Target: 10 Questions in 10 minutes

An IB Physics data booklet is helpful

1-3. A 200 kg cannon initially at rest fires a cannonball of mass 2 kg at a velocity of 100 ms-1.

cannon

 

 1. After firing, what is the momentum of the cannonball?

  • A. 50 kg m s-1
  • B. 100 kg m s-1
  • C. 200 kg m s-1
  • D. 400 kg m s-1

2. What is the total momentum of the system after firing?

  • A. 0 kg m s-1
  • B. 100 kg m s-1
  • C. 200 kg m s-1
  • D. 400 kg m s-1

3. What is the recoil velocity of the cannon?

  • A. 200 m s-1
  • B. 100 m s-1
  • C. 2 m s-1
  • D. 1 m s-1

4-6. Two 'trolleys' (blocks of wood on wheels) are collided together.

The trolleys have magnets on top so they stick together after the collision. Trolley A is moving at 3 m/s towards B, which is stationary.

colliding trolleys

 

4. What is the speed of both A+B (stuck together) after the collision?

  • A. 100 kg m s-1
  • B. 1.5 m s-1
  • C. 1 m s-1
  • D. 2 m s-1

5. What is the impulse of trolley A on Trolley B?

  • A. 0 kg m s-1
  • B. 12 kg m s-1
  • C. 6 kg m s-1
  • D. 4 kg m s-1

6. The collision of the trolleys is inelastic. This means that....

  • A. momentum is not conserved.
  • B. kinetic energy is not conserved.
  • C. the elastic energy is not conserved.
  • D. the total momentum after the collision is negative.

 

 

7. A particle of mass m is accelerated to a velocity v. The momentum of the particle is p and the kinetic energy is k.

A second particle of mass ½m has the same momentum, p. This means the kinetic energy will be...

  • A. ¼ k
  • B. ½ k
  • C. 2 k
  • D. 4 k

 

8-10. A basketball of mass m is thrown to the right at a wall with a velocity of v. It rebounds at the same speed, moving to the left.

 

basketball  

8. What is the impulse acting on the ball during the impact?

  • A. ½ mv
  • B. mv
  • C. ½ mv2
  • D. 2 mv

9. If the force acting on the ball is constant, acting for a time t, the force will be given by...

    answers for Q9

10. Which answer correctly describes the type of collision with the wall and the direction of the force of the wall on the ball?

collision direction of force on the ball
A
elastic to the left
B
elastic to the right
C
inelastic to the left
D
inelastic to the right

 

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Question 1:

Momentum of the cannonball after firing
The momentum of the cannonball is its mass multiplied by its velocity.

Mass of cannonball m = 2 kg

Velocity of cannonball v = 100 m/s

p =m × v =2 kg×100 m/s=200 kg m/s

The correct answer is C. 200 kg m s-1


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Question 2:

Total momentum of the system after firing
According to the law of conservation of momentum, the total momentum of a closed system remains constant. Since the initial momentum of the cannon-cannonball system was 0 (both were at rest), the total momentum after firing must also be 0.

The correct answer is A. 0 kg m s-1


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Question 3:

Recoil velocity of the cannon
Based on the conservation of momentum:
Initial momentum = Final momentum
0=pcannon + pcannonball

0=(mc×vc)+(mb × vb)

We know the mass of the cannon (mc) is 200 kg and the momentum of the cannonball (pb) is 200 kg m/s. We need to find the velocity of the cannon (vc):

0=(200 kg × vc)+ 200 kg m/s
200 ×vc = −200 kg m/s
vc = − 200 / 200
vc = −1 m/s

The negative sign indicates that the cannon moves in the opposite direction of the cannonball. The magnitude of the recoil velocity is 1 m/s.

The correct answer is D. 1 m/s


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Question 4:

Speed of A+B after the collision
This problem can be solved using the principle of conservation of momentum. The total momentum of the system before the collision is equal to the total momentum after the collision.

Initial Momentum:
pinitial =(mA×vA)+(mB ×vB)

pinitial =(4 kg×3 m/s)+(2 kg×0 m/s)
pinitial =12 kg m/s

Final Momentum:
Since the trolleys stick together, the final momentum is the combined mass multiplied by the new velocity (V).
pfinal =(mA×vA)+(mB ×vB)

=(4 kg+2 kg)×v= 6v kg m/s

Conservation:
pinitial =pfinal

12 kg m/s= 6v kg m/s
v= 12/6 =2 m/s

This corresponds to answer D.

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Question 5:

Impulse of trolley A on Trolley B
Impulse is defined as the change in momentum of an object. The impulse of trolley A on trolley B is equal to the change in momentum of trolley B.

Initial momentum of B:
pB,initial = mB × vB = 2 kg×0 m/s = 0 kg m/s

Final momentum of B:
pB,final = mB ×vB = 2 kg×2 m/s = 4 kg m/s

Impulse:
Impulse=Δp =pB,final - pB,initial

Impulse=4 kg m/s−0 kg m/s=4 kg m/s

This corresponds to answer D.


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Question 6:

The meaning of an inelastic collision
In an inelastic collision, kinetic energy is not conserved. During the collision, some of the kinetic energy is converted into other forms of energy, such as heat, sound, or the energy used to deform the objects (in this case, the magnets sticking together). While the kinetic energy of the system changes, momentum is always conserved in a collision, whether it is elastic or inelastic.

B. kinetic energy is not conserved.


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Question 7:

Explanation
To solve this, we can use the relationship between momentum (p) and kinetic energy (k).

First, let's define the initial values for the first particle:

Mass: m

Momentum: p=mv

Kinetic Energy: k=½mv2

We can derive a formula that links kinetic energy and momentum:

From the momentum equation, we can express velocity (v) as:
v= p/m
Substitute this expression for v into the kinetic energy equation:
k=½m(p/m)2

k= p2/2m ( *this is in the IB data book)

So, the kinetic energy of the first particle is k= p2/2m

Now, let's analyze the second particle:

Mass: ½ m

Momentum: p (same as the first particle)

Kinetic Energy: k2

Using the same derived formula for kinetic energy, we can find the kinetic energy of the second particle:
k2 = p2/2m

k2 = p2/2(½ m)

Comparing k2 to k:

k2 = p2/ m

k= p2/2m

​Therefore, k2 =2× (p2/2m)

k2=2k

The kinetic energy of the second particle is twice the kinetic energy of the first particle.

The correct answer is C. 2k.


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Question 8:

Impulse on the ball

Impulse is the change in momentum. Momentum is a vector, meaning its direction matters. Let's define the initial direction (to the right) as positive and the final direction (to the left) as negative.

Initial momentum (pinitial): The ball is moving to the right.
pinitial =m×v

Final momentum (pfinal): The ball rebounds to the left with the same speed.
pfinal =m×(−v)=−mv

Change in momentum (Δp):
Δp=pfinal −pinitial

=(−mv)−(mv)=−2mv

The magnitude of the impulse is 2mv. The negative sign indicates the direction of the impulse is to the left, which makes sense as the wall pushes the ball in that direction.

The correct answer is D. 2mv.


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Question 9:

Force acting on the ball

Impulse is also equal to the force multiplied by the time over which the force acts. The formula is:
Impulse=F×t
From the previous question, we know the magnitude of the impulse is 2mv. Therefore,
F×t = 2mv
F = 2mv /t

The correct answer is D. 2mv/t.


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Question 10:

Collision type and force direction

The correct answer is A. elastic, to the left.


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