How Do You Calculate the Speed and Retarding Force in a Momentum Problem?

AI Thread Summary
The discussion focuses on solving a momentum problem involving a boy jumping onto a stationary canoe. The initial speed of the combined system after the jump is calculated to be 4 m/s. For the retarding force, the average force acting on the canoe due to water resistance is determined using the third equation of motion, resulting in a deceleration of -0.53 m/s². The net force is then calculated as -39.75 N, but clarification is provided that the average retarding force should only consider the mass of the canoe. The final calculations emphasize the importance of correctly identifying the system in question for accurate results.
TheRedDevil18
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Homework Statement



A boy of mass 60kg, running at 5m/s jumps on a stationary 15kg canoe and they move of together.

1.1) Find their speed just after the boy lands on the canoe
1.2) If the canoe comes to rest after moving through a distance of 15m, calculate the average magnitude of the retarding force on the canoe due to the water

Homework Equations



momentum before= momentum after
Impulse = Change in momentum

The Attempt at a Solution


Unsure about 1.2

1.1) 4m/s(momentum before = momentum after)
1.2) t=d/s = 15/4 = 3.75's

f*t = m*v
3.75f = 75(combined mass)*4
F = 80N

Could someone check
 
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Hello TheRedDevil18,

TheRedDevil18 said:

Homework Statement



A boy of mass 60kg, running at 5m/s jumps on a stationary 15kg canoe and they move of together.

1.1) Find their speed just after the boy lands on the canoe
1.2) If the canoe comes to rest after moving through a distance of 15m, calculate the average magnitude of the retarding force on the canoe due to the water

Homework Equations



momentum before= momentum after
Impulse = Change in momentum

The Attempt at a Solution


Unsure about 1.2

1.1) 4m/s(momentum before = momentum after)

Correct !

1.2) t=d/s = 15/4 = 3.75's

f*t = m*v
3.75f = 75(combined mass)*4
F = 80N

Could someone check

Hmmm. Wrong. Constant resistive force is acting on the canoe system. You will have to use third equation of motion, v2=u2-2ad... Find deceleration a...
 
sankalpmittal said:
Hello TheRedDevil18,



Correct !



Hmmm. Wrong. Constant resistive force is acting on the canoe system. You will have to use third equation of motion, v2=u2-2ad... Find deceleration a...

v^2 = u^2+2ad
16+30a = 0
a = -0.53m/s^-2

Fnet = ma
= 75*-0.53
= -39.75N

Would this be correct?
 
TheRedDevil18 said:
v^2 = u^2+2ad
16+30a = 0
a = -0.53m/s^-2

Fnet = ma
= 75*-0.53
= -39.75N

Would this be correct?

It is asking for the average retarding force exerted on the canoe system, and not the "canoe+boy" system.

You need to consider the mass of canoe only.
 

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