How does a car jack multiply a force?

Answer 1

A car jack is able to multiply force by multiplying the torque.

A car jack combines two types of simple machines, a screw and the wheel and axle. For the car jack the wheel handle is very similar to lever handle. When you add force to the lever hand on the car jack that force is transferred to the turning rod.

According to simple lever physics the longer the lever arm the more the force is multiplied. So if a lever arm 1ft long, with 1lb of force applied exerts 1ft /lbs of torque, what happens when the lever arm is 2ft?

Just by doubling the length of the lever arm to 2ft we now double the force to 2ft/lbs of torque.

That torque in the rod is translated to the screws which pull the jack together to lift up the car.

The screw also is a simple machine(an inclined planed curved on itself) which multiples the force(torque) from the lever. The more screw grooves per unit of length the more the force is multiplied.

So comparing a jack that has 20 screw grooves per every 1ft to one that has 40 screw grovers per every 1ft, the one with 40 grooves would be multiplying the force twice as much. So if the 20 screw groove is multiplying the torque 100times the 40 grooves would be multiplying it 200times.

The beauty of a screw as a force multiplier comes in understanding that it is an incline plane curved on itself. Understanding how an incline plane multiples force helps to better understand the screw.

Imagine trying to pick up a 100lb box and put it on a 5ft ledge. It would take over 100lbs to over lift it straight up onto the ledge. Now imagine we add a plank that we can slide the weight onto. This plank is 5ft long, we place it on the ground and on the tip of the ledge. The plank will have a slope of 1(rise)/ 1(run), which is one. That is because it rises one foot up for every foot of distance it spans.

If we wanted to make our task easier and multiply our force even more we could make the plank even longer, making it 50 feet would give us a slope of 5(rise)/(50)run or 1/10 or 0.1 . This gives us a very long slope and we can exert way less force(though we need to exert it for a longer time) to move the 100lbs. For example if a 10 year old can only exert 25lbs of force, that 25lbs of force may now be enough to move the 100lbs given a 5ft plank providing a slope of 1 rise/1 run. We provide the 50ft plank to assist in the job, the 10 year old is still only exerting 25lbs of force but the longer plank (inclined plane) acts as a force multiplier and may multiply his 25lbs of force by 10 allow him to move the 100 lbs given a long slope.

That is the essence of how a car jacks uses a screw and level to multiply force.

-WNL

Answer 2

The physics definition of "force" is a push or pull upon an object resulting from the object's interaction with another object.

Essentially, there are two forces at work in a car jack. The first is gravity, represented by the weight of the vehicle being lifted. The second is the mechanical force applied by the jack handle to the lever that transfers the force on the handle to the jacking mechanism.

A fine point: although the mass of the vehicle is constant, the weight and thus the force required to lift it depend entirely on gravity. E.g., it would take 1/6 as much force to jack a car on the moon as on Earth, but 2.5 times as much to jack the same car on Jupiter, 28 times as much on the sun, and 200 billion times as much on a typical neutron star.