So now go back and look at the friction model. What it says is that the “static frictional force” (Ff)—that is, the amount of friction between two surfaces needed to keep them from moving relative to each other—is less than or equal to the normal force (N) times a “static friction coefficient” (μs). That sets a limit on how much sideways force the block can resist. If the sideways push exceeds the number on the right of the equation, the block will move.
The friction coefficient is a unitless value, usually between 0 and 1, that depends on the specific materials that are in contact. Steel on ice has a low coefficient, so it’s easy to push a sled on ice. Tire rubber on asphalt has a much higher value, somewhere around 0.7.
OK, what does all this have to do with the mass of a pickup truck? Suppose I have a truck that’s pulling another truck. Here is a force diagram for such a situation.
</p> <p><img alt="truck diagram on level plane with Force N meaning surface Force of truck force of gravity and mass nd Force T" class="responsive-image__image" src="https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1" srcset="https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1 1600w, https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1 1280w, https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1 1024w, https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1 768w, https://i2.wp.com/www.ultimatepocket.com/wp-content/uploads/2019/11/tesla-cybertruck-vs-ford-f-150-its-all-about-the-friction.jpg?w=640&ssl=1 640w" sizes="100vw" data-recalc-dims="1"></p> <p>
Illustration: Rhett Allain
Since the truck doesn’t accelerate up or down, the forces in the vertical direction must add up to zero newtons. That means the normal force must have the same magnitude as the downward gravitational force. And the gravitational force depends on the mass of the truck: The heavier the truck, the greater the normal force.
What about the horizontal direction? In the video, the Cybertruck has a rope attached to it. This rope pulls with some tension force to the right, and the friction between the Cybertruck’s tires and the ground push to the left. The frictional force must be at least a little bit greater than the backwards-pulling friction, so that the truck can accelerate (and start to move). But here you can see the key point. More mass means a greater normal force, which in turn produces a greater frictional force. Mass matters.
Oh, but you still don’t buy it? Wouldn’t a more powerful truck win this tug-of-war? How about a nice demonstration? Here I have two battery-powered toy cars. The red car runs on 2 C cell batteries. The blue one is running on just 1 C battery, making it not as strong. What happens when these two cars have a tug-of-war?
Video: Rhett Allain
No surprises here. The car with two batteries wins. But wait! What if I add mass to the car with only one battery? This is what happens.
Video: Rhett Allain
Check that out. The car with only one battery won. It’s the mass that matters. Oh, but what about on an incline? Ok, it’s harder to win this contest if you are pulling uphill—but mass still matters the most. Here is a diagram of a Tesla truck pulling a car up a hill.
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