Topic 6 Review Sheet

Which quantity has both a magnitude and a direction?

• energy

• impulse

• work

• power

Answer explanation

Energy, work, and power are all SCALAR quantities that only have a magnitude. Only impulse has both a magnitude and a direction.

A 3.0-kilogram object is acted upon by an impulse having a magnitude of 15 newton seconds. What is the magnitude of the object's change in momentum due to this impulse?

• 45 kg m/s

• 15 kg m/s

• 3.0 kg m/s

• 5.0 kg m/s

Answer explanation

Remember, impulse = change in momentum. So if the impulse was 15, then the change in momentum is 15!

A 5.00-kilogram block slides along a horizontal, frictionless surface at 10.0 meters per second for 4.00 seconds. What is the magnitude of the block's momentum?

• 50.0 kg m/s

• 12.5 kg m/s

• 200 kg m/s

• 20.0 kg m/s

Answer explanation

p = mv

p = (5kg)(10m/s) - 50 kg m/s

The time was extra information!

When a 1.0-kilogram cart moving with a speed of 0.50 meter per second on a horizontal surface collides with a second 1.0-kilogram cart initially at rest, the carts lock together. What is the speed of the combined carts after the collision? [Neglect friction.]

• 0 m/s

• 0.50 m/s

• 0.25 m/s

• 1.0 m/s

Answer explanation

p(before) = p(after)

(1kg)(0.50m/s) + (1.0kg)(0m/s) = (1kg +1kg)v

.5 = 2v

v = 0.25 m/s

The diagram below shows an 8.0-kilogram cart moving to the right at 4.0 meters per second about to make a head-on collision with a 4.0-kilogram cart moving to the left at 6.0 meters per second. After the collision, the 4.0-kilogram cart moves to the right at 3.0 meters per second. What is the velocity of the 8.0-kilogram cart

after the collision?

• 5.5 m/s, right

• 5.5 m/s, left

• 0.50 m/s, right

• 0.50 m/s, left

Answer explanation

p(before) = p(after)

(8kg)(4m/s) + (4kg)(-6m/s) = (8kg)v + (4kg)(3m/s)

32 + (-24) = 8v + 12

8 = 8v +12

-4 = 8v

-0.5 m/s = v

The negative means to the left!

Watch your signs!!

Which graph best represents the greatest amount of work?

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Answer explanation

Work is the area under a force vs. distance graph - so just look for the largest area - you can calculate if you feel it necessary.

How much work is done by the force lifting a 0.1-kilogram hamburger vertically upward at constant velocity 0.3 meter from a table?

• 0.4 J

• 0.1 J

• 0.03 J

• 0.3 J

Answer explanation

W = Fd

W = (0.1kg x 9.81 m/s2)(0.3m)

W = 0.3 J

You must turn the mass in kg into a force before finding the work done. Since the burger was lifted vertically, the work done was against gravity, so just find the weight!

The watt*second is a unit of what measurement?

• energy

• electric field strength

• power

• potential difference

Answer explanation

Watt is the unit for Power (P)

second is the unit for time (t)

So watt*second = P*t

REarrange the power equation

P = W/t

P*t = W

So watt*second must be a unit for work, which is the same as energy!

A shopping cart slows as it moves along a level floor. Which statement describes the energies of the cart?

• The kinetic energy decreases and the gravitational potential energy increases.

• The kinetic energy decreases and the gravitational potential energy remains the same.

• The kinetic energy increases and the gravitational potential energy decreases.

• The kinetic energy increases and the gravitational potential energy remains the same.

Answer explanation

Since the speed decreases, so must the KE. But since the ground is level, the PE is not increasing OR decreasing, but remaining the same.

Which graph represents the relationship between the kinetic energy and the speed of a freely falling object?

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Answer explanation

The equation for kinetic energy is KE = 1/2 mv²

this shows the KE has a direct squared relationship with velocity. Relate it to y = x² and the shape of that graph!