Physics: Forces and Machines Worksheet

Question 1

A student pushes a box with a force of 40 newtons, but the box does not move. How much work is done on the box? Explain your answer.

Accepted Answer

The work done on the box is 0 joules because work requires both force and movement in the direction of the force. Since the box does not move, no work is done on it.

Question 2

A wagon is pulled with a force of 25 newtons for a distance of 6 meters. How much work is done on the wagon?

Accepted Answer

The work done on the wagon is 150 joules. This is found by multiplying 25 newtons by 6 meters, which equals 150 joules.

Question 3

A ramp helps a worker move a heavy box into a truck. Explain how the ramp makes the job easier, even though the box must be moved a longer distance.

Accepted Answer

The ramp makes the job easier by reducing the force needed to lift the box. The worker applies a smaller force over a longer distance, so the total amount of work is about the same in an ideal machine.

Question 4

A simple machine has an input force of 20 newtons and produces an output force of 80 newtons. What is the mechanical advantage of the machine?

Accepted Answer

The mechanical advantage is 4. This is found by dividing the output force, 80 newtons, by the input force, 20 newtons.

Question 5

Classify each tool as a lever, pulley, wheel and axle, inclined plane, wedge, or screw: scissors, flagpole rope system, doorknob, knife, and jar lid.

Accepted Answer

Scissors are levers and wedges because the handles act as levers and the blades act as wedges. A flagpole rope system is a pulley. A doorknob is a wheel and axle. A knife is a wedge. A jar lid uses a screw thread.

Question 6

A lever has a fulcrum in the middle, an input force on one end, and a load on the other end. What class of lever is it? Give one example.

Accepted Answer

This is a first-class lever because the fulcrum is between the input force and the load. A seesaw is one example of a first-class lever.

Question 7

A student lifts a 60-newton backpack straight up 1.5 meters. How much work is done on the backpack?

Accepted Answer

The work done on the backpack is 90 joules. This is found by multiplying 60 newtons by 1.5 meters.

Question 8

A machine has a mechanical advantage of 3. If the input force is 50 newtons, what output force does the machine produce?

Accepted Answer

The machine produces an output force of 150 newtons. This is found by multiplying the input force, 50 newtons, by the mechanical advantage, 3.

Question 9

Two students move identical boxes onto the same stage. Student A lifts a box straight up 1 meter. Student B pushes a box up a ramp that is 4 meters long. Compare the force each student must use and the work done in an ideal situation.

Accepted Answer

Student B uses less force because the ramp spreads the lifting over a longer distance. In an ideal situation without friction, both students do the same amount of work because they raise the identical boxes to the same height.

Question 10

A pulley system changes the direction of a force so a person can pull down on a rope to lift a bucket up. Does this pulley system always reduce the amount of input force needed? Explain.

Accepted Answer

No, a pulley that only changes the direction of the force does not always reduce the input force needed. A fixed pulley mainly makes the force easier to apply by changing its direction.

Question 11

A machine does 200 joules of useful output work when 250 joules of input work are put into it. What is the efficiency of the machine?

Accepted Answer

The efficiency of the machine is 80 percent. This is found by dividing 200 joules by 250 joules and multiplying by 100 percent.

Question 12

Why is the efficiency of a real machine usually less than 100 percent?

Accepted Answer

The efficiency of a real machine is usually less than 100 percent because some energy is transformed into heat, sound, or vibration due to friction and other losses.

Question 13

A screw is used to hold two pieces of wood together. Explain how a screw is related to an inclined plane.

Accepted Answer

A screw is an inclined plane wrapped around a cylinder. The spiral threads let a small turning force move the screw forward into the wood.

Question 14

A person uses a crowbar to pry open a lid. The fulcrum is placed close to the lid. Why does this make the crowbar more helpful?

Accepted Answer

Placing the fulcrum close to the lid increases the mechanical advantage of the crowbar. A longer effort arm and shorter load arm allow a smaller input force to produce a larger output force on the lid.

Question 15

A wheelbarrow has the wheel as the fulcrum, the load in the bucket, and the effort applied at the handles. What class of lever is a wheelbarrow, and how does it help the user?

Accepted Answer

A wheelbarrow is a second-class lever because the load is between the fulcrum and the input force. It helps the user by reducing the force needed to lift and move the load.

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Physics: Forces and Machines

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