Force needed to push a trolley onto sidewalk

[Fibo112]

Homework Statement

Find minimum Force needed to push the trolley onto the curb.

Homework Equations

Torque=r x F

The Attempt at a Solution

The front wheel will rotate about point b if the trolley is pushed onto the sidewalk. This should mean that as soon as the wheel has a positive torque about point b the trolley will move onto the sidewalk. I think that in the moment just before the wheel begins to roll onto the sidewalk there will be no normal force acting on it. What confuses me is how should I know which part of the weight is pushing onto the wheel and if the entire force F is acting on the wheel.

 

[haruspex]

Homework Statement

Find minimum Force needed to push the trolley onto the curb.

Homework Equations

Torque=r x F

The Attempt at a Solution

The front wheel will rotate about point b if the trolley is pushed onto the sidewalk. This should mean that as soon as the wheel has a positive torque about point b the trolley will move onto the sidewalk. I think that in the moment just before the wheel begins to roll onto the sidewalk there will be no normal force acting on it. What confuses me is how should I know which part of the weight is pushing onto the wheel and if the entire force F is acting on the wheel.

At the point where the wheel is about to rotate, what forces act on the trolley? What equations can you write?

 

[Fibo112]

The forces acting are the gravitational force, the force the person imparts, the force that the curb imparts on the wheel and the normal force. Would it be correct to assume the in the moment before the wheel rotates the entire gravitational force is on the front wheels?

 

[haruspex]

the normal force

The normal force where, exactly?
What direction is the force from the kerb?

 

[Fibo112]

The force from the ground on the wheels. I'm not sure in which direction the force from the kerb is. I know it is usually normal to the surface but here is surface is a corner...

 

[haruspex]

The force from the ground on the wheels

Which wheels? Consider the situation just after it starts to mount the kerb.

I'm not sure in which direction the force from the kerb is. I know it is usually normal to the surface but here is surface is a corner...

The normal force is normal to the contact plane. What plane separates the two objects without cutting either?

 

[Fibo112]

I guess one with a 45 degree angle...I would say that there is a normal force acting on all of the wheels..

 

[haruspex]

I would say that there is a normal force acting on all of the wheels..

As I wrote, think about the situation just after it has started to mount the kerb. What are the normal forces on the wheels then?