Topic Menu

Cyber-chess a chess instruction site
Momentum - Multiple choice solutions

Multiple choice questions should take you just under 2 minutes a question! Only one of the choices is correct.

Q1. Which one of the following statements is correct?

The force acting on an object is equivalent to:

A its change of momentum.
B the impulse it receives per second.
C the energy it gains per second.
D its acceleration per metre.

Ft = p - therefore F = p/t = Ft/t = impulse/t

Q2. The graph shows how the force on a glider of mass 2000 kg changes with time as it is launched from a level track using a catapult.

Assuming the glider starts at rest what is its velocity in ms-1 after 40 s?

A 2.5
B 10
C 50
D 100

Area under graph is impulse so Ft = 0.5 x 5 x 40 = 100 kNs

That is the change in momentum - initial momentum being zero we know that final momentum = 100 kNs

mv = 100,000 N

m = 2000 kg therefore v = 100,000/2000 = 50 m/s

Q3. A gas molecule of mass m in a container moves with velocity v.

If it makes an elastic collision at right angles to the walls of the container, what is the change in momentum of the molecule?

A zero
B 0.5 mv
C mv
D 2 mv

mv - (-mv) = 2mv

Q4. The graph shows the variation with time, t, of the force, F, acting on a body.

What physical quantity does the area X represent?

A the displacement of the body
B the acceleration of the body
C the change in momentum of the body
D the change in kinetic energy of the body

Q5. Water of density 1000 kg m–3 flows out of a garden hose of cross-sectional area 7.2 × 10–4m2 at a rate of 2.0 × 10–4m3 per second.

How much momentum is carried by the water leaving the hose per second?

A 5.6 × 10–5 Ns
B 5.6 × 10–2 Ns
C 0.20 Ns
D 0.72 Ns

density = mass/volume therefore mass = density x volume.

You can then work out the mass delivered every second.

That water is delivered like a 'tube' - you are given the CSA so you can work out the length of that tube. That length is the distance the water travels in a second - the velocity!

Momentum = mv so the answer is (density x volume) x (volume / csa)

Q6. Which row, A to D, in the table correctly shows the quantities conserved in an inelastic
collision?

 
mass
momentum
kinetic energy
total energy
A conserved not conserved conserved conserved
B not conserved conserved conserved not conserved
C conserved conserved conserved conserved
D conserved conserved not conserved conserved

Inelastic collision - KE not conserved - a realy quick one to answer!

Q7. A ball of mass 2.0 kg, initially at rest, is acted on by a force F which varies with time t as shown by the graph. What is the velocity of the ball after 8.0 s?

Impulse = area under graph = 40 Ns

The impulse produces the change in momemtum, which, as the initial velocity was zero, will be equal to the final momemtum after 8 seconds.

Final mv = 40 but m = 2.0 kg so v = 20 m/s

 

A 20 ms–1

B 40 ms–1

C 80 ms–1

D 160 ms–1

Q8. A body X moving with a velocity v makes an elastic collision with a stationary body Y of equal mass on a smooth horizontal surface.

Which line, A to D, in the table gives the velocities of the two bodies after the collision?
Velocity of X
Velocity of Y
A
v/2
- v/2
B
- v/2
v/2
C
v
0
D
0
v

 

Elastic collision - therefore no loss of KE.

Initial KE = ½mv2 but if velocities halve total KE will be halved therefore A and B cannot happen.

The ball cannot continue at velocity v without pushing the other ball too - therefore C is wrong.

Answer must be D