So since I can’t put the picture I’m gonna try to describe the situation:
A bunch of guys are pushing a pickup truck in one direction. There’s also a guy *on top* of the pickup truck (in the back) pushing against the front in the same direction. Obviously he’s not applying any force, but why?
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To push something you need to push against something, because the thing you’re pushing exerts an equal and opposite force on you.
If you’re on the ground you’re using your feet to push against the ground, but if you’re ON the truck then the push you give in one direction is cancelled out by the push of your feet against the truck, in the opposite direction. With your feet on the ground you’re exerting a net force on the truck, the two forces don’t cancel out.
He is applying force, he’s just not applying net force. Imagine he’s in socks, what happens? His feet slide backwards! When he puts shoes on, the force on his feet is still there, it’s just pushing what he’s standing on. He’s pushing one direction with his hands, and the other with his feet, and the force is exactly equal, so the net is zero. They cancel each other out.
His hands are pushing the truck forward, sure…
But he’s bracing himself against the truck itself with his feet. His feet are pushing the truck backwards. This is where he’s balancing out the forces. I suppose it’s more like he’s trying to pull the truck apart.
If he pushed and jumped, he could exert some forward force against the truck. But it’ll push him backwards in the air… Assuming he lands on the truck bed, he’ll be moving backwards – skidding in this case – and the backwards movement of his body will slow the truck down. Assuming he doesn’t fall out… either he skids or a stop, or slams into the back of the truck bed to stop himself… yeah, forces largely balance out. Maybe some wind resistance but it’ll be small enough to be ignored.
If he is the Hulk ge wont be applying 0 net force. He will tear the truck in 2. In theory his force just isnt significant enough to do anything to the structural integrity of the truck.
You’re close to something there with his body mass causing higher friction, but he is still exerting net zero force on the truck. The force of friction is properties to the total normal force exerted downward (the “normal force”) on the road by the total mass of the truck, which includes the forward force exerted by the man’s mass. But the force of friction is between the road surface and the truck, not the man in the bed and the truck. The normal force and the friction force are two separate forces.
his hands are pushing the truck forwards, and at the same time, his feet are pushing it backwards.
When the truck is stationary (or moving at constant velocity) these two forces (backwards and forwards) are the same magnitude, and cancel out.
If the truck is accelerating (because the other guys are pushing), then his feet actually apply *more* backwards force than his hands apply forwards force – as a result:
* he accelerates along with the truck (the truck applies to him a net force equal and opposite to the one he is applying to the truck)
* the truck accelerates slower than if he got off and let his friends push just the truck.
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