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M1 dynamics

The engine of a van of mass 400 kg cuts out when it is moving along a straight horizontal road with speed 16m s−1. The van comes to rest without the brakes being applied.

In a model of the situation it is assumed that the van is subject to a resistive force which has constant magnitude of 200 N.

a Find how long it takes the van to stop.

Here is my drawing of the system:

Spoiler

Now with all previous questions I've done there seemed to be some sort of force moving the van forward, however looking at the solutions for this question I see that they have only used the resistive force, I'm wondering why is there no force moving the van forward?

Similarly for this question part a)

A small stone of mass 400 g is projected vertically upwards from the bottom of a pond full of water with speed 10m s−1. As the stone moves through the water it experiences a constant resistance of magnitude 3 N. Assuming that the stone does not reach the surface of the pond, find

a) the greatest height above the bottom of the pond that the stone reaches,

Is there no force moving the particle up the water? Or is F just the forces that they have given us.

If anyone could clear this up thanks

(edited 11 years ago)

Reply 1

Killjoy-

In each situation there is no force to accelerate the mass in the direction it is initially moving, the resistive force is the only force acting to change the motion of the van / stone.

Your free body diagram is correct, but note that since you have your acceleration arrow pointing to the RHS a would be negative (if you take motion to the RHS to be positive, of course).

Reply 2

kmikhaylov_101

In both situations the only force acting on the particles is the resistance. So in both cases first use f=ma and you should end up with negative accelerations assuming the direction of travel is positive. Then use the suvat equations to work out in 1) the time when v=0 and in 2) the displacement when v=0

Reply 3

thorn0123

For question 2) I don't understand how there is no force acting on the particle when it is moving against the resistance force

Reply 4

Killjoy-

Original post by thorn0123

For question 2) I don't understand how there is no force acting on the particle when it is moving against the resistance force

It is moving upwards cutting through the water so there must be a resistive force.
In real life the value of this (Viscous drag) force would be related to the speed of the moving particle, it wouldn't be constant.

Reply 5

thorn0123

Original post by Killjoy-

yes but would there not be a force moving up through the water?

Reply 6

Killjoy-

Original post by thorn0123

yes but would there not be a force moving up through the water?

No. The ball has an upward velocity, it does not need to be accelerated upwards to move against the water.
The only thing an additional upward acting force would do is make it move upwards faster (provided that the upward force has a greater magnitude than the resistive force).

If you throw a ball vertically upward it will move after leaving your hand, only falling because of gravity. There is no force to maintain the upward motion.