Practical Activity for 14-16
- Activity time 15 mins
In this activity students observe a crate being lifted by two different methods. You can use it to introduce horizontal and vertical force components.
- Crate filled with books (or other objects) to provide a total mass of approx. 2.5 kg
- Two lengths of rope each about 2 m long
Preparation & safety
Before the activity attach a length of rope to each end of the crate. Ensure that the ropes are tied securely and that the crate doesn’t tip over when lifted. During the demonstration discourage volunteers from trying to impress their classmates by pulling on the ropes with exaggerated force.
- Tuck the ropes into the crate and use your hands to lift it off the floor or bench.
- Hold crate stationary with your arms straight and then put it down.
- Ask for two volunteers to pull on the ropes to lift the crate and hold it stationary above the ground with the ropes at angles of about 45°.
- Now challenge them to try to pull firmly on the ropes until they are horizontal.
- Which forces act on the crate?
- Are the forces balanced?
- Why is it impossible to get the ropes horizontal?
Most students should be able to identify forces acting on the crate lifted by hand and explain why they balance. The forces are vertical. Each hand provides half the upward force required to balance the pull of gravity.
For the crate lifted by ropes some may struggle with the direction of the lifting forces. Explain how they arise. When the crate is lifted off the ground, the ropes stretch slightly, exerting forces along their length and at an angle of 45°.
To explain how the forces balance, introduce force components. They can think of each force as being made up of two parts one sideways and one upwards known as the horizontal and vertical components. In the horizontal direction the components are of equal size but in opposite directions and so cancel each other out. Similarly, gravity is balanced by the vertical components.
No matter how hard your students pull, it’s impossible to get the ropes completely horizontal because you always need a vertical component to balance gravity.
Students explain equilibrium situations in terms of vertical and horizontal force components.
This experiment was safety-checked in March 2020.