Mechanics (Part 1)

Speed, Displacement, Velocity & Acceleration

Displacement

is a straight-line distance in a give direction.

Speed

is distance divided by time. It’s the rate of change of distance with respect to time.

Average speed

distance/time

Velocity

is the rate of change of displacement with respect to time

Velocity

displacement/time

Acceleration

is the rate of change of velocity with respect to time

acceleration

change in velocity/time

Acceleration due to gravity

in the absence of air resistance, all object in freefall near the surface of the Earth will experience the same acceleration

Describe the steps for measurement of the acceleration of a rider on an air track using light gates & timers?

1. Measure the distance, s, between the light gates.

2. Measure the length of the card using a meter stick.

3. Set up the apparatus as shown.

4. Ensure track is level.

5. Attach rider to the weight using thread.

6. Turn on air blower. Release the rider.

7. Record times through each gate.

8. Calculate u and v.

9. Calculate a using the formula a = (v2 –u2 )/2s.

10. Repeat experiment to obtain an average value.

What apparatus is used for measurement of the acceleration of a rider on an air track using light gates & timers?

Air track, timers, thread, rider and card, metre stick, weight, light gates, air blower, retort stands and clamps.

What needs to be to make the measurement of the acceleration of a rider on an air track using light gates & timers aceturate?

• Ensure air track is level.

• Clean the air track.

• Use an air track to minimize friction.

• Avoid parallax error.

What’s the value of g on the surface of the Earth?

9.8m s^-2

What are the steps for measurement of acceleration due to gravity (g)

1. Set up the apparatus as shown. The millisecond timer starts when the ball is released and stops when the ball hits the trapdoor.

2. Measure the distance s using a metre stick.

3. Flick the switch to release the ball and record the time t from the millisecond timer.

4. Repeat for different values of s.

5. Draw a graph of s against t 2 and use the slope to find a value for g.

Conclusion

The value for g should be approximately 9.8 m s^-2

What apparatus is used for measurement of acceleration due to gravity (g)

Millisecond timer, metal ball, trapdoor and electromagnet.

Precaution for measurement of acceleration due to gravity (g)

• For each height s repeat three times and take the smallest time as the correct value for t.

• Place a piece of paper between the ball bearing and the electromagnet to ensure a quick release.

• Remember to convert from milliseconds to seconds.

Force

is anything which has the potential to cause an object to accelerate.

One newton

is that force which causes a body of mass 1 kg to undergo an acceleration of 1 m s^-2.

What apparatus is use to show that acceleration is proportional to the force causing it

Air track, rider and card, light gates, timers, metre stick, air blower, thread, masses, sticky tape, electronic balance, retort stand and clamps.

The steps to show that acceleration is proportional to the force causing it

1. Measure the mass of both riders.

2. Measure the length of card using a metre stick.

3. Set up as shown.

4. Gently push rider 1 so that it moves at constant velocity. It should move through gate 1 before colliding with the second rider.

5. Record the first time and hence find the initial velocity.

6. The rider then collides and sticks to the second rider.

7. Record the second time and hence find the final common velocity.

8. Calculate the momentum before and after.

Conclusion

The total momentum before the collision equalled the total momentum after the collision, in agreement with the theory.

Precautions to show that acceleration is proportional to the force causing it

• Ensure the air track is level.

• Use an air track to minimise friction.

• Clean the air track.

• Avoid parallax error.

• Check that the first rider is moving at constant velocity

Mass

is a measure of how difficult it is to accelerate that body.

Momentum

is equal to its mass multiplied by its velocity

Principle of conservation of momentum.

In any interaction, the total momentum before the interaction is equal to the total momentum afterwards provided no external forces act on the system.

What equation do you use for one object collides with another and the two objects coalesce (stick together) then there is only one (common) velocity after collision.

m₁u₁ + m₂u₂ = m₁v₁ +m₂v₂

m₁u₁ + m₂u₂ = (m₁ +m₂)v