Physics: Forces and Machines
Exploring force, work, mechanical advantage, and simple machines
Physics: Forces and Machines
Exploring force, work, mechanical advantage, and simple machines
Physics - Grade 6-8
- 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.
Use the idea that work equals force times distance.
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. - 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?
Work = force x distance.
The work done on the wagon is 150 joules. This is found by multiplying 25 newtons by 6 meters, which equals 150 joules. - 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.
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. - 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?
Mechanical advantage = output force divided by input force.
The mechanical advantage is 4. This is found by dividing the output force, 80 newtons, by the input force, 20 newtons. - 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.
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. - 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.
Look at the position of the fulcrum compared with the effort and load.
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. - 7
A student lifts a 60-newton backpack straight up 1.5 meters. How much work is done on the backpack?
The work done on the backpack is 90 joules. This is found by multiplying 60 newtons by 1.5 meters. - 8
A machine has a mechanical advantage of 3. If the input force is 50 newtons, what output force does the machine produce?
Output force = input force x mechanical advantage.
The machine produces an output force of 150 newtons. This is found by multiplying the input force, 50 newtons, by the mechanical advantage, 3. - 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.
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. - 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.
Think about a single fixed pulley attached above a load.
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. - 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?
Efficiency = useful output work divided by input work, then multiplied by 100 percent.
The efficiency of the machine is 80 percent. This is found by dividing 200 joules by 250 joules and multiplying by 100 percent. - 12
Why is the efficiency of a real machine usually less than 100 percent?
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. - 13
A screw is used to hold two pieces of wood together. Explain how a screw is related to an inclined plane.
Imagine unwrapping the thread of the screw into a slanted ramp.
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. - 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?
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. - 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?
In a second-class lever, the load is in the middle.
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.