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**6**. Acceleration (instantaneous or average) is of course**C**. change in speed ÷ time.

Therefore **4**. (280 km/s) / (7 s) = 40 (km/s)/s, **C**.

**1**. It is not clear whether a balls considered at rest or moving at their usual speeds. In the first case "overcome the inertia" means "make a ball move with some speed" of course mass is a measure of inertia, and the answer is **B**.

But in the second case "overcome the inertia" probably means "stop a ball moving with usual speed" and the answer depends on momentum, which may be greater of any ball. So the answer is C if it is known that "is it harder to overcome the inertia of a bowling ball than a baseball".

**2**. The friction force depends on the properties of the surfaces and the pressure force (against the surface). So A is irrelevant, in the cases B and D the friction would decrease and in C it would increase. The answer is **C**.

**5**. Momentum and acceleration aren't forces, so the answer is **A**.

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