**Problem : **
Magnetic forces are often at least as powerful as gravitational forces.
Consider a 5 kg piece of iron suspended in mid-air by a powerful magnet
above the piece of iron. How much force does the magnet exert on the
iron?

**Problem : **

The earth rotates around the sun with a constant speed. Is the earth an inertial reference frame?

At first glance, the earth seems to be an inertial reference frame, as it
retains constant speed. However, it does *not* retain constant velocity.
Recall that velocity is a vector while speed is a scalar. Though the magnitude
of the velocity remains constant, the direction changes. In fact, when we
calculate the change in velocity through vector addition we see that
*Δv*
points in the exact direction of the gravitational force exerted on the earth by
the sun, as expected by Newton's Second Law:

Solution 2

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