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

[TheRedDevil18]

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

 

[sankalpmittal]

Hello TheRedDevil18,

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...

 

[TheRedDevil18]

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?

 

[sankalpmittal]

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.