² , the stone being at rest relative to the train

Solution

Weight of the stone is balanced by the reaction of the floor. The only acceleration is provided by the horizontal motion of the train.

a = 1ms^-2

force in horizontal direction

F = ma = 0.1 x 1 = 0.1N
Question 2
1 / -0

Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg lying on the floor of a train which is accelerating with 1 m s⁻², the stone being at rest relative to the train. Neglect air resistance.

A
1.0 N in the direction of motion

B
0.1 N along the direction of motion

C
0.1 N opposite to the direction of motion

D
0.2 opposite to the direction of motion

Solution

Weight of the stone is balanced by the reaction of the floor. The only acceleration is provided by the horizontal motion of the train.

a = 1ms^-2

force in horizontal direction

F = ma = 0.1 x 1 = 0.1N
Question 3
1 / -0

A rocket with a lift-off mass 20,000 kg is blasted upwards with an initial acceleration of 5.0 m s-2. Calculate the initial thrust (force) of the blast.

A
365000 N

B
300000 N

C
378000 N

D
350000 N

Solution

Initial thrust = upthrust required to impart acceleration + uthrust to overcome gravity

= ma + mg = m(a + g )= 20000(5 + 10) = 300000N