dynamics

Each diagram illustrates a pair of forces of equal magnitude.Which diagram gives an example of a pair of forces that is described by Newton’s third law of motion?

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A helium atom of mass m collides normally with a wall. The atom arrives at the wall with speed v and then rebounds along its original path. Assume that the collision is perfectly elastic.

What is the change in the momentum of the atom during its collision?

• zero

• 0.5 mv

• mv

• 2 mv

A nucleus collides with a stationary nucleus in a vacuum. The diagrams show the paths of the nuclei before and after the collision.

No other particles are involved in the collision.

Which diagram is not possible?

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A picture is suspended from a nail by a single cord connected to two points X and Y on the picture. There is negligible friction between the cord and the nail so that the tension in both sides of the cord is the same. The picture hangs symmetrically, as shown.

The tension in the cord is T. The angle between the cord and the vertical is θ on both sides.

Which statement is correct?

• Increasing the length of the cord, with points X and Y in the same place on the picture, would reduce the tension in the cord.

• Moving points X and Y further apart on the picture while keeping the length of the cord constant would reduce the tension in the cord.

• Moving points X and Y to the top edge of the picture while keeping their distance apart constant and the length of the cord constant would reduce the tension in the cord.

• The weight of the picture is equal to T cosθ.

Two objects X and Y in an isolated system undergo a perfectly elastic collision. The velocities of the objects before and after the collision are shown.

What is the speed v of Y after the collision?

• 2.0 m s^–1

• 18 m s^–1

• 22 m s^–1

• 24 m s^–1

A solid sphere, which is less dense than water, is held completely immersed in water a few metres below the surface. The density of the water is uniform.

The sphere is released. Immediately after release, the sphere rises.

Which row correctly describes the changes in the (i) upthrust on the sphere and the (ii) resultant force on the sphere?

• (i) constant, (ii) increasing

• (i) constant, (ii) decreasing

• (i) decreasing, (ii) increasing

• (i) decreasing, (ii) decreasing

A uniform rod of length 200 cm is freely pivoted at point P. The rod is held horizontally in equilibrium by a 60 N weight that is attached to the rod by a string passing over a frictionless pulley.

What is the weight of the rod?

• 30 N

• 60 N

• 80 N

• 140 N

A resultant force of 10 N acts on a body for a time of 2.0 s.Which graph could show the variation with time t of the momentum p of the body?

• A

• B

• C

• D

The mass of a rocket-propelled truck is approximately equal to the mass of the fuel in its tank.

The fuel is ignited and the truck is propelled along horizontal tracks by a constant force. The effect of air resistance is negligible.

During a test run the fuel is consumed at a constant rate.

Which statement describes the acceleration of the truck during the test run?

• The acceleration of the truck decreases as the fuel is consumed.

• The acceleration of the truck increases as the fuel is consumed.

• The acceleration of the truck remains constant.

• The acceleration of the truck is zero and the truck moves at a constant velocity.

A particle with mass and charge is moving from left to right in a uniform gravitational field and a uniform electric field. The gravitational field is downwards. The gravitational force and the electric force on this particle act in opposite directions.

What could be the (i) sign of the charge on the particle and the (ii) direction of the electric field?

• (i) negative, (ii) down

• (i) negative, (ii) up

• (i) positive, (ii) left

• (i) positive, (ii) right